Section 14.3 Different Charging Methods

Section 14.3 Different charging methods • Charging insulator by rubbing • Charging conductor by sharing • Charging conductor by induction © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 11)

Conductor and insulator Materials can be classified as: Insulators

Conductors copper

glass aluminium

iron

plastic air Go to

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Activity 3

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14.3 Different charging methods (SB p. 11)

Conductor Conductors loosely held electrons

-

- - - + + +

-

+

+

+

+

+

+ -

-

-

- + + + - -

charges move easily from one place to another

can conduct electricity © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 11)

Insulator Insulators

charges cannot move easily

cannot conduct electricity © Manhattan Press (H.K.) Ltd.

electrons held tightly by nuclei

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ 4

14.3 Different charging methods (SB p. 12)

Charging insulator by rubbing Rubbing a polythene rod with a neutral dry cloth neutral dry cloth

polythene rod

polythene rod

electrons

Friction provides energy to electrons to transfer © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 12)

Charging insulator by rubbing Rubbing an acetate rod with a neutral dry cloth

acetate rod

electrons

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14.3 Different charging methods (SB p. 12)

Charging insulator by rubbing • insulators can be charged by rubbing • direction of flow of electrons depends on materials

neutral dry cloth

polythene rod

• charges reallocated • no charges created / destroyed © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 13)

Class Practice 2: 1. Two plastic strips repel as shown. (a) How can you obtain the result shown in the figure?

Ans wer

You are given two neutral plastic strips.

Pull three fingers down the pair of plastic strips, with one finger between them. © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 13)

Class Practice 2: (Cont) (b) Explain briefly the phenomenon in terms of the charges on the strips. rubbed by fingers

Ans wer They repel each other because they carry like charges. © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 13)

Class Practice 2: (Cont) 2.

Fig. (a)

Fig. (b)

You are given two neutral plastic balloons, which are made of the same material.

You are given two neutral plastic balloons, which are made of different material. (a) How can you obtain the results Ans shown in Fig. (a) and Fig. (b)? wer

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14.3 Different charging methods (SB p. 13)

Class Practice 2: (Cont)

In Fig. (a), rub the balloons with a piece of woollen cloth. In Fig. (b), rub the balloons against each other.

Fig. (a)

Fig. (b)

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14.3 Different charging methods (SB p. 13)

Class Practice 2: (Cont) (b) Explain briefly the phenomena shown in Fig. (a) and Fig. (b) in terms of the charges on the balloons. Ans wer In Fig. (a), they repel each other because they carry like charges. In Fig. (b), they attract each other because they carry unlike charges.

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Fig. (a)

Fig. (b)

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14.3 Different charging methods (SB p. 14)

Charging conductor by sharing insulated conducting rod

metal sphere

electron

sphere carries -ve charges

insulated stand

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14.3 Different charging methods (SB p. 14)

Uneven sharing of charges conductor

electrons less charges

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more charges

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14.3 Different charging methods (SB p. 15)

Van de Graaff generator Van de Graaff generator

Van de Graaff generator → for generating charges

foam board © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 15)

Van de Graaff generator metal dome

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-

-

-

Van de Graaff generator

© Manhattan Press (H.K.) Ltd.

-

The hairs carry like charges & stand on their ends!

- -

roller

-

-

-

- - -

--

plastic conveyor belt – charged by rubbing

Thinking 4

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14.3 Different charging methods (SB p. 16)

Earthing Earthing — charge sharing between a body & the earth symbol

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14.3 Different charging methods (SB p. 16)

Earthing

electrons

electrons

size of earth >> size of sphere → electrons go to sphere / earth → sphere becomes electrically neutral © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 17)

Charging conductor by induction insulated positively charged metal rod +ve charges on rod attract –ve charges on sphere

metal sphere

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electrons flow from earth

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14.3 Different charging methods (SB p. 17)

Charging conductor by induction -ve charges are not free to move

_ _ _

_ _

sphere carries -ve charges

_ _ _ _ _

Thinking 5 © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 17)

Example 1: A metal-coated ball hanging on a nylon string is brought near a negatively charged dome of a Van de Graaff generator as shown. Describe briefly what happens to the ball.

