Unit 9 Mag Nest Ism

Unit IX: Magnetism        

Magnets Magnetic field Electromagnets Dc motor Hall effect Magnetic induction Ac generator Transformer

The magnetic force A

free suspended magnet line up with the Earth’ north-south line Navigational compass like this since 11th century A compass is a small magnet that can points the direction by aligning its needle under the presence of the magnetic force of the earth.

The Magnet Compass woks because : • The Earth is like a giant magnet • It has two magnetic poles a north pole, a south pole • Its magnetic north pole is nearly aligned with the south geographical pole of the Earth

Rules of magnetic force

• There are only two magnetic poles: north pole and south pole • Like poles repel, unlike poles attract • In magnetic substance, like iron, each atom is a small magnet • A larger magnetic force is produced when these tiny magnet are aligned in the same direction

Magnetic fields Existence of magnetic fields: • Magnets attract small iron particles • Compass needle rotates with the Earth poles  The magnetic field is stronger at the pole  The direction pointing by the trace of magnetic needle: magnetic field lines

Magnetic field created by a wire carrying current

• A wire carrying current can create a magnetic field. • The direction of such created field is determined by right hand rule.

Electromagnets

• The magnetic force can be increased if the wire is coiled, or solenoid. • The direction of such created field is determined by right hand rule • Putting an iron bar into the coil strengthens the magnet even more. • This is a simple electromagnet. The strength of an electromagnet can be enhanced by • Increasing the number of turn of the coil • Increasing the current intensity

Current: The source of all magnetism

(a) In the planetary model of the atom, an electron orbits a nucleus, forming a closed current loop and creating a magnetic field with a north and south pole.

Current: The source of all magnetism

(b) Electrons have spin and can be crudely pictured as rotating charge, forming a current that produces a magnetic field with a north and south pole.

Circuit breaker

The circuit breaker is a typical application of the electromagnet. The electromagnet can create a strong magnetic field. The electromagnet of the circuit breaker is usually not strong enough to attract the iron bolt under the normal current range. However, if there is a fault which causes a current surge, the iron bolt is pulled out of the plunger by the electromagnet. Hence, the circuit is broken.

Magnetic force on current-carrying wire F I

Fleming’s left-hand (motor) rule

F is proportional to I, B and l

Magnetic force on current-carrying wire

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(a) (b) (c) The magnetic field B (directed into the plane) exerts a force on the current- carrying wires. (a) I=0, (b) I upward, (c) I downward. The magnetic force on wire carrying current is the basis for dc motor.

The motor principle

a rectangular loop carrying a current I, in the presence of a uniform magnetic field B. The force on both side of a will be cancelled. The magnitude of force on the sides b is not zero. They are the same magnitude but opposite in direction. These two forces will produce a torque about O that rotates the loop clockwise.

The motor principle

•When the coil at vertical, the current should change its direction, the coil continue to turn. •A split ring ensures that the current flow changes direction at the right time. •This is the principle of a dc motor.

Hall effect Principle The magnetic force creates the separation of charge in nonmetallic conductor, which builds up until it is balanced by the electric force, an equilibrium that is quickly reached.

ε = Blv

Blood velocity Measurement

The blood tube is a conductor-carrying vessel

E = Blv E is the emf in volts, B is the magnetic strength in Tesla (T) l =2a is the vessel’s diameter in m

Doppler ultrasound The frequency shift of the reflected ultrasound :

2 fv cos θ ∆f = c Where f is the source frequency, v the speed of the moving blood, and c is the speed of sound in the tissue.

By measuring the frequency shift ∆ f, the average speed of the blood can be calculate.

