Electromagnetic Effects

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Notes

Electromagnetic Induction

An **e.m.f.** is induced in a conductor when there is **relative movement** between the conductor and a magnetic field (conductor moves in stationary field OR conductor stationary in changing field).
If the conductor is part of a complete circuit, an induced current flows.
**Lenz's law** (Extended): The direction of the induced e.m.f. always **opposes the change** causing it (e.g., induced magnetic field repels approaching magnet).
**Right-hand dynamo rule** (Extended): First finger = Field, thuMb = Motion, seCond = Current (direction of induced e.m.f.).
Factors increasing induced e.m.f.: **faster motion**, **more turns** on coil, **stronger magnet**, **larger coil area**.

Demonstrating Induction

**Experiment 1**: Move a bar magnet in/out of a coil connected to a sensitive voltmeter. Stationary → zero reading; moving in → deflection; moving out → opposite deflection.
**Experiment 2**: Move a wire between magnetic poles, connected to a voltmeter. Stationary → zero; moving → deflection; reverse direction → opposite deflection.
Induced e.m.f. only occurs while there is **relative motion** (cutting field lines).
Factors increasing induced e.m.f.: **faster movement**, **more turns**, **stronger magnet**, **longer wire** (for wire experiment).

The A.C. Generator

An **a.c. generator** converts mechanical energy to electrical energy using a **rotating coil** in a magnetic field.
Components: **permanent magnet**, **rotating coil**, **slip rings** and **carbon brushes** to connect to external circuit.
As the coil rotates, it cuts field lines, inducing an alternating e.m.f. (and current).
Maximum e.m.f. when coil is **parallel** to field (moving perpendicular); zero e.m.f. when coil is **perpendicular** to field (moving parallel).
Output graph is a **sine/cosine wave**; frequency equals rotation frequency.
To increase maximum e.m.f.: **increase rotation speed**, **more turns**, **stronger magnet**, **add soft iron core**.

Magnetic Effect of a Current

A current-carrying wire produces a magnetic field of **concentric circles** around the wire.
**Right-hand grip rule**: Thumb points in current direction; curled fingers show field direction.
A **solenoid** (coiled wire) produces a field similar to a **bar magnet** (one end north, other south).
Field strength increases with **larger current**, **more turns**, and **inserting a soft iron core** (electromagnet).
Applications: **relay circuits** (electromagnet switches a second circuit), **electric bells**, **loudspeakers** (motor effect).

Force on a Current-Carrying Conductor

A current-carrying conductor in a magnetic field experiences a **force** (motor effect) if current is **perpendicular** to field.
**Fleming's left-hand rule**: Thumb = Force (Thrust), First finger = Field, Second finger = Current.
Reversing **current** or **magnetic field** reverses the force direction.
**Charged particles** (e.g., electrons) in a magnetic field also experience a force, deflecting perpendicular to both velocity and field.

Electric Motors

A **d.c. motor** uses the motor effect to rotate a coil continuously.
Components: **coil** in magnetic field, **split-ring commutator** and **carbon brushes** connected to a d.c. supply.
The split-ring commutator **reverses current** every half turn, so the coil rotates in the same direction.
Forces on opposite sides of the coil are opposite, causing rotation.
To increase speed/turning effect: **increase current**, **stronger magnet**, **more turns** on coil.
To reverse rotation: reverse **current direction** or **magnetic field**.

Transformers

A transformer changes the size of an **alternating voltage** using electromagnetic induction.
Structure: **primary coil**, **secondary coil**, and a **soft iron core** (easily magnetised).
**Step-up transformer**: more turns on secondary than primary → increases voltage (Vs > Vp).
**Step-down transformer**: fewer turns on secondary than primary → decreases voltage (Vs < Vp).
Transformer equation: Vp / Vs = Np / Ns (where N = number of turns).
High-voltage transmission reduces **energy loss** (lower current for same power, less heating in wires).

Simple circuit to demonstrate electromagnetic induction (e.g., moving magnet near coil).

Refraction of light (for comparison with electromagnetic induction concepts).

Practice questions

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1.Which of the following metals can be permanently magnetised?
Easy
- AAluminium
- BCopper
- CSteel
- DTungsten
2.What is the name of the component in a transformer that is made of soft iron?
Easy
- APrimary coil
- BSecondary coil
- CCore
- DInsulation
3.In a simple a.c. generator, what is the function of the slip rings?
Easy
- ATo reverse the current every half turn
- BTo allow the coil to rotate continuously
- CTo provide a uniform magnetic field
- DTo connect the coil to the external circuit
4.A transformer has 100 turns on the primary coil and 500 turns on the secondary coil. The input voltage is 20 V. What is the output voltage?
Medium
- A4 V
- B100 V
- C500 V
- D1000 V
5.Which change would increase the strength of an electromagnet?
Medium
- ADecreasing the current in the coil
- BRemoving the iron core
- CIncreasing the number of turns on the coil
- DUsing a shorter wire
6.A coil is connected to a sensitive voltmeter. A bar magnet is pushed into the coil and then held stationary inside it. What does the voltmeter show?
Hard
- AA constant deflection while the magnet moves, then zero
- BA deflection only while the magnet is moving
- CNo deflection at any time
- DA constant deflection while the magnet is stationary inside
7.In a d.c. motor, what is the purpose of the split-ring commutator?
Medium
- ATo increase the magnetic field strength
- BTo reverse the direction of the current in the coil every half turn
- CTo connect the coil to the power supply continuously
- DTo reduce friction
8.A wire carrying a current is placed in a magnetic field. The direction of the force on the wire can be found using:
Hard
- AFleming's right-hand rule
- BFleming's left-hand rule
- CThe right-hand grip rule
- DLenz's law