Solut ion

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14.3 Different charging methods (SB p. 17)

Example 1: (Cont)

Fig. (b)

Fig. (c)

The ball is attracted towards the dome at first. This is due to the attraction between the negative charges on the dome and the positive charges induced on the left hand side of the ball (Fig. (b)). But when the ball touches the dome, it shares the negative charges with the dome. The dome and the ball then carry like charges and the ball is repelled (Fig. (c)).

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14.3 Different charging methods (SB p. 18)

Class Practice 3: 1. Two insulated spheres A and B are in contact as shown. You are given a positively charged rod. How do you charge sphere A positively and sphere B negatively by induction after both of them are separated? Explain briefly with the help of two diagrams. Ans wer

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14.3 Different charging methods (SB p. 18)

Class Practice 3: (Cont)

Fig. (a)

Fig. (b)

Bring the positively charged rod near sphere B. Negative and positive charges are induced on Separate the spheres and then take away spheres B and A respectively. the rod. Now, sphere A carries positive charges and sphere B carries negative charges. © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 18)

Class Practice 3: (Cont) 2. The figure shows an insulated neutral metal sphere. How do you charge it positively by (a) a positively charged rod, and (b) a negatively charged rod? Explain briefly. Ans wer (a) By sharing: Touch the sphere with the positively charged rod. (b) By induction: Place the negatively charged rod near the sphere. Touch the sphere momentarily with a finger.Then take away the rod. © Manhattan Press (H.K.) Ltd.

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To section 14.4

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14.3 Different charging methods (SB p. 10)

Activity 3 Charging methods — (A) Rubbing

Let’s start: 1. Cut a piece of newspaper into very small pieces and put them on a table. 2. Bring a plastic ruler close to the paper pieces (Fig. (a)). Fig. (a) Ans What happens? wer There is no observable change. © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 10)

Activity 3 (Cont) Charging methods — (A) Rubbing

3. Then rub the plastic ruler many times with a woollen cloth. 4. Bring the ruler close to the paper pieces Ans again. What happens? wer

The paper pieces are attracted by the ruler. © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 10)

Activity 3 (Cont) Charging methods — (A) Rubbing

5. Try rubbing a metal ruler with a woollen cloth. Then, bring it close to the paper pieces. What happens? There is no observable

Ans change. wer

6. Which kind of materials, metal or plastic, can attract the paper pieces after Ans rubbing? Plastic. © Manhattan Press (H.K.) Ltd.

wer

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14.3 Different charging methods (SB p. 10)

Activity 3 Charging methods — (B) Sharing

Let’s start: 1. Ask your classmate to stand on a foam board. 2. Ask him to touch the metal dome of a Van de Graaff generator with his hand (Fig. (b)). 3. Turn on the generator.

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Fig. (b)

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14.3 Different charging methods (SB p. 11)

Activity 3 (Cont) Charging methods — (B) Sharing

4. What happens to your classmate’s Answe hairs? The hairs stand on their ends. r

Answe 5.The Explain briefly the result in step 4. charges on the metal dome flowr to his body through his hand. Therefore, his hairs carry like charges and repel each other. This makes the hairs stand on their ends. Return to © Manhattan Press (H.K.) Ltd.

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14.3 Different charging methods (SB p. 15)

Thinking 4 de Refer to Fig. 14.8, what is the use Van Graaff generator of the foam board under the student? If the board is taken away, what happens to the student? The foam board is used to isolate Ans the body from the ground. If it is wer taken away, charges flow through the student to the ground. Therefore, he will get an Return to electric shock! Text

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foam board

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14.3 Different charging methods (SB p. 17)

Thinking 5

In Fig. 14.12(b), if the finger touches the sphere without the presence of the charged rod, can the sphere be charged? Ans No.werIf

the sphere is touched without the presence of the rod, all the positive and negative charges on the sphere will recombine. © Manhattan Press (H.K.) Ltd.

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