Mphg2009-chapter 7 Electricity

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Physics Module Form 5 Chapter 7: Electricity

CHAPTER 7: ELECTRICITY 7.1 CHARGE AND ELECTRIC CURRENT Van de Graaf 1. What is a Van de Graaff generator? Fill in each of the boxes the name of the part shown. A device that ……………….. and ………………….. at high voltage on its dome

+

+

+

+

dome

+

+

+

+

+ +

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Physics Module Form 5 Chapter 7: Electricity

2. How are electrical charges produced by a Van de Graaff generator? And what type of charges is usually produced on the dome of the generator? ˆ When the motor of the Van de Graaff generator is switched on, it drives the ……………………….. ˆ This causes the rubber belt to against the …….……… and hence becomes …..……… ˆ The charge is then carried by the moving belt up to the ……………

……….

where it is collected. ˆ A large amount of ……………. is built up on the dome ˆ ……………………. charges are usually produced on the dome of the generator.

3. What will happen if the charged dome of

+ +

the Van de Graaff is connected to the earth via a micrometer? Explain. ˆ There is a …………………….. of the pointer of the microammeter. ˆ This indicates an electric current …………………….

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+ + +

+

+

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

4. Predict what will happen if a discharging metal sphere to the charged dome.

+ +

+

ˆ When the discharging metal sphere is brought

near

the

charged

dome,

…………………………… occurs. ˆ An electric current ……………

5.

Predict what will happen if hair of a student is brought near to the charged dome. Give reasons for your answer. ˆ The metal dome …………. the hair and the hair stand ……………….. ˆ This is because of each strand of hair receives ……………….. charges and …………………….. each other.

6. The flow of electrical charges produces ………………….

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+ + + + +

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Physics Module Form 5 Chapter 7: Electricity

Electric Current 1. Electric current consists of a flow of …………...... 2. The more charges that flow through a cross section within a given time, the ……………… is the current. 3. Electric current is defined as the rate of flow of …………………………. Each second, 15 coulombs of charge cross the plane. The current is I = 15 amperes. One ampere is one coulomb per second.

4. In symbols, it is given as:

I=

where I = …………………….…

Q = …………………….… t = …………..................... (i) The SI unit of charge is (Ampere / Coulomb / Volt) (ii) The SI unit of time is (minute / second / hour) (iii)The SI unit of current is (Ampere / Coulomb / Volt) is equivalent to (Cs // C-1s // Cs-1)

I (iv) By rearranging the above formula, Q = ( It / t

t

/ I

)

4. If one coulomb of charge flows past in one second, then the current is …………………. ampere. 5. 15 amperes means in ………………second, …………….. coulomb of charge through a cross section of a conductor. 6. In a metal wire, the charges are carried by…………………. 7. Each electron carries a charge of ……………………….. 8. 1 C of charge is……………………………..

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Physics Module Form 5 Chapter 7: Electricity

Electric Field a) An electric field is a ………………. in which an……………… experiences a………….. b) An electric field can be represented by a number of lines indicate both the……………. and ……………….. of the field c) The principles involved in drawing electric field lines are : (i) electric field lines always extend from a ……………… - charged object to a ………………..-charged object (ii) electric field lines never ………………….. each other, (ii) electric field lines are ……………….. in a ………………….. electric field.

Demo: To study the electric field and the effects of an electric field. Apparatus & materials Extra high tension (E.H.T) power supply (0 – 5 kV), petri dish, electrodes with different shapes (pointed electrode and plane electrode), two metal plates, talcum powder, cooking oil, polystyrene ball coated with conducting paint, thread and candle. Method DEMO A)

1. Set up the apparatus as shown in the above figure 2. Switch on the E.H.T. power supply and adjust the voltage to 4 kV 3. Observed the pattern formed by the talcum powder for different types of electrodes. 4. Draw the pattern of the electric field lines.

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→

Physics Module Form 5 Chapter 7: Electricity

Draw the pattern of the electric field lines. ELECTRIC FIELD AROUND A POSITIVE CHARGE

ELECTRIC FIELD AROUND A NEGATIVE CHARGE

ELECTRIC FIELD AROUND A POSITIVE AND NEGATIVE CHARGE

ELECTRIC FIELD AROUND TWO NEGATIVE CHARGES

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Physics Module Form 5 Chapter 7: Electricity

ELECTRIC FIELD AROUND TWO POSITIVE CHARGES

ELECTRIC FIELD AROUND A NEGATIVE CHARGE AND A POSITIVELY CHARGED PLATE

ELECTRIC FIELD AROUND A POSITIVE CHARGE AND A NEGATIVELY CHARGED PLATE

ELECTRIC FIELD BETWEEN TWO CHARGED PARALLEL PLATES

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Physics Module Form 5 Chapter 7: Electricity

EFFECT OF AN ELECTRIC FIELD ON A POLYSTYRENE BALL Observation: The polystyrene ball oscillated between the two plates, touching one plate after another. Explanation: ˆ When the polystyrene ball touches the negatively charged plate, the ball receives negative charges from the plate and experiences a repulsive force.

1. Place the polystyrene ball between the two metal plates.

ˆ The ball will then move to the positively charged plate.

2. Switch on the E.H.T and displace the polystyrene ball slightly so that it

ˆ When the ball touches the plate, the ball loses some of its negative charges to the

touches one of the metal plates

plate and becomes positively charged. It then experiences a repulsive force. This process continues.

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Physics Module Form 5 Chapter 7: Electricity

EFFECT OF AN ELECTRIC FIELD ON A CANDLE FLAME C)

Observation: The candle flame splits into two portions in opposite direction. The portion that is attracted to the negative plate is very much larger than the portion of the flame that is attracted to the positive plate. Explanation: 1) Switch of the E.H.T and replace the

ˆ The heat of the flame ionizes the air

polystyrene ball with a lighted candle.

molecules to become positive and

2) Sketch the flame observed when the E.H.T. is switched on.

negative charges. ˆ The positive charges are attracted to the negative plate while the negative charges are attracted to the positive plate. ˆ The flame is dispersed in two opposite directions but more to the negative plate. ˆ The positive charges are heavier than the negative charges. This causes the uneven dispersion of the flame.

