6.3 Electromagnetism

Question 1

What causes magnetic fields? (2)

Answer 1

Moving charges

Permanent magnets

Question 2

How do magnetic field lines represent magnetic fields? (2)

Answer 2

The arrow points from north to south

The spacing represents the field strength

Question 3

How is a uniform magnetic field represented?

Answer 3

Equally spaced, parallel lines

Question 4

Magnetic field patterns - current-carrying wires

Answer 4

Field lines are concentric circles centred on the wire and perpendicular to it

Question 5

What is the right-hand grip rule? (2)

Answer 5

In a current carrying wire, the thumb points in the direction of the conventional current and fingers point in the direction of the field
In a solenoid, the thumb points in the direction of the field and the fingers point in the direction of the conventional current

Question 6

What is Fleming’s left hand rule? (3)

Answer 6

Thumb gives direction of force
First finger gives the direction of the external field
Second finger gives direction of conventional current

Question 7

What does the magnitude of the force experienced by a wire in an external magnetic field depend on? (4)

Answer 7

Angle of wire to field
Current
Length of wire in field
Strength of the magnetic field

Question 8

When is the force experienced by a wire in an external magnetic field at a maximum?

Answer 8

When the wire is perpendicular to the magnetic field

Question 9

Define magnetic flux density

Answer 9

The strength of the field

Question 10

How is magnetic flux density determined experimentally? (5)

Answer 10

Place two magnets on top of each other on a top pan balance with a stiff copper wire between the two poles
Measure the length of the wire in the field with a ruler
Connect the wire in series with an ammeter and variable power supply using crocodile clips
When a current flows through the wire, an upwards force is exerted on it so, according to Newton’s third law, the magnets experience an equal downwards force which can be calculated from the mass reading on the balance
Magnetic flux density can be calculated using B = F/IL

Question 11

How does a charged particle move in a magnetic field? (4)

Answer 11

As the charged particles enter the field, they experience a force according to Fleming’s left hand rule
They change direction but the force always remains perpendicular to the velocity
The speed of the electrons remains the same as there is no component of force in that direction
This causes the charged particles to move in circular motion until they leave the field and move in a straight line

Question 12

What is a velocity selector? (2)

Answer 12

A device that uses an electric field and a perpendicular magnetic field to select charged particles of a specific velocity
The forces from the electric and magnetic fields act in opposite directions and only one specific velocity will cause the two forces to cancel out, allowing these particles to travel in a straight line and emerge through the opposite narrow slit

Question 13

Why does a magnet moving in a coil cause electromagnetic induction? (2)

Answer 13

Some of the work done to move the magnet is transferred into electrical energy
The relative motion of the coil causes the electrons to move as they experience a force from the field, causing a current to be induced

Question 14

Define magnetic flux

Answer 14

The product of the component of the magnetic flux density perpendicular to the cross-sectional area

Question 15

Define magnetic flux linkage

Answer 15

The product of the number of turns in the coil and the magnetic flux

Question 16

When is an EMF induced?

Answer 16

Whenever there is a change in the magnetic flux linking the circuit

Question 17

State Faraday’s law

Answer 17

The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux linkage

Question 18

Explain Lenz’s law (3)

Answer 18

When a magnet and coil are brought close together, the current induced in the end of the coil is such that the polarity at the end of the coil is the same as the polarity of the pole of the magnet
The work done on the magnet = electrical energy produced in the coil
The induced pole cannot be the opposite pole because the principle of conservation of energy would be violated, producing electrical energy out of nothing

Question 19

State Lenz’s law

Answer 19

The direction of the induced current is always such as to oppose the change producing it

Question 20

What is a simple AC generator?

Answer 20

A rectangular coil rotating in a uniform magnetic field

Question 21

What does an flux linkage-time graph show for an AC generator? (2)

Answer 21

The gradient is equal to the induced EMF

If the coil rotates at a steady frequency, the variation is sinusoidal

Question 22

What does maximum induced EMF depend on? (4)

Answer 22

Magnetic flux density
Cross-sectional area of the coil
Number of turns
Frequency of rotating coil

Question 23

How does a simple transformer work? (2)

Answer 23

An alternating current in the primary coil produces a varying magnetic flux in the iron core
The varying magnetic flux induces a varying EMF across the secondary coil

Question 24

What is a step-up transformer?

Answer 24

A transformer with more turns on the secondary coil than the primary coil

Question 25

What is a step-down transformer?

Answer 25

A transformer with fewer turns on the secondary coil than the primary coil

Question 26

How are transformers made more efficient? (3)

Answer 26

Low-resistance windings to reduce power loss die to the heating effect of the current
Laminated core minimises eddy currents induced in the core itself
Use soft iron as it is easy to magnetise and demagnetise

Question 27

Why is electrical power transmitted at high voltage?

Answer 27

To minimise heat losses