Electromagnetism

Question 1

What are the key points about

non-magnetic materials?

(and some examples)

Answer 1

• can’t be magnetised
• not attracted or repelled by other magnets

(e.g. copper, aluminium, diamond, salt, plastic, wood)

Question 2

What are the key points about

magnetic materials?

(and some examples)

or ferromagnetic “ferromagnets”

Answer 2

• can be magnetised and demagnetised
• two types - “soft” and “hard”
• when demagnetised, are attracted and not repelled

(e.g. iron, steel, cobalt, nickel)

Question 3

What makes a magnetic material

soft?

Answer 3

it is easy to magnetise

(and demagnetise)

Question 4

What makes a magnetic material

hard?

Answer 4

it is difficult to magnetise

(and demagnetise)

Question 5

What are the key points about

magnets?

Answer 5

• attracted and repelled by other magnets
• two types - “permanent” and “non-permanent”
• has magnetic poles that attract or repel other magnets

(these poles are called “north” and “south”)

Question 6

How are magnetic fields represented?

Answer 6

Using “field lines” in space which show the strength and direction of the magnetic field.

Question 7

What are the key points about

field lines?

Answer 7

• they can’t stop/start in mid air
• they can’t touch/cross
• they point from north to south
• their density shows the strength of the field

Question 8

How can a ferromagnetic material be magnetized?

Answer 8

• put it in a magnetic field
• stroke it repeatedly in the same direction with another magnet
• put it inside a long coil of wire which has a direct current passing through it

Question 9

What makes iron magnetic?

Answer 9

• inside a piece of iron, each atom acts as a small magnet called a dipole
• dipoles are grouped together into domains
• in an unmagnetised piece of iron the domains are arranged in random directions and their effects cancel out
• when the iron is exposed to a magnetic field, the domains line up in the direction of the field with all the north poles pointing in the same direction

(when all the domains are lined up the magnet cannot get any stronger and is saturated)

Question 10

How can you

demagnetise a magnet?

Answer 10

• heat it
• drop it
• be generally rough with it

each time a magnet is dropped or heated, more domains will become arranged in random directions and the magnet will become weaker

Question 11

Describe the magnetic field for a

wire coming out of a board.

Answer 11

perfect circles around the wire which are not evenly spaced

(less dense further away from the wire)

Question 12

What is the right hand rule and when is it used?

Answer 12

It is used when determining the field direction around a current carrying wire.
Your thumb points in the direction of the wire’s conventional current. The direction in which your fingers wrap is the direction of the field.

Question 13

Describe the magnetic field around a

coil of wire carrying current.

(a solenoid)

Answer 13

like a bar magnet

Question 14

How can electromagnets be made stronger?

Answer 14

• by increasing the number of turns
• by increasing the current
• by adding a (soft) iron core

(the core is soft so that the ferromagnet is quicker to turn on and off)

Question 15

How does an (old-fashioned) electric bell work?

Answer 15

1. The battery is turned on - completing the circuit.
2. The coil (which now has a current through it) magnetises the electromagnet.
3. The striker is attracted (which hits the bell and creates a sound).
4. This breaks the contact - so the circuit is no longer complete.
5. The spring pushes the striker back (now that the solenoid is no longer magnetised).
6. This process repeats (until the battery is turned off).

Question 16

How is the motor effect created?

Answer 16

When a current carrying wire is placed in between two flat magnets - so the two different fields interact and create a force.

Question 17

What is the EXAM EXPLANATION of the

motor effect?

Answer 17

The magnetic field due to the current-carrying wire [1] interacts with the field due to the magnets [1] causing a force perpendicular to both the current and the field [1].

Question 18

What is the left hand rule and when is it used?

Answer 18

The left hand rule is used to determine one of the factors in the motor effect.
The THumb represents the force (or THrust).
The First Finger represents the magnetic Field.
The seCONd finger represents the CONventional current (positive to negative).

Question 19

Explain what the split ring commutator does.

Answer 19

The split ring commutator rotates with the coil, while the brushes remain stationary and make electrical contact with the two halves of the commutator but don’t prevent it from rotating. One brush is connected to the negative terminal of the power supply and one is connected to the positive terminal. Every half a turn, side A swaps from touching the negative brush to the positive brush (or vice versa), so the direction of current in side A reverses. This reverses the direction of the force on side A, ensurng that this force causes continuous rotation.