Conclusion 1. Electric field is a ……………………………………………………………………….. 2. Like charges ………………. each other but opposite charges …………… each other. 3. Electric field lines are ……………………in an electric field. The direction of the field lines is from …………………….. to …………………………

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Physics Module Form 5 Chapter 7: Electricity

Exercise 7.1 1. 5 C of charge flows through a wire in 10 s. What is the current in the wire?

2.

A charge of 300 C flow through a bulb in every 2 minutes. What is the electric current in the bulb?

3.

The current in a lamp is 0.2 A. Calculate the amount of electric charge that passes through the lamp in 1 hour.

4.

If a current of 0.8 A flows in a wire, how many electrons pass through the wire in one minute? (Given: The charge on an electron is 1.6 x 10-19 C)

An electric current of 200 mA flows through a resistor for 3 seconds, what is the (a)

electric charge

(b)

the number of electrons which flow through the resistor?

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Physics Module Form 5 Chapter 7: Electricity

Ideas of Potential Difference (a)

(b)

X

water Y P

Q

⇒ ˆ Pressure at point P is ………………

ˆ Gravitational potential energy at X is ……………

than the pressure at point Q

than the gravitational potential energy at Y.

ˆ Water will flow from ……to ……

ˆ The apple will fall from …… to …… when the

when the valve is opened.

apple is released.

ˆ This due to the ……………….. in the

ˆ This due to the …………………….. in the

pressure of water

gravitational potential energy.

(c) Similarly, ˆ Point A is connected to…………………. terminal ˆ Point B is connected to …………………. terminal ˆ Electric potential at A is

……………. than the electric

potential at B.

Bulb

ˆ Electric current flows from A to B, passing the bulb in the

A

B

circuit and ……………….. the bulb. ˆ This is due to the electric ………………. between the two terminals. ˆ As the charges flow from A to B, work is done when electrical energy is transformed to ………….and …………… energy. ˆ The potential difference, V between two points in a circuit is defined as ……………………………………………………... ………………………………………………………………… ………………………………………………………………… ˆ The potential difference,V between the two points will be given by:

W Work V = Quantityofch arg e = Q

where W is …………………………. Q is ………………………….

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Device and symbol Cells

ammeter voltmeter

Switch

connecting wire

Constantan wire // eureka wire bulb

resistance rheostat

Measuring Current and Potential Difference/Voltage

Measurement of electricity

Measurement of potential difference/voltage

(a) Electrical circuit

(a) Electrical circuit

(b) Circuit diagram

(b) Circuit diagram

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1. Name the device used to measure electrical current.

2. (a) What is the SI unit for current?

Physics Module Form 5 Chapter 7: Electricity

1. Name the device used to measure potential difference.

2. (a) What is the SI unit for potential difference?

(b) What is the symbol for the unit of current?

(b) What is the symbol for the unit of potential difference?

3. How is an ammeter connected in an electrical circuit?

3. How is an voltmeter connected in an electrical circuit?

4. The positive terminal of an ammeter is connected to which terminal of the dry cell?

4. The positive terminal of a voltmeter is connected to which terminal of the dry cell?

5. What will happen if the positive terminal of the ammeter is connected to the negative terminal of the dry cell?

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Experiment: To investigate the relationship between current and potential difference for an ohmic conductor.

(a)

(b)

Figure (a) and figure (b) show two electrical circuits. Why do the ammeters show different readings? Why do the bulbs light up with different intensity? Referring to the figure (a) and (b), (i) Make one suitable inference. (ii) State one appropriate hypothesis that could be investigated. (iii) Design an experiment to investigate the hypothesis. (a) Inference

The current flowing through the bulb is influenced by the potential difference across it.

(b) Hypothesis (c) Aim

To determine the relationship between current and potential difference for a constantan wire. (i)

(d) Variables

manipulated variable

:

(ii) responding variable

:

(iii) fixed variable

:

Apparatus / materials

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JPN Pahang Student’s Copy Method

Physics Module Form 5 Chapter 7: Electricity : 1.

Set up the apparatus as shown in the figure.

2.

Turn on the switch and adjust the rheostat so that the ammeter reads the current, I= 0.2 A.

Tabulation of

3.

Read and record the potential difference, V across the wire.

4.

Repeat steps 2 and 3 for I = 0.3 A, 0.4 A, 0.5 A, 0.6 A and 0.7 A.

:

data

Analysis of data

:

Discussion

:

Current,I/A

Volt, V/V

0.2

1.0

0.3

1.5

0.4

2.0

0.5

2.5

0.6

3.0

0.7

3.5

Draw a graph of V against I . 1.

From the graph plotted. (a) What is the shape of the V-I graph? The graph of V against I is a straight line that passes through origin (b) What is the relationship between V and I? This shows that the potential difference, V is directly proportional to the current, I. (c) Does the gradient change as the current increases? The gradient ≡ the ratio of

2.

V I

is a constant as current increases.

The resistance, R, of the wire used in the experiment is equal to the gradient of the V-I graph. Determine the value of R.

3.

What is the function of the rheostat in the circuit? It is to control the current flow in the circuit

Conclusion

:

The potential difference, V across a conductor increases when the current, I passing through it increases as long as the conductor is kept at constant temperature.

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Physics Module Form 5 Chapter 7: Electricity

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Physics Module Form 5 Chapter 7: Electricity

Ohm’s Law (a) Ohm’s law states that the electric current, I flowing through a conductor is directly proportional to the potential difference across the ends of the conductor, if temperature and other physical conditions remain constant (b) By Ohm’s law:

V

∝

I

= constant × I or

V = constant I

(c) The constant is known as ………………………………. of the conductor. (d) The resistance, R is a term that describes …………………………………………….. ………………………………………………………………………………………….. It is also defined as the ratio……………………………………………………………. ………………………………………………………………………………………….. That is R=

and

V=

(e) The unit of resistance is ………………………………… (f) An ……………………….. is one which obeys Ohm’s law, while a conductor which does not obey Ohm’s law is known as a ……………………….conductor

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Factors Affecting Resistance 1. The resistance of a conductor is a measure of the ability of the conductor to (resist / allow) the flow of an electric current through it. 2. From the formula V = IR, the current I is (directly / inversely) proportional to the resistance, R. 3. When the value of the resistance, R is large, the current, I flowing in the conductor is (small / large) 4. What are the factors affecting the resistance of a conductor? a) ……………………………………………………………. b) ……………………………………………………………. c) ……………………………………………………………. d) ……………………………………………………………. 5. Write down the relevant hypothesis for the factors affecting the resistance in the table below.

