SP12 and 13

Question 1

What happens to unlike magnetic poles?

Answer 1

• They attract

Question 2

What happens to like magnetic poles?

Answer 2

• They repel

Question 3

What is a permanent magnet?

Answer 3

• A magnet that is always magnetic and cannot be turned on and off

Question 4

What is an induced magnet?

Answer 4

• A magnet that can be turned off and on and is only magnetic when placed inside a magnetic field

Question 5

What are some uses of magnetic materials?

Answer 5

• Compasses
• Motors
• Knife holders
• Loudspeakers

Question 6

Which way do arrows point in a magnetic field when drawing lines?

Answer 6

• Field lines go in from the South pole and out from the North pole

Question 7

How do field lines show strength of a magnetic field?

Answer 7

• When field lines are denser (more close together), the field is stronger

Question 8

How can plotting compasses be used to show shape of magnetic field?

Answer 8

• The tiny bar magnet points in the direction of the magnetic field, which can show the direction of the field at different points around a magnet

Question 9

How do compasses show that the Earth’s core is magnetic?

Answer 9

• The tilt by different amounts, relative to the horizontal. This shows how the magnetic field of the Earth causes a shift in the compass position

Question 10

What is the shape of the magnetic field produced when a current is passed through a wire?

Answer 10

• Magnetic field made of multiple circles around each other, closest together nearest to the wire
• The ‘Right hand grip’ rule can be used to how the direction of the field compared to the current flow
• As distance from the wire increases, the strength of the magnetic field decreases

Question 11

What is a solenoid and describe the magnetic field in it?

Answer 11

• A solenoid is a series of coils of wire, one after the other
• The magnetic field is strong and uniform within a solenoid, because they add together
• The magnetic field is weaker on the outside as they cancel each other

Question 12

What are four ways of increasing the strength of an electromagnet?

Answer 12

• Increase the current flowing through the solenoid
• Increase the number of turns (or coil) in the solenoid
• Decrease the length of the solenoid, whilst keeping number of turns the same
• Add a soft iron core in the middle of the solenoid

Question 13

What happens when a current carrying conductor is placed near a magnet?

Answer 13

• It experiences a force, whilst an equal and opposite force acts on the magnet (this is known as the Motor Effect)

Question 14

How does a magnetic force occur?

Answer 14

• It is due to interactions between magnetic fields, where it will either cause attraction or repulsion

Question 15

What is Fleming’s left-hand rule?

Answer 15

• A method of determining the force on a conductor carrying a current
• Can be done by holding your left hand, with the first three fingers at right angles from each other
• The direction of the thumb represents Force, the index finger represents the Magnetic field and the middle finger represents the Current

Question 16

How do you calculate the strength of the force?

Answer 16

F = B x I x L
- F= force and is measured in Newtons (N)
- B=magnetic flux density and is measured in Teslas (T)
- I= current and is measured in Amperes (A)
- L=length and is measured in metres (M)

Question 17

How does an electric motor spin?

Answer 17

• There is a magnetic field caused by two magnets on either side of the motor
• There is a current flowing through the motor, which flows in opposite directions at either end
• Due to Fleming’s left-hand rule, each side of the motor will experience an opposite force, causing it to spin clockwise

Question 18

What is the role of a split-ring commutator?

Answer 18

• Every 180 degrees that the motor turns, the currents will now be facing in opposite directions, resulting in the forces acting in opposite directions and the motor turning anticlockwise
• To overcome this, a split-ring ring commutator swaps the positive and negative connections every half-turn, allowing the currents to swap so that the motor can turn a full 360 degrees as the forces will always act in the same direction

Question 19

What are 3 ways of increasing the speed of rotations in a motor?

Answer 19

• Increase the current
• Increase the number of coils
• Increase magnetic flux density by increasing magnet strength

Question 20

What is electromagnetic induction?

Answer 20

• The effect seen as the movement of a wire or magnet in a field will induce a voltage
• Each time direction changes, the direction of the potential difference changes

Question 21

What factors affect the size of the induced potential difference- and therefore the current?

Answer 21

• Change strength of magnetic field by using stronger magnets
• Move wire/magnets quicker, to faster change the field
• Shape wire into a coil, as more turns mean an increased potential difference

Question 22

What factors affect the direction of the potential difference?

Answer 22

• Direction by which magnet-wire is moved into/out of the field
• Direction of poles in the magnet

Question 23

How does a generator produce an alternating current?

Answer 23

• Consists of a coil of wire rotated inside a magnetic field
• As the coil rotates, a potential difference is induced
• The coils are connected to slip rings, which come into contact with carbon brushes, which press on them and allow a current to flow
• These types of generators are called alternators

Question 24

How does a generator produce a direct current?

Answer 24

• It uses a split-ring commutator, which switches the connections every half-turn and allows a direct current to flow
• This is known as a dynamo

Question 25

How does a microphone work?

Answer 25

• A person singing causes sound waves and air pressure that hit the diaphragm and causes vibrations
• This causes the coil of wire within the microphone to move backwards and forwards
• This results in electromagnetic induction, causing a current and allowing the signal to be amplified

Question 26

How does a loudspeaker work?

Answer 26

• Similar to microphone, yet it starts out with current and ends with air pressure
• An alternating current and magnetic field within the speaker causes a force to be achieved (Fleming’s left-hand rule)
• The AC means the force moves backwards and forwards, causing a paper cone to do so too, causing vibrations and amplifying the noise

Question 27

What is the role of a transformer?

Answer 27

To increase (step-up) or decrease (step-down) the size of an alternating voltage

Question 28

How does a transformer work?

Answer 28

• An alternating potential difference is applied across the primary coil, causing a current to flow and generate a magnetic field around the coil
• This induces an alternating magnetic field in the iron core
• This induces a potential difference in the secondary coil
• If this is part of a complete circuit, it causes current to flow

Question 29

How does a transformer step-up or step-down the voltage?

Answer 29

• By increasing or decreasing the number of turns in the secondary coil, it increases or decreases the potential difference, respectively

Question 30

Why and where are transformers used?

Answer 30

• An increase in voltage means that the wire gets hitter, and so more energy is lost through heating
• Transformers step down the voltage before entering a transmission line, to reduce energy lost as electricity travels over a long distance
• They then step up the voltage at the end, so more electricity can travel to homes and power buildings