Using Magnetic Fields to Keep Things Moving

Question 1

What is a magnetic field?

Answer 1

The region around a magnet in which a magnetic material (e.g. iron) will be attracted to the magnet

Question 2

Define ‘line of force’

Answer 2

Line in a magnetic field along which a magnetic compass points.

Also called magnetic field lines.

Question 3

Describe the force between two magnets

Answer 3

Like poles repel, same poles attract

Question 4

Give 5 examples of where electromagnets are used.

Answer 4

• Scrapyard cranes
• Circuit breakers
• Electric bells
• Relays
• Fire alarms

Question 5

What happens when a current flows through a wire?

Answer 5

A magnetic field is created

Question 6

How are electromagnets made?

Answer 6

• Insulated wire is wrapped around a piece of soft iron, this will become the core.
• When a current flows through the wire the iron becomes strongly magnetised
• When the current is switched off the iron loses its magnetism

This temporary magnetism makes electromagnets very useful

Question 7

What is the motor effect?

Answer 7

When a current is passed along a wire that has been placed inside a maganetic field, a force is exerted on the wire by the magnetic field.

This only works if the wire is not parallel to the lines of the magnetic field

Question 8

When is the force felt by the wire in a magnetic field greatest?

Answer 8

When the wire is at 90° to the magnetic field (perpendicular)

Question 9

When is the force felt by the wire in a magnetic field smallest?

Answer 9

When the wire is parallel to the magnetic field

Question 10

What is Fleming’s Left Hand Rule used to determine?

Answer 10

The direction of the force felt by a wire expiriencing the motor effect (in a magnetic field)

Question 11

In what position are the fingers in Fleming’s Left Hand Rule at in accordance to one another?

Answer 11

They are at right angles to each other (perpendicular)

Question 12

What do the fingers in Fleming’s Left Hand Rule represent?

Answer 12

• Thumb - Thrust (direction of force)
• First Finger - Field (north to south)
• Second Finger - Current (positive to negative)

Question 13

How can the force felt through the motor effect be increased?

Answer 13

Increasing the strength of the magnetic field

Increasing the size of the current

Question 14

How can the direction of the force on the wire undergoing the motor effect reversed?

Answer 14

The direction of the current is reversed

The direction of the magnetic field is reversed

Question 15

What happens to the direction of the force on a wire carrying a current in a magnetic field if:

Both the current and magnetic field are reversed?

Answer 15

It stays the same

Question 16

What happens to the direction of the force on a wire carrying a current in a magnetic field if:

The direction of the current changed?

Answer 16

It is revesed

Question 17

What happens to the direction of the force on a wire carrying a current in a magnetic field if:

The direction of the magnetic field is changed?

Answer 17

It reverses

Question 18

How can the direction of a motor be changed?

Answer 18

By reversing the direction of the current (or magnetic field, but current is more easy/more common)

Question 19

How can the speed of a motor be increased?

Answer 19

By increasing the size of the current (or magnetic field, but current is easier)

Question 20

When a current passes through the coil of a motor, why does the motor spin?

Answer 20

Because a force acts on each side of the coil due to the motor effect.

The force on one side of the coil is in the opposite direction to the force on the other side.

This causes the motor to spin.

Question 21

What is a split-ring commutator?

Answer 21

Metal contacts on the coil of a DC motor that connects the rotating coil continuously to its electrical power supply.

Question 22

What does a split-ring commutator do?

Answer 22

Reverses the direction of the current around the coil every half-turn.

Because the sides swap over every half-turn, the coil is always pushed in the same direction.

Question 23

What happens when an electrical conductor ‘cuts’ through magnetic field lines?

Answer 23

A potential difference (voltage) is induced across the ends of the conductor.

Question 24

What is electromagnetic induction?

Answer 24

The process of inducing a potental difference (voltage) in a wire by moving the wire so it cuts across the lines of force of a magnetic field

Question 25

What happens when a maget is moved into a coil of wire?

Answer 25

A potential difference (voltage) is induced across the ends of the coil. This is electromagnetic induction. If the wire or coil is part of a circuit, the current passes through it.