The temperature of the conductor

The type of the material of the conductor

The cross-sectional area of the conductor, A

Length of the conductor, l

Factors

Diagram

Hypothesis The …………… the conductor, the …………….. its resistance Resistance is …………………. proportional to the length of a conductor The ……………….….. the cross sectional area, the …….………… the its resistance Resistance is ……………...…….. proportional to the cross-sectional area of a conductor Different conductors with the same physical conditions have ……………………. resistance

The …………………. The temperature of a conductor, the …………………... the resistance

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Graph

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Physics Module Form 5 Chapter 7: Electricity

6. From, the following can be stated: ˆ Resistance of a conductor,

R

∝

length

ˆ Resistance of a conductor,

R

∝

1 cross-sectional area

ˆ Hence, resistance of a conductor, R

∝

length cross-sectional area

Or

R∝

l

or

R= ρ l

A

A

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where ρ =

resistivity of the substance

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Exercise 7.2 1.

Tick (√) the correct answers True

False

(a) Unit of potential difference is J C-1 (b) J C-1 ≡ volt, V The potential difference between two points is 1 volt if 1 joule (c) of work is required to move a charge of 1 coulomb from one point to another. (d)

2 volt is two joules of work done to move 2 coulomb of charge from one to another in an electric field.

(e) Potential difference ≡ Voltage

2.

t I / ) t I

i) Electric charge,

Q = ( It /

ii) Work done,

W = (QV /

V Q / ) Q V

iii) Base on your answer in 2(i) and (ii) derive the work done, W in terms of I, V and t. W

=

QV

=

ItV

3. If a charge of 5.0 C flows through a wire and the amount of electrical energy converted into heat is 2.5 J. Calculate the potential differences across the ends of the wire. W

=

QV

2.5

=

5.0 (V)

V

=

0.5 V

4. A light bulb is switched on for a period of time. In that period of time, 5 C of charges passed through it and 25 J of electrical energy is converted to light and heat energy. What is the potential difference across the bulb? W

=

QV

20

=

6 (V)

V

=

3.33 V

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Physics Module Form 5 Chapter 7: Electricity

5. The potential difference of 10 V is used to operate an electric motor. How much work is done in moving 3 C of electric charge through the motor? W

=

QV

=

3 (10)

=

30 J

6. When the potential difference across a bulb is 20 V, the current flow is 3 A. How much work done to transform electrical energy to light and heat energy in 50 s? W

=

VIt

=

20 (3) (50)

=

3000 J

Bulb

3A 20 V

7. What is the potential difference across a light bulb of resistance 5 Ω when the current that passes through it is 0.5 A? V

=

IR

=

0.5 (5)

=

2.5 V

8. A potential difference of 3.0 V applied across a resistor of resistance R drives a current of 2.0 A through it. Calculate R. V

=

IR

3.0

=

2.0 (R)

R

=

1.5 Ω

9. What is the value of the resistor in the figure, if the dry cells supply 2.0 V and the ammeter reading is 0.5 A? V

=

IR

2.0

=

0.5 (R)

R

=

4Ω

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Physics Module Form 5 Chapter 7: Electricity

10. If the bulb in the figure has a resistance of 6 Ω, what is the reading shown on the ammeter, if the dry cells supply 3 V? V

=

IR

3.0

=

6 (R)

R

=

0.5 Ω

11. If a current of 0.5 A flows through the resistor of 3 Ω in the figure, calculate the voltage supplied by the dry cells? V R

=

IR

=

0.5 (3)

=

1.5 Ω

12. The graph shows the result of an experiment to determine the resistance of a wire. The resistance of the wire is From V-I graph, resistance

V/V 1.2

= gradient = = 2.4Ω

0 13. An experiment was conducted to measure the current, I flowing through a constantan wire when the potential difference V across it was varied. The graph shows the results of the experiment. What is the resistance of the resistor? From V-I graph, resistance

= gradient = = 2.0 x 10-3Ω

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5

I/A

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

14. Referring to the diagram on the right, calculate (a) The current flowing through the resistor. V

=

IR

12

=

I (5)

I

=

2.4 A

I

5Ω 12 V

(b) The amount of electric charge that passes through the resistor in 30 s Q

=

It

=

2.4 (30)

=

72 C

(c) The amount of work done to transform the electric energy to the heat energy in 30 s. W

=

QV

or

W

=

72 (12)

= 12(2.4)(30)

=

864 C

= 864 C

15. Figure shows a torchlight that uses two 1.5 V dry cells. The two dry cells are able to provide a current of 0.3 A when the bulb is at its normal brightness. What is the resistance of the filament? V

=

IR

3.0

=

0.3(R)

I

=

10Ω

16. The diagram shows four metal rods of P, Q, R and S made of the same substance. a) Which of the rod has the most resistance? P

b) Which of the rod has the least resistance? S

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= VIt

+ 1.5 V -

+ 1.5 V -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

17. The graph shows the relationship between the

V/V

potential difference, V and current, I flowing through two conductors, X and Y.

X 8 Y

a) Calculate the resistance of conductor X. From V-I graph, resistance

= gradient =

2 0 0

I/A

2

= 4Ω

b) Calculate the resistance of conductor Y. From V-I graph, resistance

= gradient = = 1Ω

c) If the cross sectional area of X is 5.0 x 10-6 m2, and the length of X is 1.2 m, calculate its resistivity.