Slides

Electromagnetic Induction

An e.m.f. is induced in a conductor when there is relative movement between the conductor and a magnetic field.
This can happen when the conductor moves in a stationary magnetic field, or when the conductor is stationary in a changing magnetic field.
If the conductor is part of a complete circuit, a current is induced.

When is an e.m.f. induced in a conductor?

Only when the conductor moves in a stationary magnetic field
Only when the conductor is stationary in a changing magnetic field
When there is relative movement between the conductor and a magnetic field
Only when the conductor is part of a complete circuit

Demonstrating Induction

A magnet moved into a coil induces an e.m.f., detected by a voltmeter.
When the magnet is stationary, no e.m.f. is induced.
Moving the magnet faster, using more turns, or a stronger magnet increases the induced e.m.f.

When a magnet is pushed into a coil, an ____ is induced. If the magnet is stationary, the voltmeter reads ____.

Lenz's Law

Lenz's law states: The direction of an induced e.m.f. always opposes the change causing it.
When a magnet is pushed into a coil, the induced current makes the coil act like a magnet repelling the incoming magnet.
When pulled out, the coil attracts the magnet to oppose the motion.

Which statements about Lenz's law are true?

The induced e.m.f. opposes the change that causes it.
When a magnet is pushed into a coil, the coil attracts the magnet.
Lenz's law is a consequence of conservation of energy.

The A.C. Generator

An a.c. generator converts mechanical energy into electrical energy.
It consists of a rotating coil in a magnetic field, with slip rings and brushes to connect to an external circuit.
The induced e.m.f. alternates as the coil rotates, producing an alternating current.

What is the function of slip rings in an a.c. generator?

To reverse the current every half turn
To allow the alternating current to flow between the coil and the external circuit
To increase the induced e.m.f.
To provide a uniform magnetic field

Magnetic Field Around a Current-Carrying Wire

A current-carrying wire produces a magnetic field with concentric circular field lines around the wire.
The direction of the field is given by the right-hand grip rule: thumb points in current direction, fingers curl in field direction.
Reversing the current reverses the field direction.

The direction of the magnetic field around a current-carrying wire can be found using the ____ rule. If the current flows upward, the field lines are ____ clockwise as seen from above.

Force on a Current-Carrying Conductor (Motor Effect)

A current-carrying conductor in a magnetic field experiences a force.
The force is maximum when the current is perpendicular to the magnetic field.
Fleming's left-hand rule gives the direction: thumb = force, first finger = field, second finger = current.

Using Fleming's left-hand rule, if the magnetic field is from north to south and the current flows from left to right, what is the direction of the force?

Upwards
Downwards
Into the page
Out of the page

The D.C. Motor

A d.c. motor uses the motor effect to convert electrical energy into mechanical energy.
It consists of a coil in a magnetic field, a split-ring commutator, and brushes.
The split-ring commutator reverses the current every half turn so the coil rotates continuously in one direction.

Which of the following increase the speed of rotation of a d.c. motor?

Increasing the current in the coil
Using a weaker magnet
Adding more turns to the coil

Transformers

A transformer changes the size of an alternating voltage using electromagnetic induction.
It consists of a primary coil, a secondary coil, and a soft iron core.
A step-up transformer has more turns on the secondary coil (Vs > Vp); a step-down transformer has fewer turns on the secondary coil (Vs < Vp).

A transformer has 100 turns on the primary coil and 200 turns on the secondary coil. The input voltage is 12 V a.c. What is the output voltage?

6 V
12 V
24 V
48 V

Transformer Calculations

The transformer equation: Vp / Vs = Np / Ns, where V is voltage and N is number of turns.
For an ideal transformer, power in primary equals power in secondary: Vp × Ip = Vs × Is.
Step-up transformers increase voltage but decrease current; step-down transformers decrease voltage but increase current.

For a step-up transformer, the number of turns on the ____ coil is greater than on the ____ coil. The output voltage is ____ than the input voltage.

Applications of Electromagnets

Electromagnets are used in relay circuits, electric bells, and scrapyard cranes.
A relay uses a small current to control a larger current in another circuit.
In a relay, current in the primary circuit creates an electromagnet that attracts a switch to close the secondary circuit.

In a relay, what happens when the switch in the primary circuit is closed?

The electromagnet is deactivated
The armature pivots and closes the contacts in the secondary circuit
The secondary circuit is broken
The current in the primary circuit stops

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