3.0 Electromagnetism

7/28/2019 3.0 Electromagnetism

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7/23/2013 Hjh Sulastri Kadiman 1

3.0 ELECTROMAGNETISM


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3.1 Magnetic effect of acurrent-carrying conductor


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Electromagnets

Electromagnet is a magnet inwhich a magetic field is producedby the flow electric current

A simple electromagnet can beformed by sending an electriccurrent through coil of wire wuond

around an iron core


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Magnetic field pattern

Rule to determine the direction ofthe mgnetic field

1. Right-hand grip rule for a straightwire and coil

Thumb of a clenched pointing inthe direction of the conventional

current and the fingers indicate thedirection of the magnetic

2. digrram


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Solenoid

Diagram

The thumb points towards the north

pole of the magnetic field while thefingers indicate the direction of thecurrent in the solenoid


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Magnetic field strength

The field strength incereases withcurrent. Thus,a stronger magneticfield exists near appliances anddecreases with distance

The field strength incereases with anumberof turns of wire


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Use of electromagnets

Electric bell

Electromagnetic relay

Maglev train


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3.2 The force on a current-carryingconductor in a magnetic field


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Force produceed by the combined

magnetic field Catapult force

When a current carrying conductoris place in the magnetic field of apermanent magnet

The two magnetic field involved are

(i) magnetic field of the permanentmagnet

(ii)mangetic field produced by thecurrent-carrying conductor


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The two magnetic field interact toproduce a resultant field known as acatapult field

Diagram


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Flemings left-hand rule

Diagram

The thumb given the direction of

the force The first finger shows the direction

of the magnetic field

The center finger points in thedirection of the current


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Turning Effect of a current-carrying coil

in a Magnetic Field

Two sides of the coil producecircular magnetic field lines inopposite direction

The interation between themagnetic field of the current andthe magneticfield of the permanent

magnet produces a resultantmagnetic field


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The catapult field produced pushesthe upward direction and pushes

downwards The two force which are in opposite

direction constitute a couple whichproducea turning effect and thecoil rotates


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Turning force on a current-carrying coil

in magnetic field

Ammeter

Direct current motor

Factor that affect the speed ofrotation of an electric motor

a. increasing the current

b. using the strong magnet

c. increasing the number of turns onthe coil


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3.3 Electromagnetic induction


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How induced e.m.f. is produced

The production of an electric currentby a changing magnetic field

The induced current is producedonly when these is relative motionbetween the conductor or coil andthe magnetic field line


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Faradays laws

The magnitude of the induced e.m.f isdirectly proportional to the rate of changeof magnetic flux experienced by theconductor

The magnitude of e.m.f increases when1. the wire is moved faster2. a stronger magnet is used3. the length of the wire in the magnetic

field is increases4. the number of turns on coil5. the cross-sectional area of coil


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Lenzs laws

Lenzs law state that the direction of

the induced e.m.f. is such that itsmagnetic effect alawys oppose thechange producing

diagram


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Application of electromagnetic

induction

Direct current Altenatingcurrent

1. Sinusoidal trace

2. Magnitude change


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3.4 Transformers


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Function of a Transformer

The equipment used to raise orlower the potential difference of analternating currrent supply

Important role in the transmissionand distribution of electrical energyfrom one circuit to another through

electromagnetic induction


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Constructing of a transformer

Diagram

A simple transfomer consists of two coilswound on a laminated iron core

The coil which is connected to the powersupply whose voltage is called primarycoil, NP and supply voltage is called theprimary voltage, VP

The other coil which connected to theelectrical equipment or resistor is calledsecondary coil.NS and voltage accros itis called secondary voltage, VS


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Operating principle of aTransformer

The operating principle of a transformer isbase on electromagnetic induction

The current from power supply that is

conneted to primary coil is ac Ac produce a flux or magnetic field lines

which link the primary coil and secondarycoil. The magnetic flux produced varies in

magnitude and direction


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Step-down and Step-up transformer

Step-down

1. VP > VS

2. NP > NS

Step-up

1. VP < VS2. NP < NS


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Relationship between Voltage and

number of turns

For ideal transfomer (efficiency of100%)

Secondary = number of secondary

voltage turns

primary number of primary

voltage turns

VS = NsVp Np


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Efficiency and ideal of a transformer

Efficiency = output power x 100%

input power

Ideal transformer which has anefficiency of 100%

Output power =Input power

Po =Pi Vs Is =Vp Ip


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Factors that affect the Efficiency and

Ways to improve

Factor that causeenergy

1. Resistance of

coil:Energy is lostas heat in the coil

2. Magnetisationanddemagnetiisationof iron core

Way to minimiseenergy losses

1. Thick copper wire

to reduce theresistance of thecoil

2. A soft iron corewhich can be

magnetised anddemagnetisedeasily


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3. Eddy currents iniron core

4. Leakage ofmagnetic flux

3. The iron core islaminated

4. Secondary coilwound over theprimary coil


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3.5 The Generation andTransmission of Electricity


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3.0 Electromagnetism

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