18. The graph shows a graph of I against V for three conductors, P, Q and R. i) Compare the resistance of conductor P, Q and R. Q

I/A

P

Q

R

ii) Explain your answer in (a) From V-I graph, resistance = gradient The greater the gradient, the greater the resistance Gradient of P > Gradient of Q > Gradient of R

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V/V

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Physics Module Form 5 Chapter 7: Electricity

19. Figure shows a wire P of length, l with a crosssectional area, A and a resistance, R. Another wire, Q is a conductor of the same material with a length of 3l and twice the cross-sectional area of P. What is resistance of Q in terms of R? Conductor P

R

=

Conductor Q

R’

=

(notes: P and R have the same resistivity, ρ)

=

=

R

20. PQ, is a piece of uniform wire of length 1 m with a resistance of 10Ω. Q is connected to an ammeter, a 2 Ω resistor and a 3 V battery. What is the reading on the ammeter when the jockey is at X? Resistance in the wire R is directly proportional to l = 10 Ω

100 cm

Hence, 20 cm = R

(10)

= 2Ω

Total resistance 2Ω + 2Ω = 4Ω Current, I

= =

= 0.75 A

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Physics Module Form 5 Chapter 7: Electricity

21. Figure shows the circuit used to investigate the relationship between potential difference, V and current, I for a piece of constantan wire. The graph of V against I from the experiment is as shown in the figure below.

(a)

What quantities are kept constant in this experiment? Length // cross-sectional area // type of material // temperature of the wire

(b)

State the changes in the gradient of the graph, if i) the constantan wire is heated R ↑, gradient ↑ // the resistance increases, hence the gradient increases

ii) a constantan wire of a smaller cross-sectional area is used R ↑, gradient ↑ // the resistance increases, hence the gradient increases

iii) a shorter constantan wire is used R ↓, gradient ↓ // the resistance decreases, hence the gradient decreases

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Physics Module Form 5 Chapter 7: Electricity

7.3 SERIES AND PARALLEL CIRCUITS Current Flow and Potential Difference in Series and Parallel Circuit SERIES CIRCUIT

PARALLEL CIRCUIT

I

V

1

the current flows through each bulb/resistor is

1

the same.

bulb/resistor

I = I1 = I2 = I3 2

the potential difference is the same across each V = V1 = V2 = V3

the potential difference across each bulb /

2

the current passing through each bulb / resistor is

resistor depends directly on its ………………….

inversely proportional to the resistance of the

The potential difference supplied by the dry cells

resistor. The current in the circuit equals to the

is shared by all the bulbs / resistors.

sum of the currents passing through the bulbs / resistors in its parallel branches.

V = V1 + V2 + V3

where V is the potential I = I1 + I2 + I3

difference across the

from the battery

battery 3

If Ohm’s law is applied separately to each bulb /

where I is the total current

3

If Ohm’s law is applied separately to each bulb /

resistor, we get :

resistor, we get :

V = V1 + V2 + V3 IR = IR1 + IR2 + IR3

I = I1 + I2 + I3 V V V V R = R1 + R2 + R3

If each term in the equation is divided by I, we

If each term in the equation is divided by V, we

get the effective resistance

get the effective resistance 1 1 1 R = R1 + R2 +

R = R1 + R2 + + R3

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1 R3

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Identify series circuit or parallel circuit

(a)

(b)

(c)

Ammeter reading ≡ Current

Voltmeter reading ≡ Potential difference ≡ Voltage

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(d)

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Effective resistance, R

(a) (b)

(c)

(d)

(e)

(f)

(h)

(g)

(i)

(j)

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Solve problems using V = IR

Exercise 7.3 1.

The two bulbs in the figure have a resistance of 2Ω and 3Ω respectively. If the voltage of the dry cell is 2.5 V, calculate (a) the effective resistance, R of the circuit Effective R = 2 + 3 = 5 Ω (b) the main current, I in the circuit

(c) the potential difference across each bulb. 2Ω: V = IR = (0.5)(2) = 1V 3Ω: V = IR = (0.5)(3) = 1.5 V

V = IR 2.5 =I(5) = 0.5 A 2.

There are two resistors in the circuit shown. Resistor R1 has a resistance of 1Ω. If a 3V voltage causes a current of 0.5A to flow through the circuit, calculate the resistance of R2. V = IR 3=0.5(1+R2) R2 = 5Ω

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Physics Module Form 5 Chapter 7: Electricity The electrical current flowing through each branch, I1 and I2, is 5

3.

A. Both bulbs have the same resistance, which is 2Ω. Calculate the voltage supplied. Parallelcircuit;V =V1=V2 = IR1 or = IR2 = 5(2) = 10 V 4. The voltage supplied to the parallel is 3 V. R1 and R2 have a resistance of 5Ω and 20Ω. Calculate (a) the potential difference across each resistor 3 V (parallel circuit) (b) the effective resistance, R of the circuit 1/R = 1/5 + 1/20 =1/4 R=4Ω (c) the main current, I in the circuit

(d) the current passing through each resistor 5Ω: V = IR 3 =I(5) I = 0.6 A

V = IR 3 =I(4) = 0.75 A 5.

In the circuit shown, what is the reading on the ammeter when switch, S (a) is open? Effective R = 6 Ω V = IR 12 =I(6) I=2A

6.

20 Ω: V = IR 3 =I(20) I = 0.15 A

(b) is closed? Effective R = 4 Ω V = IR 12 =I(4) I=3A

Determine the voltmeter reading.

Determine the ammeter reading.

(a)

(a)

(b)

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Physics Module Form 5 Chapter 7: Electricity

7.

Calculate

(d) (i) The potential difference across 8Ω

(a) The effective resistance, R

resistor.

R = 12 Ω

V = IR

(b) The main current, I

= 2(8) = 16 V

I=2A

(ii) The potential difference across 2.5Ω

(c) The current passing through 8Ω and 2.5Ω

resistor.

resistors.

