Topic P4- Magnetism and Magnetic Fields

Question 1

What is a magnetic field?

Answer 1

• A region where a magnetic material (iron, nickel, cobalt) experiences a force.
• magnetic field lines are used to show the size and direction of magnetic fields.
• They always point from NORTH to SOUTH
• The closer the field lines are to each other, the stronger the magnetic field at the point.

Question 2

What is magnetic flux density?

Answer 2

• The strength of the magnetic field is the magnetic flux density.
• It is measure din teslas (T)

Question 3

How can you use a compass to plot magnetic field patterns?

Answer 3

• Put a magnet on a piece of paper and place the compass on the paper, next to the magnet
• The compass needle will point in the direction of the field line at this position.
• Mark the direction that the compass needle is pointing in by marking two dots on the paper,
• one at each end of the needle.
• Move the compass so that the tail end of the needle is where the tip of the needle was previously
• Repeat this and then join up the marks made
• this will create one field line around the magnet.
• Repeat method at different points around the magnet to get several field lines.

Question 4

Give evidence that the earth has a magnetic core:

Answer 4

• Compasses will always point north when they aren’t near a magnet.
• This is evidence that the Earth has a magnetic north and south pole
• and therefore must have a magnetic core.

Question 5

What is the difference between induced magnets and permanent magnets?

Answer 5

• Induced magnets will usually lose their magnetism once the magnet has been moved away
• whilst permanent magnets keep their magnetism regardless of it moving near/away from a magnetic material.

Question 6

How do magnetic field behave around a straight wire?

Answer 6

• A magnetic field around a straight, current-carrying wire is made up of concentric circles
• with the wire in the centre
• use the “right-hand rule”
• You could show that there’s a magnetic field around a current-carrying wire by using a compass
• they can be used to find the direction of the magnetic field
• The further away from the wire, the weaker the magnetic field (field lines getting further apart)

Question 7

How does a magnetic field behave around a flat circular coil?

Answer 7

• The magnetic field in the centre of a flat circular coil of wire is similar to that of a bar magnet
• There are concentric ellipses of magnetic field lines around the coil.

Question 8

How does a magnetic field behave around a solenoid?

Answer 8

• Magnetic effect is increased (lots of coils of wire joined together)
• Magnetic field in a solenoid (current-carrying) is strong and uniform
• Outside the coil, the field is just like the one around a bar magnet
• Ends of a solenoid act like the north pole and south pole of a bar magnet
• This is a type of magnet called electromagnet

Magnetic effect at the ends of solenoid will increase if:

• current in the wire is increased
• number of turns (coils) of wire is increased, but length stays the same
• the length of the solenoid is decreased (number of turns stay the same)
• an iron core is added inside the solenoid

Question 9

What is the equation used for calculating the force on a current-carrying conductor when it is at right angles to a magnetic field?

Answer 9

Force on a magnetic current length
conductor = flux density x (A) x (m)
carrying a (T)
current (N)

Question 10

How does a simple electric motor work?

Answer 10

• A simple electric motor uses magnets and a current-carrying coil
• a current-carrying coil sits between two opposite poles of a magnet
• The current in flowing in a different directions on each side of the coil,
• each side of the coil is perpendicular to the magnetic field,
• each side will experience forces in opposite directions
• Because the coil is on a spindle, and the forces act in opposite directions on each side
• it rotates
• The split-ring commutator is a smart way of swapping 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 d.c. supply (reversing the current)
• or by swapping the magnetic poles over (reversing the field)

Question 11

How can you speed up a simple electric motor?

Answer 11

• Increase the current
• adds more turn to the coil
• or increases the magnetic flux density

Question 12

How does a loudspeaker work? What makes it vibrate to create sound?

