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.
The conductor can move in a stationary field, or the field can change around a stationary conductor.
If the conductor is part of a complete circuit, an **induced current** flows.
Factors increasing induced e.m.f.: **faster movement**, **more turns** on the coil, **larger coil area**, **stronger magnetic field**.
Reversing the magnet poles or the direction of movement **reverses the direction** of the induced e.m.f.

The A.C. Generator

An **a.c. generator** converts mechanical energy into electrical energy using a rotating coil in a magnetic field.
It uses **slip rings** and **carbon brushes** to connect the coil to an external circuit.
The induced e.m.f. is **alternating** (sine/cosine wave) as the coil rotates.
Maximum e.m.f. when coil is **horizontal** (moving perpendicular to field); zero e.m.f. when **vertical** (moving parallel).
Output e.m.f. can be increased by: **faster rotation**, **more turns**, **stronger magnet**, **soft iron core**.

Magnetic Effect of a Current

A current-carrying wire produces a **circular magnetic field** around it (concentric circles).
Use the **right-hand grip rule**: thumb = current direction, fingers = field direction.
A **solenoid** (coil of wire) produces a field like a **bar magnet** with north and south poles.
Field strength increases with **higher current**, **more turns**, or adding a **soft iron core** (electromagnet).
The field direction reverses if the current direction is reversed.

Force on a Current-Carrying Conductor

A current-carrying conductor in a magnetic field experiences a **force** (motor effect).
The force is maximum when the current is **perpendicular** to the field; zero when parallel.
**Fleming's left-hand rule**: thumb = force (thrust), first finger = field (N→S), second finger = current (+ to -).
Reversing either the current or the magnetic field **reverses the force direction**.

Electric Motors

A **d.c. motor** uses the motor effect to produce continuous rotation.
Key parts: **coil**, **permanent magnet**, **split-ring commutator**, **carbon brushes**.
The split-ring commutator **reverses the current** every half-turn to keep rotation in the same direction.
Turning effect increased by: **more turns**, **higher current**, **stronger magnet**.
Direction of rotation reversed by reversing **current** or **magnetic field**.

Transformers

A **transformer** changes the voltage of an alternating supply using electromagnetic induction.
It consists of a **primary coil**, **secondary coil**, and a **soft iron core**.
A **step-up transformer** increases voltage $(Ns >$ Np); a **step-down transformer** decreases voltage $(Ns < Np)$ .
Transformer equation: $\frac{V_p}{V_s} = \frac{N_p}{N_s}$ .
For an **ideal transformer** (100% efficient): $I_p V_p = I_s V_s$ .

High-Voltage Transmission

Electricity is transmitted at **high voltage** and **low current** to reduce energy loss in cables.
Power loss in cables: $P = I^2 R$ . Lower current means less heat loss.
**Step-up transformers** increase voltage before transmission; **step-down transformers** decrease voltage for safe use.
This system improves **efficiency** of power distribution.

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

Practice questions

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1.A coil of wire is connected to a sensitive voltmeter. A magnet is pushed into the coil and the voltmeter needle deflects to the right. What will happen when the magnet is stationary in the centre of the coil?
Easy
- AThe needle will deflect to the left
- BThe needle will deflect to the right
- CThere will be no deflection of the needle
- DThe needle will deflect to the left and then to the right
2.State two factors that affect the magnitude of the induced e.m.f. in a coil when a magnet is moved into it.
Easy
3.A step-up transformer has more turns on the secondary coil than on the primary coil.
Easy
True or false?
4.A transformer has 20 turns on the primary coil and 800 turns on the secondary coil. The voltage across the primary coil is 500 V. Calculate the output voltage of the secondary coil.
Medium
- A18000
- B20000
- C22000
- D24000
5.Complete the sentence about Fleming's left-hand rule.
Easy
In Fleming's left-hand rule, the thumb points in the direction of $the ____ on$ the conductor.
6.In a simple a.c. generator, the induced e.m.f. is zero when the coil is:
Medium
- Ahorizontal and moving perpendicular to the magnetic field
- Bvertical and moving parallel to the magnetic field
- Chorizontal and moving parallel to the magnetic field
- Dvertical and moving perpendicular to the magnetic field
7.Match each component of a simple a.c. generator to its function.
Medium
- Permanent magnet
- Slip rings
- Carbon brushes
- Allow alternating current to flow between coil and external circuit
- Provide a uniform magnetic field
- Provide a good electrical connection between coil and external circuit
8.State two ways to reverse the direction of the force on a current-carrying conductor in a magnetic field.
Medium

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.