V = IR

I=2A

= 2(2.5) = 5 V (e) The current passing through 6 Ω resistor. V = V8 + V2.5 +Vparallel 24 = 16 + 5 + Vparallel Vparallel = 3V V = IR 3 = I(6) I = 0.5 A

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Physics Module Form 5 Chapter 7: Electricity

8.

The electrical components in our household appliances are connected in a combination of series and parallel circuits. The above figure shows a hair dryer which has components connected in series and parallel. Describe how the circuit works. ƒ ƒ ƒ ƒ ƒ ƒ

The hair dryer has three switches A, B and C When switch A is switched on, the dryer will only blow air at ordinary room temperature When switches A and B are both switched on, the dryer will blow hot air. As a safety feature to prevent overheating, the heating element will not be switched on if the fan is not switched on The hair dryer has an energy saving feature. Switch C will switch on the dryer only when it is held by the hand of user The body of the hair dryer must be safe to hold and does not get hot easily

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Physics Module Form 5 Chapter 7: Electricity

7.4 ELECTROMOTIVE FORCE AND INTERNAL RESISTANCE Electromotive force Figure (a)

Figure (b)

Voltmeter reading, potential difference, V < e.m.f.,

Voltmeter reading, e.m.f.

E,r

R Current flowing

No current flow

1. An electrical circuit is set up as shown in figure (a). A high resistance voltmeter is connected across a dry cell which labeled 1.5 V. a) Figure (a) is (an open circuit / a closed circuit) b) There is (current flowing / no current flowing) in the circuit. The bulb (does not light up / lights up) c) The voltmeter reading shows the (amount of current flow across the dry cell / potential difference across the dry cell) d) The voltmeter reading is (0 V / 1.5 V / Less than 1.5 V) e) The potential difference across the cell in open circuit is (0 V / 1.5 V / Less than 1.5 V). Hence, the electromotive force, e.m.f., E is (0 V / 1.5 V / Less than 1.5 V) f) It means, (0 J / less than 1.5 J / 1.5 J / 3.0 J) of electrical energy is required to move 1 C charge across the cell or around a complete circuit. - 34 -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

2. The switch is then closed as shown in figure (b). a) Figure (b) is (an open circuit / a closed circuit) b) There is (current flowing / no current flowing) in the circuit. The bulb (does not light up / lights up) c) The voltmeter reading is the (potential difference across the dry cell / potential difference across the bulb / electromotive force). d) The reading of the voltmeter when the switch is closed is (lower than/ the same as / higher than) when the switch is open. e) If the voltmeter reading in figure (b) is 1.3 V, it means, the electrical energy dissipated by 1C of charge after passing through the bulb is (0.2 J / 1.3 J / 1.5 J) f) The potential difference drops by (0.2 V/ 1.3 V / 1.5 V). It means, the potential difference lost across the internal resistance, r of the dry cell is (0.2 V/ 1.3 V / 1.5 V). g) State the relationship between e.m.f , E , potential difference across the bulb, VR and drop in potential difference due to internal resistance, Vr. Electromotive force, e.m.f., E = Potential Difference + Drop in Potential Difference across resistor, R = VR + Vr

due to internal resistance,r

where VR = IR and Vr = Ir

= IR + Ir

= I (R + r)

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

3.

i.

Why is the potential difference across the resistor not the same as the e.m.f. of the battery? The potential drops as much as 0.4

ii.

V across the internal resistance

Determine the value of the internal resistance. Since

E

=

1.5 = r

=

V

+

1.1 +

Ir

0.5 r

0.8 Ω

Therefore, the value of the internal resistance is 0.8

iii.

Ω

Determine the value of the external resistor. Since

V

=

IR

1.1 =

0.5 R

R

2.2 Ω

=

Therefore, the value of the external resistance is 2.2

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Ω

JPN Pahang Student’s Copy

Activity :

Physics Module Form 5 Chapter 7: Electricity

To determine the values of the electromotive force (e.m.f.) and the internal resistance, r of the cell Voltmeter V Internal resistance

+ Ammeter

Dry cell

Switch

Rheostat

To determine the values of the electromotive force (e.m.f.) and

Aim

the internal resistance, r of the cell

Apparatus /

Dry cells holder, ammeter (0 – 1 A), voltmeter(0 – 5 V), rheostat (0 – 15 Ω), connecting

materials

wires, switch, and 2 pieces of 1.5 V dry cell.

Method

: a)

Set up the circuit as shown in the figure.

b) Turn on the switch, and adjust the rheostat to give a small reading of the ammeter, I, 0.2 A. c)

Read and record the readings of ammeter and voltmeter respectively

d) Adjust the rheostat to produce four more sets of readings, I = 0.3 A, 0.4 A, 0.5 A and 0.6 A. Tabulation of data

: Current,I/A

Volt, V/V

0.2

2.6

0.3

2.5

0.4

2.4

0.5

2.2

0.6

2.0

0.7

1.9

- 37 -

JPN Pahang Student’s Copy Analysis of data

Physics Module Form 5 Chapter 7: Electricity :

Based on the above data, draw a graph of V against I

Discussion

:

1. From the graph plotted, state the relationship between the potential difference, V across the cell and the current flow, I? The potential difference, V across the cell decreases as the current flow increases. 2. A cell has an internal resistance, r. This is the resistance against the movement of the charge due to the electrolyte in the cell. With the help of the figure, explain the result obtained in this experiment. When the current flowing through the circuit increases, the quantity of charge flowing per unit time increased. Hence, more energy was lost in moving a larger amount of charge across the electrolyte. Because of this, there was a bigger drop in potential difference measured by the voltmeter. 3. By using the equation E = V + Ir (a) write down V in terms of E, I and r. V = -rI + E (b) explain how can you determine the values of E and r from the graph plotted in this experiment. E = the vertical intercept of the V – I graph R = the gradient of the V – I graph (c) determine the values of E and r from the graph. By extrapolating the graph until it cuts the vertical axis, E = 2.9 V r = - gradient = 1.4 Ω

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Exercise 7.4 A voltmeter connected directly across a battery gives a reading of 1.5 V.