Answer 12

• As well as rotation, the force between a current-carrying coil of wire and a magnetic field can be used to make things move back and forth
• like a loudspeaker
• loudspeaker contains a coil of wire which is surrounded by one magnet
• another magnet is inside the coil
• a.c. (alternating current) electrical signals are fed to the coil of wire, which is wrapped around the base of the cone
• The interaction between the magnetic field and the current in the coil forces the coil to move in one direction
• because it is a.c., the current changes direction, forcing the coil back in the other direction
• as the current continues to alternate, the coil moves back and forth
• these movements make the cone vibrate
• this creates pressure variations in the air
• i.e. sound.

Question 13

What is electromagnetic induction?

Answer 13

• Electromagnetic induction is when a potential difference is induced across a conductor
• which is experiencing a change in its external magnetic field
• this happens when it passes though magnetic field lines
• if the conductor is part of a complete circuit
• the induced p.d. will result in a current in the circuit
• this current produces its own magnetic field too
• the p.d. is always induced so that the magnetic filed produced by the current will OPPOSE the original change in the external magnetic field.

Question 14

What are the two different situations where electromagnetic induction is created?

Answer 14

• an electrical conductor and a magnetic field move relative to each other
• the magnetic field through an electrical conductor changes (bigger or smaller or reverses)

Question 15

What is a generator?

Answer 15

• Generator are the opposite to motors
• they use relative motion of a conductor and magnetic field to induce a p.d. and a current.
• For any generator:
• If direction of rotation is reversed, the direction of the induced p.d./current reverses too
• the current induced in an alternator or dynamo will be greater if there are more turns on the coil, the magnetic flux density is increased
• or if the speed of rotation is increased.

**ALTERNATORS AND DYNAMOS are types of generators.

Question 16

How do alternators work?

Answer 16

• some rotate a magnet in a coil of wire
• as the magnet spins, an alternating p.d. is induced across the end of the coil.
• the p.d. changes direction every half-turn because the direction of the field changes as the magnet rotates
• this produces an a.c. if the coil is part of a complete circuit
• a.c. can also be generated by rotating a coil in a magnetic field
• slip rings remain in contact with the brushes that are connected to the rest of the circuit
• this means the contacts don’t swap every half-turn, so they produce a.c.

Question 17

How do dynamos work?

Answer 17

• rotate a coil in a magnetic field
• output p.d. and current change direction with every half rotation of the coil
• producing a.c.
• the coil is part of a complete circuit
• the split-ring commutator swaps the connection every half turn to keep the current flowing in the same direction
• so it changes from a.c. to d.c.

Question 18

How do microphones work?

Answer 18

• dynamic microphone’s structure is like a loudspeaker, but the cone is replaced by a diaphragm.
• sound waves (pressure variations) cause the diaphragm to move back and forth when hit by them
• as the diaphragm moves, the coil of wire moves,
• inducing a p.d. across the ends of the coil of wire
• the coil is part of a circuit, so the induced p.d. means variations in current in the electrical circuit.

Question 19

How do transformers work?

Answer 19

• They change the size of the p.d. of an a.c.
• They all have two coils: primary and secondary, joined with an iron core
• When an alternating p.d. is applied across the primary coil, it produces an alternating magnetic field.
• As iron is a magnetic material, the core also becomes magnetised
• The coil is producing an alternating magnetic field, the magnetisation in the core also alternates
• A changing magnetic field induces a p.d. in the secondary coil
• Transformers are almost 100% efficient
• so power in primary coil = secondary coil

Question 20

Describe the difference between step-down transformers and step-up transformers:

Answer 20

• step-down: step the voltage down. They have more turns on the primary coil than the secondary coil
• step-up: step the voltage up. They have more turns on the secondary coil than the primary coil.

Question 21

What is the transformer equation?

Answer 21

p.d. across
primary coil (V) number of turns in primary coil
—————– = —————————————–
p.d. across number of turns in the secondary secondary coil (V) coil

Equation can be used either way up- less rearranging to do if you put whatever you are trying to calculate (unknown) on the top (of fraction)