A coil of wire is connected to a sensitive voltmeter. A magnet is pushed into the coil. Which action would produce a larger deflection on the voltmeter?

Using a weaker magnet
Moving the magnet more slowly
Using a coil with more turns
Keeping the magnet stationary inside the coil

Factors Affecting Induced e.m.f.

The magnitude of induced e.m.f. increases with: speed of movement, number of turns on the coil, area of the coil, and strength of the magnetic field.
The direction of induced e.m.f. reverses if the poles of the magnet are swapped or if the direction of movement is reversed.

When a magnet is moved into a coil, $an ____ is$ induced. If the magnet is moved faster, the induced e.m.f. $is ____$ .

The A.C. Generator

An a.c. generator converts motion into electrical output, producing an alternating e.m.f.
It consists of a rotating coil in a magnetic field, connected to an external circuit via slip rings and carbon brushes.
The induced e.m.f. is maximum when the coil is horizontal (moving perpendicular to the field) and zero when vertical (moving parallel).

Which statements about an a.c. generator are true?

The coil rotates in a uniform magnetic field.
Slip rings allow the current to reverse direction in the external circuit.
The induced e.m.f. is zero when the coil is vertical.
Carbon brushes provide electrical connection to the rotating coil.

Magnetic Field Around a Current-Carrying Wire

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

Using the right-hand grip rule, if the conventional current in a straight wire flows upwards, what is the direction of the magnetic field lines?

Clockwise as seen from above
Anticlockwise as seen from above
Radially outward
Radially inward

Magnetic Field of a Solenoid

A solenoid (coil of wire) produces a magnetic field similar to a bar magnet, with a north and south pole.
The field lines emerge from the north pole and return to the south pole.
The right-hand grip rule: curled fingers show current direction, thumb points to north pole.

The magnetic field inside a solenoid is similar to that of $a ____$ magnet. The end where the current flows anticlockwise is $the ____$ pole.

Force on a Current-Carrying Conductor

A current-carrying conductor in a magnetic field experiences a force if the current is perpendicular to the field.
The direction of the force is given by Fleming's left-hand rule: thumb = force, first finger = field, second finger = current.
Reversing either the current or the magnetic field reverses the force direction.

A wire carrying current is placed between the poles of a magnet. Using Fleming's left-hand rule, the thumb points in the direction of the:

Magnetic field
Current
Force on the wire
Motion of electrons

The D.C. Motor

A d.c. motor uses the motor effect to rotate a coil in a magnetic field.
It consists of a coil, a permanent magnet, a split-ring commutator, and carbon brushes.
The split-ring commutator reverses the current direction every half turn so the coil rotates continuously in one direction.

Which of these increase the turning effect of a d.c. motor?

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

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 increases voltage $(Ns >$ Np); a step-down transformer decreases voltage $(Ns < Np)$ .

A transformer has 100 turns on the primary and 500 turns on the secondary. The primary voltage is 230 V. What is the secondary voltage?

46 V
1150 V
230 V
500 V

Transformer Calculations

For an ideal (100% efficient) transformer: $Vp / Vs = Np / Ns$ and Ip $Vp = Is$ Vs.
Power is conserved: input power = output power.
Electricity is transmitted at high voltage and low current to reduce energy loss in wires $(P = I^{2}R)$ .

In an ideal transformer, the power in the primary coil equals the power in $the ____$ coil. The equation $is ____$ .

Why is electricity transmitted at high voltage?

To increase the current
To reduce power loss in the cables
To make it safer for homes
To increase the resistance

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