1

The voltmeter reading drops to 1.35 V when a bulb is connected to the battery and the ammeter reading is 0.3 A. Find the internal resistance of the battery. E = 3.0 V, V = 1.35 V, I = 0.3 A Substitute in :

E = V + Ir 1.5 = 1.35 + 0.3(r) r = 0.5 Ω

2. A circuit contains a cell of e.m.f 3.0 V and internal resistance, r. If the external resistor has a value of 10.0 Ω and the potential difference across it is 2.5 V, find the value of the current, I in the circuit and the internal resistance, r. E = 3.0 V, R = 10 Ω, V = 2.5 V Calculate current : V = IR Calculate internal resistance : E = I(R + r) r = 2.0 Ω

3

A simple circuit consisting of a 2 V dry cell with an internal resistance of 0.5Ω. When the switch is closed, the ammeter reading is 0.4 A. Calculate (a) the voltmeter reading in open circuit The voltmeter reading = e.m.f. = 2 V (b) the resistance, R

(c) the voltmeter reading in closed circuit

E = I(R + r)

V = IR

2 = 0.4(R + 0.5)

= 0.4 (4.5)

R = 4.5 Ω

= 1.8 V Find the voltmeter reading and the resistance, R of the

4

resistor. E = V + Ir e.m.f.

12 = V + 0.5 (1.2) V = 11.4 V V = IR 11.4 = 0.5 (R) R = 22.8 Ω

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

5

/V

A cell of e.m.f., E and internal resistor, r is connected to a rheostat. The ammeter reading, I and the voltmeter 6

reading, V are recorded for different resistance, R of the rheostat. The graph of V against I is as shown.

2

From the graph, determine a)

the electromotive force, e.m.f., E E = V + Ir

Rearrange

:V = E - Ir

Equivalent

: y = mx + c

/A

2 b) the internal resistor, r of the cell r = - gradient = - (6 - 2) 2 =2Ω

Hence, from V – I graph : E = c = intercept of V-axis =6V

6

V/V

The graph V against I shown was obtained from an experiment.

1.5 a) Sketch a circuit diagram for the experiment 0.2 5

1/A

b) From the graph, determine i) the internal resistance of the battery

ii) the e.m.f. of the battery

r = -gradient

E = c = intercept of V-axis

= 0.26Ω

7

= 1.5 V A graph of R against 1/I shown in figure was obtained

R/Ω

from an experiment to determine the electromotive force,

1.3

e.m.f., E and internal resistance, r of a cell. From the graph, determine a)

the internal resistance of the cell

E = I(R + r) 1 (A ) Rearrange I - r, -1

0.5 - 0.2

:R=

Hence, r = -gradient = -(-0.2) = 0.2Ω b) the e.m.f. of the cell e.m.f. = gradient = 3 V

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

7.5 ELECTRICAL ENERGY AND POWER Electrical Energy 1.

Energy Conversion battery (chemical energy)

(a)

current

(b)

battery (chemical energy)

current

current

current

Light and heat

Energy Conversion: Electrical energy → Light energy + Heat energy

Energy Conversion: Electrical energy → Kinetic energy

2. When an electrical appliance is switched on, the flows and the .............................. energy supplied by the source is ................................... to other forms of energy. 3. Therefore, we can define electrical energy as : ................................................................................................................................................ ................................................................................................................................................ ................................................................................................................................................

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Electrical Energy and Electrical Power 1. Potential difference, V across two points is the ............................ dissipated or transferred by a coulomb of charge, Q that moves across the two points. 2. Therefore,

Potential difference, V = Electrical energy dissipated, E Charge, Q 3. Hence, E = VQ 4. Power is defined as the rate of energy dissipated or transferred. 5. Hence, Power, P = Energy dissipated, E

time, t Electrical Energy, E From the definition of potential

Electrical Power, P Power is the rate of transfer of electrical energy,

difference, V

P = VQ t

V = VQ

Electrical energy converted, E

E = VQ

Hence,

E = VI t

Hence,

E = I2R

Hence,

E=

SI unit : Joule (J)

P = VQ t

; where Q = It

; where V = IR

P = VI

2 P= I R t

; where I = V R

V2 t R

P = I2 R

-1

SI unit : Joule per second // J s // Watt(W)

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Power Rating and Energy Consumption of Various Electrical Appliances 1. The amount of electrical energy consumed in a given period of time can be calculated by Energy consumed E

=

Power rating x

Time

=

Pt

energy, E is in Joules

where

power, P is in watts time, t is in seconds 2. The unit of measurement used for electrical energy consumption is the ………………………………………...

1 kWh

=

1000 x 3600 J

=

3.6 x 106 J

=

1 unit

3. One kilowatt-hour is the electrical energy dissipated or transferred by a ….. kW device in ……... hour 4. Household electrical appliances that work on the heating effect of current are usually marked with, ……………… and ………………….. 5. The energy consumption of an electrical appliance depends on the ……………… and the………………………., , E = Pt 6. Power dissipated in a resistor, three ways to calculate:

R= 100Ω, I=0.5 A, P=? P = I2R = (0.5)2 100 = 25 watts

R= 100Ω, V=50 W, P=? P = (V/R)2 R = V2/R = (50)2 /100 = 2500/100 = 25 watts

- 43 -

V=50 V, I=0.5 A, P=? P = I2(V/I) = IV = (0.50)50 = 25 watts

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Cost of energy Appliance

Quantity

Power / W

Power / kW

Time

Energy Consumed (kWh)

Bulb

5

60

8 hours

Refrigerator

1

400

24 hours

Kettle

1

1500

3 hours

Iron

1

1000

2 hours

Electricity cost: RM 0.28 per kWh

Total energy consumed, E

= (0.48 + 9.6 + 4.5 + 2.0) = 16.58 kWh = 16.58 kWh x RM 0.28

Cost

= RM 4.64 Comparing Various Electrical Appliances in Terms of Efficient Use of Energy 1. A tungsten filament lamp changes ...............................to useful ................ energy and unwanted ................energy 2. A fluorescent lamp or an ‘energy saving lamp’ produces less heat than a filament lamp for the same amount of light produced. 3. a) Efficiency of a filament lamp : Efficiency

=

Output power x 100 Input power

=

3 x 100 60

=

5%

b) Efficiency of a fluorescent lamp and an ‘energy saving lamp’ Efficiency

=

Output power x 100 Input power

=

3 x 100 12

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Exercise 7.5

1. How much power dissipated in the bulb? R = 10Ω

(a)

5V

(b)

R = 10Ω R = 10Ω 5V

2.