Comparison of Blood Velocity Measurement The electromagnetic and ultrasound techniques are two most used methods for the the measurement of the blood flow rate.. • The features of ultrasound technique are as follows: •In clinical application, it is most frequently utilized to detect the presence or location of blood flow rather than to measure its magnitude accurately •The frequency shift is in the audio range and is made audible with loudspeaker. •The popularity of the magnetic technique is the result of the following factors: •Utilized normally during surgical procedures in which blood vessels are exposed . •Producing accuracies up to 5% •Accommodation of blood vessels of diameters from 1mm to 20 mm

Electromagnetic Induction: Phenomena Phenomena of electric induction

We have learned : •A current-carrying wire in a magnetic field will experience a force, •A current loop in a magnetic field will experience a torque How about a torque in a magnetic field to create a current? The answer is YES. The induction phenomena deals with the creation of an electric current (or electro-motif force emf) in a loop by varying the magnetic fields. (either direction or magnitude).

Electromagnetic Induction: 1st experiment

A moving magnet can induce an current in a loop even there is no battery in the loop.

Electromagnetic Induction: 2nd experiment

The current meter registers a current in the left hand loop just as the switch S is closed or opened. No motion of the coils is involved.

Electromagnetic Induction: 3nd experiment

A current is induced when the rod moves to the right in a uniform constant magnetic field.

Electromagnetic Induction: Faraday’s law Faraday’s law of induction An potential difference can be induced in a loop, if there is a change of  magnetic field  loop area  angle between magnetic field and loop 

∆( BA cos θ ) ε =− ∆t

BA cos θ

is called mangetic flux.

Magnetic flux B θ

BA cos θ 

A 



represents the product of a area A with the perpendicular component of a magnetic field B passing through it. θ is the angle between the direction of the magnetic field B and the normal the area A. The unit of magnetic flux is T ·m2

Electromagnetic Induction: Lenz law

Lenz law An induced current has a direction such that it induces a magnetic field which opposes the changes in the magnetic flux i.e. BA cos θ 

The Generation of Alternative Current Faraday’s law is the basis of an ac current generation ∆ ε = − BA (cos θ ) = BAω sin θ ∆t

In order to generate ac currents • not necessary to move magnet, • rotation of a coil of wire between poles of magnet The induced potential difference (or current) is increased, if - the coil rotate faster, - the area of the coil is increased, - there are more turns on the coil, - the strength of the magnet is increased

Simple ac generator • It has a fixed magnet and a rotating coil • Coil connected to a conducting ring • The conducting rings rotate together with coil • The rings come into contact with two fixed carbon brushes As the coil turns, the induced voltage changes direction for each half turn of the coil, this creates an alternating current

Energy Transmission How a household circuit wired? Electricity produced in power stations is first step-up to high voltage (> 10 kV) by a transformer and delivered to local area through high tension cable towers. A transformer later step-down the voltage to domestic level (rms 220 V in Hong Kong).

Power transmission requirements For safety reason, low voltages are required at both generating end and receiving end in an energy transmission. •energy loss I2R in the transmission line •output power is IV, •in order to minimize I, we have to rise V during transmission. The device with which we can rise and lower the voltage is called the transformer. transformer

The Transformer Principle

• There is a current in the 2nd coil only as the switch is on or off in the 1st coil. • A changing magnetic field in a fixed coil will induce a current in a second fixed coil. • The iron core provide a magnetic link between the two coil.

Transformers

They use magnetic link between two coils to step-up or stepdown alternating voltage: •primary coil must uses alternating current, •which produces a changing magnetic field in the iron core •an alternating current induced in the secondary coil •Transformer works with ac current only

Rule of voltage transformation

•V1 and V2 are the primary and secondary voltages, •N1 and N2 are number of turns on primary and secondary coils

Rule of current transformation

•I1 and I2 are the primary and secondary currents, •N1 and N2 are number of turns on primary and secondary coils

Example: A transformer is designed to step-down 230 V to 11.5 V. There are 1000 turns of wire on the primary coil. Calculate 1) the number of turns on the secondary coil 2) For a input current of 0.01 A, what is the output current?

1)

V1 V2 = N1 N 2

230 11.5 ∴ = 1000 N 2

N2= 50 Turns 2)

I1 N1 = I 2 N 2 ∴ 0.01 ⋅1000 = I 2 ⋅ 50 I2=0.2 A