Unit 9 Magnetism

8/14/2019 Unit 9 Magnetism

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Unit 9: Magnetism

9.1 Magnets

Working principle of compass

Magnets were used as navigational compasses. Since the Earth is

like a giant magnet with a north and a south pole, the magnetic

north pole of the compass is closely aligned with the geographic

south pole of the Earth.

Rules of magnetic force

There are several rules for magnetic force:

There are only two magnetic poles: the north and south pole

Like poles repel, unlike poles attract

In magnetic substances, like iron, each atom is a small

magnet

A larger magnetic force is produced when these tiny magnets

are aligned in the same direction

Therefore, for all practical purposes, a magnet can be divided

indefinitely.


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9.2 Magnetic field

Since magnets attract small iron particles and a compass needle is

affected by the Earth's poles, magnetic fields are associated with

magnets.A magnetic field is stronger at the poles. The magnetic needle

points in the same direction as the magnetic field lines.

Magnetic field created by a wire carrying current

In addition to magnets, a magnetic field can also be generated by

a wire carrying current. The direction of such a field is determined

by the right hand rule.

9.3 Electromagnets


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The strength of the magnetic field can be increased if the wire is

coiled, or solenoid. The direction of such a field is also determined

by the right hand rule. When an iron bar is put into the coil, the

magnet becomes even stronger. This is a simple electromagnet.

The strength of the electromagnet can be also enhanced by

increasing the number of turns in the coil or the current intensity.

The cause of magnetism

The cause of magnetism has been proposed to be the electron

spin or orbit.

(a) In the planetary model of the atom, the electron spins around


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the nucleus, creating a closed current loop, along with a

magnetic field with north and south poles.

(b) Electron spin model: Electrons have spin, and can be roughly

depicted as a rotating charge which creates a current along with

a magnetic field with north and south poles.

The theory of magnetism proposed by modern physics, is different

from both the planetary model and the electron spin model.


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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.

9.4 Dc motor

Magnetic force on current-carrying wire


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Flemings left-hand (motor) rule

According to the Flemings left-hand (motor) rule, Fis proportional

to B, I and l.

(a) (b) (c)

The magnetic field B (directed into the plane) exerts a force on

the current-carrying wires. There are three cases of force exertion.(a) I=0, (b) I upward, (c) I downward.

The magnetic force on the current carrying wire is the basis for

the dc motor.

The motor principle


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The rectangular loop carrying a current I is in the presence of the

uniform magnetic field B. The forces on the two horizontal sides "a"

will cancel each other. However, the magnitude of force on the b

sides is not zero. They are the same magnitude but opposite in

direction. Hence, these two forces will produce a torque aboutO

that will rotate the loop in a clockwise direction.

When the coil is vertical, the current should change its direction

and then the coil will continue to turn. A split ring ensures that the

current flow changes direction at the right time. This is the principle

of the dc motor.


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9.5 Hall effect

Magnetic force creates a separation of charge which builds up until

it is balanced by the electric force. An equilibrium is quicklyreached.

Blood velocity Measurement

The Hall effect can be applied in blood velocity measurement,

assuming the blood is a conductor-carrying fluid. The velocity of the

blood can be described by the following equation where

is Hall emf.

The electromagnetic and ultrasound techniques are two most used methods for the

the measurement of the blood velocity:

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:

Blv=

Blv=


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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


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9.6 Electromagnetic induction

The phenomenon of electromagnetic induction

We have learnt that a current-carrying wire in a magnetic field willexperience a force and that a current loop in a magnetic field will

experience a torque. Now a torque in a magnetic field can create a

current.

The induction phenomenon deals with the creation of an electric

current (or electro-motif force emf) in a loop by varying the

magnetic fields (either in direction or magnitude).

Experiments of electromagnetic induction

First experiment:

A moving magnet can induce a current in a loop even if there is no

battery in the loop.


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Second experiment:

The current meter registers a current in the left hand loop at the

moment the switch S is opened or closed. No motion of the coils is

involved.

Third experiment:

A current is induced when the rod moves to the right in a uniform

constant magnetic field.


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Laws of electromagnetic induction

Faradays law of induction is one of two important laws ofelectromagnetic induction. A potential difference can be induced in

a loop if there is a change in the strength of magnetic field, loop

area, or angle between the magnetic field and the loop.

Faradays law of induction:

Another important law is Lenz's law. It states: An induced current

has a direction such that it induces a magnetic field which opposes

the changes in the magnetic flux.

9.7 AC generator

The Generation of Alternative Current

Faradays law is the basis of ac current generation. In order to

generate an ac current, it is not necessary to move the magnet.

Instead, one can rotate the coil of wire between the poles of the

magnet.

The induced potential difference (or current) is increased, if

the coil rotates faster,

the area of the coil is increased,

t

BA

=

)cos(

sin)(cos BAt

BA =

=


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there are more turns on the coil,

the strength of the magnet is increased.

Simple ac generator

The generator has a fixed magnet and a rotating coil. The coil is

connected to a conducting ring. Two conducting rings rotate

together with the 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.

9.8 Transformer

Energy Transmission

For a household circuit, electricity produced in power stations is


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first stepped up to a high voltage (> 10 kV) by a step-up

transformer, and then delivered to a local area through high tension

cable towers. A step-down transformer is later used to step down

the voltage to a domestic level (rms 220 V in Hong Kong).For safety reasons, low voltages are required at both the

generating and receiving end in energy transmission.

Besides, the energy loss in the transmission line is I2Rand the

power output is IV. Hence, we have to raise V during transmission in

order to minimize I and thereby reduce the power loss in the

transmission line.

The device with which we can raise and lower the voltage is called

the transformer.

The Transformer Principle

A current in the 2nd coil is generated only when the 1st coil is

turned on or off.A changing magnetic field in a fixed coil will induce a current in a

second fixed coil.

The iron core provides a magnetic link between the two coils.

Transformers


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Transformers use a magnetic link between two coils to step-up or

step-down alternating voltage. Transformers work with alternating

current only. The primary coil must use an alternating current to

produce a changing magnetic field in the iron core; an alternating

current is induced in the secondary coil.

There are two types of transformation: voltage and current.

Voltage transformation formula:

where V1 and V2 are the primary and secondary voltages, and

N1 and N2 are the number of turns on the primary and

secondary coils

Current transformation formula:

where I1 and I2 are the primary and secondary currents, and

N1 and N2 are the number of turns on the primary and

secondary coils

Example: A transformer is designed to step-down from 230 V to 11.5


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V. There are 1000 turns of wire on the primary coil.

Calculate:

1) the number of turns on the secondary coil

2) the output current for an input current of 0.01 A1)

N2= 50 Turns

2)

I2=0.2 A

2

2

1

1

N

V

N

V=

2

5.11

1000

230

N=

2211NINI = 50100001.0 2 = I

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Unit 9 Magnetism