I

V= 15V

R1=2Ω

R2=4Ω

R3=4Ω

Calculate (a) the current, I in the circuit

(b) the energy released in R 1 in 10 s.

(b) the electrical energy supplied by the battery in 10 s.

- 45 -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

2. A lamp is marked “12 V, 24 W”. How many joules of electrical energy does it consume in an hour?

3. A current of 5A flows through an electric heater when it is connected to the 24 V mains supply. How much heat is released after 2 minutes?

4. An electric kettle is rated 240 V 2 kW. Calculate the resistance of its heating element and the current at normal usage.

5. An electric kettle operates at 240 V and carries current of 1.5 A. (a) How much charge will flow through the heating coil in 2 minutes.

(b) How much energy will be transferred to the water in the kettle in 2 minutes?

(c) What is the power dissipated in the kettle?

- 46 -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

6. An electric kettle is labeled 3 kW, 240 V. (a) What is meant by the label 3 kW, 240 V? The electric kettle dissipates electrical power 3 kW if it operates at 240 V

(b) What is the current flow through the kettle?

(c) Determine the suitable fuse to be used in the kettle. 12 A

(d) Determine the resistance of the heating elements in the kettle.

7. Table below shows the power rating and energy consumption of some electrical appliances when connected to the 240 V mains supply. Appliance

Quantity

Power rating / W

Time used per day

Kettle jug

1

2000

1 hour

Refrigerator

1

400

24 hours

Television

1

200

6 hours

Lamp

5

60

8 hours

Electricity cost: RM 0.218 per kWh Calculate (a) Energy consumed in 1 day Energy consumed Kettle jug,

= Quantity x Power rating (kW) x Time used =1x2x1

= 2 kWh

Refrigerator

= 1 x 0.4 x 24 = 9.6 kWh

Television

= 1 x 0.2 x 6 = 1.2 kWh

Lamp

= 5 x 0.06 x 8 = 2.4 kWh

Total energy consumed

= 15.2 kWh

- 47 -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

(b) How much would it cost to operate the appliances for 1 month? Cost

= 16.58 kWh x 30 x RM 0.218 = RM 108.43

8. A vacuum cleaner consumes 1 kW of power but only delivers 400 J of useful work per second. What is the efficiency of the vacuum cleaner?

9. An electric motor is used to lift a load of mass 2 kg to a height 5 m in 2.5 s. If the supply voltage is 12 V and the flow of current in the motor is 5.0 A, calculate (a) Energy input to the motor

(b) Useful energy output of the motor

(c) Efficiency of the motor

- 48 -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Reinforcement Exercise Chapter 7 Part A: Objective Questions 1. What is the unit of electric charge?

3. Which of the following graphs shows

A. Ampere, A

the correct relationship between the

B. kelvin,K

potential difference, V and current, I

C. Coulomb, C

for an ohmic conductor?

D. Volt, V

A.

2. Which of the following diagrams shows the correct electric field? A. B.

B.

C.

C.

D.

- 49 -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

4. A small heater operates at 12 V, 2A. How much energy will it use when it is run for 5 minutes? A. 90 J

C.

B. 120 J C. 1800 J D. 7200 J D. 5. The electric current supplied by a battery in a digital watch is 3.0 x 10-5 A. What is the quantity of charge that flows in 2 hours? A. 2.5 x 10-7 C B. 1.5 x 10-5 C

7. Why is the filament made in the

C. 6.0 x 10-5 C

shape of a coil?

D. 3.6 x 10-3 C

A. To increase the length and produce

E. 2.2 x 10-1 C

a higher resistance. B. To increase the current and produce

6. Which of the following circuits can be used to determine the resistance of the bulb?

more energy. C. To decrease the resistance and produce higher current

A.

D. To decrease the current and produce a higher potential difference 8. Which of the following will not affect the resistance of a conducting wire.

B.

A. temperature B. length C. cross-sectional area D. current flow through the wire

- 50 -

JPN Pahang Student’s Copy

9. The potential difference between two

Physics Module Form 5 Chapter 7: Electricity

12. Which two resistor combinations have the same resistance between X and Y?

points in a circuit is A. the rate of flow of the charge from one point to another B. the rate of energy dissipation in moving one coulomb of charge from one point to another C. the work done in moving one coulomb of charge from one point to another D. the work done per unit current flowing from one point to another

A. P and Q 10.

B. P and S

An electric kettle connected to the

C. Q and R

240 V main supply draws a current

D. R and S

of 10 A. What is the power of the

E.

kettle? A. 200 W B. 2000 W C. 2400 W D. 3600 W E. 4800 W

13. In the circuit above, what is the 11. An e.m.f. of a battery is defined as

ammeter reading when the switch S is turned on?

A. the force supplied to 1 C of charge

A. 1.0 A

B. the power supplied to 1 C of charge

B. 1.5 A

C. the energy supplied to 1 C of

C. 2.0 A

charge

D. 9.0 A

D. the pressure exerted on 1 C of

E. 10.0 A

charge

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

16. An electric motor lifts a load with a

14. A 2 kW heater takes 20 minutes to heat a pail of water. How much

potential difference 12 V and fixed

energy is supplied by the heater to

current 2.5 A. If the efficiency of the

the water in this period of time?

motor is 80%, how long does it take

6

to lift a load of 600 N through a

6

vertical height of 4 m

A. 1.2 x 10 J B. 1.8 x 10 J C. 2.4 x 106 J

A. 20 s

D. 3.6 x 106 J

B. 40 s

E. 4.8 x 106 J

C. 60 s D. 80 s E. 100 s

15. All bulbs in the circuits below are identical. Which circuit has the

17. The kilowatt-hour (kWh) is a unit of

smallest effective resistance?

measurement of

A.

