1. Chapter 3: Electromagnetism
Form 5
Physics
Next >
The study of matter
Chapter 3: Electromagnetism 1

2. Physics: Chapter 3
Objectives: (what you will learn) 1) magnetic effect of current-carrying conductor 2) force on current-carrying conductor in magnetic field 3) electromagnetic induction 4) transformers 5) generation & transmission of electricity
< Back
Next > 2

3. Line of Force
A line of force in magnetic field represents path of free N-pole in magnetic field. Direction of line of force: N-pole S-pole
Line of force
Magnetic field around the Earth
< Back
Next >
Magnetic field around a bar magnet
Magnetism and the Magnetic Compass
A MAGNET IN SPACE
Pilotsweb.com
Stargazers 3

4. Magnetic effect
When 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.
< Back
Next >
The direction of field can be obtained by moving a compass around the wire. 4

5. Magnetic effect
The 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
< Back
Next >
As distance from wire increases, magnetic field gets weaker (as shown by increasing distance between lines). 5

6. Magnetic effect
Without compass, the direction of magnetic field can be obtained using Right-Hand Grip Rule.
< Back
Next >
Right-Hand Grip Rule Grip 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. 6

7. Solenoid
Current, I in circular coil creates magnetic field where it is strongest along the axis.
Solenoid is formed from many circular coils of wire uniformly wound in the shape of a cylinder through which electric current flows.
< Back
Next >
Magnetic field pattern produced by a current in a solenoid is almost identical to that of a bar magnet.
The direction of the field, B is determined using right-hand grip rule (R.H.). 7

8. Solenoid
Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets.
< Back
Next >
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. 8

9. Solenoid 9 The magnetic field inside a solenoid is given by: B = µnI
This slide for extra information only.
Solenoid
The 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 / h
N = number of turns
h = length of solenoid (meters)
< Back
Next >
µ = ku0
magnetic constant or permeability of free space, µ0 = 4π x 10-7 H/m
k = relative permeability 9

10. Electromagnet
An 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.
< Back
Next >
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. 10

11. Electromagnet 11 The strength of the electromagnet increases
significantly with the use of soft iron core (µ)
when the number of turns per unit length of the coil is increased (n)
< Back
Next >
when the current in the coil is increased (I)
B = µnI
where µ = ku0
µ0 = 4π x 10-7 H/m (or N/A2)
k = relative permeability of iron is about 200, steel over 800 11
Electromagnets are used in electric bells, circuit breakers, electromagnetic relays, telephone earpieces, etc.

12. Magnetic force
Force, F
(Motion)
Field, B
Current, I
The direction of the force F on the conductor can be obtained using Fleming’s left-hand motor rule.
< Back
Next > 12

13. Electromagnetic induction
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 induction
The e.m.f. induced in a conductor is directly proportional to the rate of change of magnetic flux through the conductor.
< Back
Next >
An e.m.f. is induced if wire cuts across magnetic field. 13
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.

14. Electromagnetic induction
Force, F
(Motion)
Field, B
Current, I
The direction of e.m.f. induced or the induced current I can be obtained using Fleming’s right-hand dynamo rule.
< Back
Next > 14

15. Electromagnetic induction
Lenz’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.
< Back
Next > 15
Induced current produces N-pole to repel the N-pole of magnet

16. Transformers
Transformer is an application of electromagnetic induction.
It consists of a primary coil and a secondary coil wound on a soft iron core.
< Back
Next > 16
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.

17. Generation of Electricity
Many sources of energy are used to generate electricity, each with their own advantages and disadvantages.
Examples:
Hydro
Potential energy of water in a dam converted to kinetic energy
Natural gas, diesel, coal
Used as fuel to heat water in boilers to produce steam
Biomass
Waste material used as fuel, or decomposition of waste for methane gas for use as fuel.
Nuclear energy
Nuclear fission of uranium releases heat used to heat water.
Sunlight
Solar cells convert sunlight into electricity.
Wind
Strong wind rotates windmill-like blades to rotate turbines.
< Back
Next > 17

18. Generation of Electricity
Many sources of energy are used to generate electricity, each with their own advantages and disadvantages.
Examples:
Hydro
Potential energy of water in a dam converted to kinetic energy
Natural gas, diesel, coal
Used as fuel to heat water in boilers to produce steam
Biomass
Waste material used as fuel, or decomposition of waste for methane gas for use as fuel.
Nuclear energy
Nuclear fission of uranium releases heat used to heat water.
Sunlight
Solar cells convert sunlight into electricity.
Wind
Strong wind rotates windmill-like blades to rotate turbines.
< Back
Next > 18

19. Transmission of Electricity
Alternating 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.
< Back
Next >
Transmission at high voltage reduces current in cables; thus reducing power loss greatly.
Power loss as heat in cables = I2R 19

20. Transmission of Electricity
Voltage is stepped down in stages to, say 240 V using transformers before supplying to consumers.
< Back
Next >
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 20

21. Summary 21 What you have learned: Thank You
magnetic effect of current-carrying conductor
2. force on current-carrying conductor in magnetic field
< Back
3. electromagnetic induction
4. transformers
5. generation & transmission of electricity 21
Thank You