Electromagnetism

Question 1

What do magnets produce?

Answer 1

A magnetic field

Question 2

What is a magnetic field?

Answer 2

A region where magnets, magnetic materials and also wires carrying a current will experience a force

Question 3

What is the direction of the arrows in a magnetic field diagram?

Answer 3

The arrows will always go out of the north side and into the south side

Question 4

How is the strength of a magnetic field shown in a magnetic field diagram?

Answer 4

The closer the field lines are together, the stronger the magnetic field

Question 5

What are three examples of magnetic materials?

Answer 5

Iron, nickel and cobalt

Question 6

What happens when current flows through a wire?

Answer 6

A magnetic field is created around the wire

Question 7

What does the strength of the magnetic field depend on?

Answer 7

The wire current and the distance from the wire

Question 8

What is flemming’s right hand rule?

Answer 8

If the direction of the magnetic field is anti-clockwise, then the electrical current will go up (north pole)
If the direction of the magnetic field is clockwise, the electrical current will go down (south pole)

Question 9

What does a solenoid do?

Answer 9

Increases the strength of a magnetic field

Question 10

How can you increase the size of the magnetic field produced by a solenoid?

Answer 10

Increasing the current
Increasing the number of coils
Adding an iron core

Question 11

What is an electromagnet?

Answer 11

A solenoid with a metal core; usually it’s an insulated wire wrapped around an iron bar

Question 12

What is the motor effect?

Answer 12

When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other

Question 13

How can the size of the force created by the motor effect be increased?

Answer 13

Increasing the current
Using a stronger magnet

Question 14

When is the force created by the motor effect the strongest and the weakest?

Answer 14

The force is the greatest when the wire is perpendicular to the magnetic field and weakest when the wire is parallel to the magnetic field

Question 15

What does each finger represent in flemming’s left hand rule?

Answer 15

Thumb: movement of the wire
First finger: magnetic field
Second finger: direction of current

Question 16

How can the direction of the force created by the motor effect be reversed?

Answer 16

Changing the direction of the current
Changing the direction of the magnetic field

Question 17

What is magnetic flux density?

Answer 17

A measure of the strength of the magnetic field

Question 18

How does an electric motor work?

Answer 18

A loop of wire (coil) is placed in a magnetic field
When the current is turned on, each side of the coil experiences a force but in opposite directions because the flow of current is opposite on each side of the loop
The force on the left side of the wire causes it to move up and the force on the right side of the wire causes it to move down
This makes the motor rotate

Question 19

What is the role of a split ring commutator in an electric motor?

Answer 19

Every half a turn, the split ring commutator reverses the direction of current around the loop, meaning that the motor will always rotate in the same direction

Question 20

What would happen without a split ring commutator in a motor?

Answer 20

The loop would oscillate and then stop, coming to rest perpendicular to the magnetic field lines

Question 21

What are graphite brushes for in an electric motor?

Answer 21

To connect the split ring to the battery

Question 22

Why is graphite used in an electric motor?

Answer 22

It’s an excellent conductor
It causes very little friction on the conductor ring

Question 23

What is the generator effect?

Answer 23

The term used to describe how electricity is created in a wire

Question 24

How can we increase the potential difference and current in a generator?

Answer 24

Increasing the speed at which the magnet / conductor moves
Increasing the strength of the magnetic field
Increasing the number of loops if we are using a coil / solenoid

Question 25

What are the two types of generator?

Answer 25

Alternators - produce alternating current
Dynamos - produce direct current

Question 26

How does an alternator work?

Answer 26

In an alternator, a coil of wire is rotated in a magnetic field so when the coil rotates, alternating current is induced because the direction of the field alternates
Every half turn of the coil, the current changes direction
Alternators don’t use slip rings which is why the current alternates

Question 27

Why does the magnitude of the induced potential difference and current vary as the coil rotates in the magnetic field of an alternator?

Answer 27

Because the rate at which the coil cuts field lines as it rotates changes depending on the orientation of the coil to the field; it is maximum when the coil is parallel to the field

Question 28

What other way can potential difference be induced in an alternator?

Answer 28

Using a rotating magnet within a fixed coil of wire

Question 29

How does a dynamo work?

Answer 29

In a dynamo, a coil of wire is rotated in a magnetic field and as it rotates, potential difference is generated in one direction
This is because a dynamo does use a split ring commutator which reverse the direction of the coil every half turn
This ensures the output current always moves in the same direction as the coil rotates

Question 30

Why does the magnitude of the induced potential difference and current vary in a dynamo?

Answer 30

Because the rate at which the coil cuts field lines changes depending on the orientation of the coil to the field

Question 31

What effect is used that makes microphones work?

Answer 31

The generator effect

Question 32

How do microphones work?

Answer 32

The sound waves that hit the microphone make a coil inside vibrate relative to a magnet
The coil then vibrates at the same frequency as the sound wave
This induces a potential difference and current in the coil

Question 33

What does a step up transformer do?

Answer 33

Increases voltage and decreases current after the power station

Question 34

What does a step down transformer do?

Answer 34

Decreases voltage and increases current before electricity gets to your house

Question 35

What three main parts do transformers consist of?

Answer 35

Soft iron core
Primary coil
Secondary coil

Question 36

Why is an iron core used in transformers?

Answer 36

Because it’s easily magnetised and demagnetised

Question 37

Why are the coils insulated in transformers?

Answer 37

Because otherwise current from the primary coil would flow through the core to the secondary coil

Question 38

How do transformers work?

Answer 38

The alternating current in the primary coil induces a changing magnetic field in the iron core
Because the current is constantly alternating, it creates a constantly changing magnetic field in the iron core
This changing magnetic field ‘cuts’ the secondary coil, causing an alternating potential difference and current to be induced in the secondary coil

Question 39

How can you tell if a transformer is a step up transformer?

Answer 39

If the number of coils on the secondary coil is higher, the potential difference will increase and it’s a step up transformer

Question 40

How can you tell if a transformer is a step down transformer?

Answer 40

If the number of coils on the secondary coil is lower, the potential difference will decrease and it’s a step down transformer