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Chapter 3: Chapter 3: ElectromagnetismElectromagnetism

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PhysicsNext >

The study of The study of mattermatter

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Objectives: Objectives: ((what you will learnwhat you will learn)) 1) magnetic effect of current-carrying conductor2) force on current-carrying conductor in

magnetic field3) electromagnetic induction4) transformers5) generation & transmission of electricity

Physics: Chapter Physics: Chapter 33

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Line of ForceLine of ForceA line of force in magnetic field represents path of free N-pole in magnetic field. Direction of line of force: N-pole S-pole

Magnetic field around a bar magnet

Magnetic field around the Earth

Line of force

Pilotsweb.comStargazers

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Magnetic effectMagnetic effectWhen current flows in a conductor, a magnetic field is produced around it.

Magnetic field can be observed by sprinkling iron filings around wire on a piece of cardboard.

The direction of field can be obtained by

moving a compass

around the wire.

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Magnetic effectMagnetic effectThe 2-dimensional view of magnetic field due to current in straight wire is easier to draw.

Current up: Current coming out of paper

Current down: Current going into paper

As distance from wire increases, magnetic field gets weaker (as shown by

increasing distance between lines).

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Magnetic effectMagnetic effect

Right-Hand Grip RuleGrip wire with the right hand and with the thumb pointing in the direction of current. The other fingers point in the direction of magnetic field.

Without compass, the direction of magnetic

field can be obtained

using Right-Hand Grip

Rule.

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SolenoidSolenoidCurrent, I in circular coil creates magnetic field where it is strongest along the axis.

The direction of the field, B is

determined using right-hand grip rule (R.H.).

Solenoid is formed from many circular coils of wire uniformly wound in the shape of a cylinder through which electric current flows.

Magnetic field pattern produced by a current in a solenoid is almost identical to that of a bar magnet.

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SolenoidSolenoid

To find the N-pole of solenoid, grip it with right hand, the fingers curl in the direction of current, and the thumb points in the direction of N-pole.

Solenoids are important

because they can create controlled magnetic

fields and can be used as

electromagnets

.

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SolenoidSolenoidThe magnetic field inside a solenoid is given by:

B = µnI

B = magnetic field magnitude (teslas)µ = magnetic permeability (henries/meter or newtons/ampere2)n = turns density (number of turns/meter) I = current (amperes)

n = N / hN = number of turnsh = length of solenoid (meters)

This slide for extra information only.

µ = ku0magnetic constant or permeability of free space, µ0 = 4π x 10-7 H/mk = relative permeability

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ElectromagnetElectromagnetAn

electromagnet is made by winding a coil of wire around

a soft iron core, which

loses its magnetism when the current is

switched off, unlike steel

which is magnetized

permanently.In electromechanical devices, direct current is used to create strong magnetic field for drawing iron core or plunger into it, such as in switches and relays.

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• when the number of turns per unit length of the coil is increased (n)

Electromagnets are used in electric bells, circuit breakers, electromagnetic relays, telephone earpieces, etc.

• significantly with the use of soft iron core (µ)

The strength of the electromagnet increases

• when the current in the coil is increased (I)

ElectromagnetElectromagnet

B = µnIwhere µ = ku0µ0 = 4π x 10-7 H/m (or N/A2)k = relative permeability of iron is about 200, steel over 800

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Magnetic forceMagnetic forceThe direction of the force F on the conductor can be obtained using Fleming’s left-hand motor rule.

Force, F(Motion)

Field, BCurrent, I

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Electromagnetic Electromagnetic inductioninduction

Electromagnetic induction is the production of induced e.m.f. in conductor when there is relative motion between conductor and magnetic field.

Faraday’s law of electromagnetic inductionThe e.m.f. induced in a conductor is directly proportional to the rate of change of magnetic flux through the conductor.An e.m.f. is induced if wire cuts across magnetic field.No e.m.f. is induced if the wire moved parallel to magnetic field; the magnetic lines of forces are not cut by the wire.

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Electromagnetic Electromagnetic inductioninduction

The direction of e.m.f. induced or the induced current I can be obtained using Fleming’s right-hand dynamo rule.

Force, F(Motion)

Field, B

Current, I

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Electromagnetic Electromagnetic inductioninductionLenz’s law

The direction of the induced current produces an effect that opposes the change in the magnetic flux.An e.m.f. is induced in a solenoid when a magnet is moved into or out of solenoid. The direction of induced current is obtained using Lenz’s law.

Induced current produces N-pole to repel the N-pole of magnet

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TransformersTransformersTransformer is an application of electromagnetic induction. It consists of a primary coil and a secondary coil wound on a soft iron core.

Transformer is used to step-up or step-down the voltage of an a.c. supply, depending on where the a.c. source is applied.

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Generation of Generation of ElectricityElectricityMany sources of energy are used to generate

electricity, each with their own advantages and disadvantages.

Examples:HydroPotential energy of water in a dam converted to kinetic energyNatural gas, diesel, coalUsed as fuel to heat water in boilers to produce steamBiomassWaste material used as fuel, or decomposition of waste for methane gas for use as fuel.Nuclear energyNuclear fission of uranium releases heat used to heat water.SunlightSolar cells convert sunlight into electricity.WindStrong wind rotates windmill-like blades to rotate turbines.

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Generation of Generation of ElectricityElectricityMany sources of energy are used to generate

electricity, each with their own advantages and disadvantages.

Examples:HydroPotential energy of water in a dam converted to kinetic energyNatural gas, diesel, coalUsed as fuel to heat water in boilers to produce steamBiomassWaste material used as fuel, or decomposition of waste for methane gas for use as fuel.Nuclear energyNuclear fission of uranium releases heat used to heat water.SunlightSolar cells convert sunlight into electricity.WindStrong wind rotates windmill-like blades to rotate turbines.

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Transmission of Transmission of ElectricityElectricityAlternating voltage is generated at power station as

its voltage can be transformed with transformers.

A step-up transformer changes voltage to 320 kV or 500 kV.

Transmission at high voltage reduces current in cables; thus reducing power loss greatly.

Power loss as heat in cables = I2R

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Transmission of Transmission of ElectricityElectricityVoltage is stepped down in stages to, say 240 V using

transformers before supplying to consumers.

The National grid network is an interconnection of various power stations in the country.

It ensures:• minimal disruption to power supply through fast backups• efficient power generation by matching demand with supply• that power stations can shut down for regular maintenance

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SummarySummary

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What you have learned:What you have learned:1. magnetic effect of current-carrying

conductor

Thank YouThank You

2.2. force on current-carrying conductor inmagnetic field

3.3. electromagnetic induction4.4. transformers5.5. generation & transmission of electricity