Question 26

In an electromagnet, what happens if the movement of the wire/coil is reversed?

Answer 26

The direction of the induced potential difference (voltage) is also reversed.

Question 27

In an electromagnet, what happens if the polarity of the magnet is reversed?

Answer 27

The direction of the induced potential difference (voltage) is also reversed.

Question 28

When can a potential difference only be induced?

Answer 28

When there is movement.

Question 29

How can the size of the induced potential difference (in an electromagnet) be increased?

Answer 29

By increasing: * The speed of **movement** * The **strength** of the magnetic field * The **number of turns** on the coil

Question 30

Why is there no potential difference induced when a bar magnet is held stationary inside a coil of wire?

Answer 30

Because no magnetic field lines are being cut

Question 31

What is the effect on the induced potential difference of reversing the direction of the current in a conductor cutting magnetic field lines?

Answer 31

The direction of the induced potential difference is reversed

Question 32

When an electromagnet is being used to induce a potetial difference, what state does it need to be in?

Answer 32

On.

Question 33

What does a transformer consist of?

Answer 33

* Two coils of insulated wire called the primary and secondary coils. * These are wrapped around a soft iron core

Question 34

How do transformers work?

Answer 34

* When an AC passes through the primary coil, it **produces an alternating magnetic field in the core**. This field continually expands and collapses. * The **alternating magnetic field lines** pass through the secondary coil and **induce an alternating potential difference across its ends**. * If the secondary coil is part of a complete circuit, an AC is produced.

Question 35

Why does a transformer only work on AC?

Answer 35

Because a changing/alternating magnetic field is needed to induce AC in the secondary coil.

Question 36

Do all transformers have the same type of core?

Answer 36

No. A switch-mode transformer has a ferrite core

Question 37

What type of core does a switch-mode transformer have?

Answer 37

A ferrite core

Question 38

Why are the wires in a transformer insulated?

Answer 38

So that the current does not short across either the iron core or the adjacent turns of wire, but instead flows around the whole coil.

Question 39

Why is the core of a transformer made from soft iron?

Answer 39

Because it is easily magnetised

Question 40

Where are transformers used?

Answer 40

The National Grid

Question 41

What is a step-up transformer/ what does it do?

Answer 41

Makes the **potential difference across the secondary coil greater than the potential difference across the primary coil**. Its **secondary coil has more turns** than its primary coil.

Question 42

What is a step-down transformer/ what does it do?

Answer 42

Makes the **potential difference across the secondary coil less than the potential difference across the primary coil**. Its **secondary coil has fewer turns** than its primary coil.

Question 43

What are the features of a switch-mode transformer compared to a traditional transformer?

Answer 43

* Operates at a much **higher frequency** * **Lighter and smaller** * **Uses very little power when there is no device connected** across its output terminals

Question 44

What does the National Grid use step-up transformers for?

Answer 44

Stepping-up the potential difference from power stations.

Question 45

Why does the National Grid use step-up transformers?

Answer 45

To **increase the potential difference** at which electrical energy is transmitted. This is done because the **higher the pd at which electrical energy is transmitted, the smaller the energy wasted in the cables**.

Question 46

Why are step-down transformers used in the National Grid?

Answer 46

To reduce the potential difference so that it is safe to be used by consumers.

Question 47

Does a step-down transformer have more or less turns on the secondary coil than the primary?

Answer 47

A **step-down** transformer has **fewer turns on the secondary coil than the primary**

Question 48

Does a step-up transformer have more or less turns on the secondary coil than the primary?

Answer 48

A **step-up** transformer has **more turns on the secondary coil than the primary**.

Question 49

How efficient are transformers (when used in calculations - specifically the formula *V_p X I_p = V_s X I_s*)

Answer 49

100% efficient

Question 50

In the equation: *V_p X I_p = V_s X I_s*, what are *V*, *I*, and _s and _p?

Answer 50

* V* = Voltage/pd across either primary or secondary coil in volts * I* = Current across either primary or secondary coils in amperes _p and _s = Primary or secondary coils repsectively