A. Power B. Electrical energy C. Electromotive force B.

C.

D.

18. The circuit above shows four identical bulbs to a cell 6 V. Which bulb labeled A, B, C and D is the brightest?

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

19. A 24 Ω resistor is connected across the terminals of a 12 V battery.

20. Which of the following quantities can

Calculate the power dissipated in the

be measured in units of JC-1

resistor.

A. Resistance

A. 0.5 W

B. Potential difference

B. 2.0 W

C. Electric current

C. 4.0 W D. 6.0 W E. 8.0 W

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Part B: Structured Questions 1.

The figure above shows a graph of electric current against potential difference for three different conductors X, Y and Z. (a) Among the three conductors, which conductor obeys Ohm’s law? Conductor Y

(b) State Ohm’s law. The potential difference across a conductor is directly proportional to the current that flows through it, if the temperature and other physical quantities are kept constant.

(c) Resistance, R is given by the formula R = V/I. What is the resistance of X when the current flowing through it is 0.4 A? Show clearly on the graph how is the answer obtained. From the graph I against V; resistance, R

= reciprocal of gradient, 1/gradient 1 = 0.11

= 9.09 Ω

(d) Among X, Y and Z, which is a bulb? Explain your answer. X, because as I increases, the gradient decreases. Hence, the resistance X increases as I increases which is a characteristic of a bulb.

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JPN Pahang Student’s Copy

2.

Physics Module Form 5 Chapter 7: Electricity

The figure below shows an electric kettle connected to a 240 V power supply by a flexible cable. The kettle is rated “240 V, 2500 W”.

The table below shows the maximum electric current that is able to flow through wires of various diameters.

(a)

diameter of wire / mm

maximum current / A

0.80

8

1.00

10

1.20

13

1.40

15

What is the current flowing through the cable when the kettle is switched on? P = IV I = P/V

(b)

= 2500 / 240 = 10.4 A

Referring to the table above, i. What is the smallest diameter wire that can be safely used for this kettle? 1.20 mm

ii. Explain why it is dangerous to use a wire thinner than the one selected in b(i) As resistance is inversely proportional to cross-sectional area, a thinner wire will have a higher resistance thus the wire will become very hot. This could probably cause a fire to break out.

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JPN Pahang Student’s Copy

(c)

Physics Module Form 5 Chapter 7: Electricity

State one precautionary measure that should be taken to ensure safe usage of the kettle. Do not operate kettle with wet hands.

(d)

Mention one fault that might happen in the cable that will cause the fuse in the plug to melt. Short circuit might occur if the insulating materials of the wires in the cable are damaged.

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Part C: Essay Questions 1.

Figure 1 shows the reading of the voltmeter in a simple electric circuit Figure 2 shows the reading of the same voltmeter

(a) What is meant by electromotive force (e.m.f.) of a battery? (b) Referring to figure (a) and figure (b), compare the state of the switch, S, and the readings of the voltmeter. State a reason for the observation on the readings of the voltmeter. (c) Draw a suitable simple electric circuit and a suitable graph, briefly explain how the e.m.f. and the quantity in your reason in (b) can be obtained. (d)

The figure above shows a dry cell operated torchlight with metal casing (i)

What is the purpose of the spring in the torchlight?

(ii)

Why it is safe to use the torchlight although the casing is made of metal?

(iii)

What is the purpose of having a concave reflector in the torchlight?

- 57 -

JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Answer 1. (a) The work done by a battery to move a unit charge around a complete circuit. (b) - Switch in figure 1 is turned off - Switch in figure 2 in turned on - Reading of voltmeter in figure 1 is higher than in figure 2 - This is due to the presence of an internal resistance in the battery (c)

e.m.f = intercept on the v-axis internal resistance = -(gradient of the graph) (d) (i) (ii) (iii)

To improve the contact between the dry cells and the terminals of the torchlight Current flowing through the torchlight is very small, will not cause electric shock To converge the light rays to obtain increase the intensity of the light rays projected by the torchlight.

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

2. A group of engineers were entrusted to choose a suitable cable to be used as the transmitting cable for a long distance electrical transmission through National Grid Network. Four different cables and their characteristic of the cables were given. The length and diameter of all the cables are similar. (a) Define the resistance of a conductor. (b) The table below shows the characteristic of the four cables, A, B, C and D. Resistivity / Ωm

Maximum load before breaking/ N

Density / kgm-3

Rate of expansion

A

0.020

500

2800

Low

B

0.056

300

3200

Low

C

0.031

400

5600

Medium

D

0.085

200

3800

High

Base on the above table: (i)

Explain the suitability of each characteristic of the table to be used for a long distance electricity transmission

(ii)

Determine the most suitable wire and state the reason

(c) Suggest how three similar bulbs are arranged effectively in a domestic circuit. Draw a diagram to explain your answer. Give two reasons for the arrangement. (d) An electric kettle is rated 2.0 kW. (i)

Calculate how long would it take to boil 1.5 kg of water from an initial temperature of 280 C. [specific heat capacity of water = 4200 J kg-1 0C-1]

(ii)

What is the assumption made in the calculations above?

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JPN Pahang Student’s Copy

Physics Module Form 5 Chapter 7: Electricity

Answer 2.(a) Resistance is the ratio of potential difference to current flowing in an ohmic conductor. (b) Characteristics

Explanations

A low resistivity

Energy loss during transmission is reduced

Max load before

Mass or weight reduced. Can be supported by transmission

braking is high

tower

A low density

Cable will not slag when it heated during transmission

Cable A is chosen because it has low resistivity, high max load before breaking, low density and low expansion rate. (c) (i) If one bulb is burnt the others is still be lighted up (ii) Each bulb can be switch on and off independently (d) (i)

Pt = mcθ (2000)(t) t

(ii)

=

(1.5)(4200)(100-28)

=

226.8 s

No heat is lost to the surroundings and absorbed by the kettle

END OF MODULE - 60 -