P15

Question 1

Which metals are magnetic?

Answer 1

Colbalt, iron, nickel

Question 2

How can you stop the effect of a magnet ?

Answer 2

You can stop the effect of a magnet by shielding it with a magnetic material.

Question 3

What happens when a current is passed through a coil of wire ?

Answer 3

When a current is passed through a coil of wire, the coil has a magnetic field around it. This is called an electromagnet.

Question 4

What do all magnets have?

Answer 4

A north and south pole

Question 5

What is a magnetic field ?

Answer 5

A region where other magnets or magnetic materials experience a force ( this is a non-contact force )

Question 6

What do all magnetic materials produce?

Answer 6

All magnetic materials produce a magnetic field

Question 7

The lines in a magnetic field diagram travel from which end to which end?

Answer 7

The lines travel from north to south

Question 8

Where is the magnetic field strongest at ?

Answer 8

It is strongest at the poles

Question 9

What does a current carrying wire produce ?

Answer 9

It produces a magnetic field

Question 10

What are the 3 things that can effect the power of an electromagnet

Answer 10

-the core materials
-number of coils
-size of the current

Question 11

What is a solenoid?

Answer 11

It is a long coil of insulated wire

Question 12

How does an electric bell work ?

Answer 12

1. When the current flows through the circuit, the electromagnet makes a magnetic field.
2. The electromagnet attracts the springy metal arm.
3. The arm hits the gong, which makes a sound.
4. The circuit is broken now the arm is out of position.
5. The electromagnet is turned off and the springy metal arm moves back.
6. The circuit is complete again.
7. The cycle repeats as long as the switch is closed.

Question 13

What is the motor effect ?

Answer 13

When a current-carrying wire (or any other conductor) is put between magnetic poles, the magnetio field around the wire interacts with the magnetic field it has been placed in. This causes the magnet and the conductor to exert a force on each other. This is called the motor effect and can cause the wire to move.

Question 14

How can you increase the motor effect?

Answer 14

The size of the force can be increased by:
-increasing the current
-using a stronger magnet

Question 15

How can the wire experience the full force of the motor ?

Answer 15

To experience the full force, the wire has to be at 90 degrees to the magnetic field. If the wire runs parallel to the magnetic field, it won’t experience any force at all. At angles in between, it will feel some force

Question 16

Where does the force always act at?

Answer 16

The force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire

Question 17

The force acting on a conductor in a magnetic field depends on which three things ?

Answer 17

• the magnetic flux density - how many field (flux) lines there are in a region. This shows the strength of the magnetic field
• the size of the current through the conductor
• the length of the conductor that’s in the magnetic field

Question 18

Whats the equation of the force that acting when the current is at 90 degrees to the magnetic field it is in ?

Answer 18

Force (N) = Magnetic flux density (T) x Current (A) x Length (m)

Question 19

How can you find the direction of the force ?

Answer 19

You can find the direction of the force with Flemings left hand rule

Using your left hand point your index finger in the direction of the Field
Point your middle finger in the direction of the current
Your thumb will then point in the direction of the force (motion)

Question 20

How does a current-carrying coil of wire rotate in a magnetic field ?

Answer 20

the coil is on a spindle and the forces act one up and one down, it rotates.

The split-ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction.

The direction of the motor can be reversed either by swapping the polarity of the de supply (reversing the current) or swapping the magnetic poles over (reversing the field).

Question 21

What does it mean by induced potential difference ? What is the generator effect

Answer 21

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor.

If the conductor is part of a complete circuit, a current is induced in the conductor - this is called the generator effect.

Question 22

What is the definition of the generator effect ?

Answer 22

The induction of a potential difference (and current if there’s a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field

Question 23

How can you do the generator effect ?

Answer 23

-You can do this by moving a magnet in a coil of wire OR moving a conductor (wire) in a magnetic field (“cutting” magnetic field lines).

-Shifting the magnet from side to side creates a little “blip” of current if the conductor is part of a complete circuit.

-If you move the magnet (or conductor) in the opposite direction, then the potential difference/current will be reversed. Likewise if the polarity of the magnet is reversed, then the potential difference/current will be reversed too.

-If you keep the magnet (or the coil) moving backwards and forwards, you produce a potential difference that keeps swapping direction - an alternating current.

Question 24

Where is the north and south for a solenoid ?

Answer 24

The south pole is the end where current is clockwise

The north pole is the end where current is anti-clockwise

Question 25

What do transformers change the size of ?

Answer 25

Transformers change the size of the potential difference of an alternating current

Question 26

What do all transformers have ?

Answer 26

They have two coils of wire, the primary and secondary, joined with an iron core

Question 27

What happens when a alternating pd is applied across a coil ?

Answer 27

When an alternating pd is applied across the primary coil, the iron core magnetises and demagnetises quickly. This changing magnetic field induces an alternating pd in the secondary coil

Question 28

The ratio between the primary and secondary potential difference is the same as the ratio…

Answer 28

Between the number of turns on the primary and secondary coils

Question 29

What do step up transformers do? What do they have more of on the secondary coil

Answer 29

Makes the pd across the secondary coil greater than the pd across the primary coil
There are more turns in the secondary coil than in the primary coil

Question 30

What do step down transformers do ?

Answer 30

Makes the pd across the secondary coil less than the pd across the primary coil
There are fewer turns in the secondary coil than in the primary coil

Question 31

What does it mean when it says that the core is laminated (layered) ?

Answer 31

It means that it cuts out induced currents in the iron and this increases the efficiency of the transformer

Question 32

What is the transformer equation ?

Answer 32

Vp/Vs = np/ns
Np meaning the number of turns on the primary coil
Ns meaning the number of turns on the secondary coil

Question 33

How efficient are transformers ? And why ?

Answer 33

Transformers are almost 100% efficient

Becuasw When a device is connected to the secondary coil, almost all of the electrical power supplied to the transformer is delivered to the device.

Question 34

How does a loudspeaker work ?

Answer 34

• The moving coil loudspeaker creates sound waves when an alternating p.d is applied to its coil
• The coil is in an magnetic field, the current in the coil cause a force on the coil due to the motor effect
• The alternating current causes the force to constantly change direction, making the coil and the diaphragm to vibrate
• The vibration causes sound waves to be produced of the same frequency as the alternating p.d source

Question 35

How do microphones work ?

Answer 35

1) Microphones are basically loudspeakers in reverse.

2) Sound waves hit a flexible diaphram that is attached to a coil of wire, wrapped around a magnet.

3) This causes the coil of wire to move in the magnetic field, which generates a current.

4) The movement of the coil (and so the generated current) depends on the properties of the sound wave (louder sounds make the diaphragm move further).

5) This is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit.