P4 Magnetism

Question 1

What happens when we pass a current through a wire?

Answer 1

A magnetic field is produced around the wire

Question 2

What happens when you put 2 magnets near each other?

Answer 2

Their fields interact producing a force, attracting or repelling

Question 3

what happens when we put a wire carrying current near a magnet?

Answer 3

The fields interact to produce a force on the wire

Question 4

The motor effect

Answer 4

when the magnetic field surrounding the wire interacts with the fixed magnetic field between the poles of the magnet, the resulting attractive or repelling force is strong enough to move the wire

Question 5

Flemmings left hand grip rule

Answer 5

First finger - field
Second finger - current
Thumb - movement / force

Question 6

F = BIL

Answer 6

F = Force (N)
B = Magnetic field strength / Magnetic flux density (Tesla, T) 
I = Current in wire (Amps, A)
L = length of wire in field (m)

Question 7

Permanent magnets

Answer 7

do not easily lose their magnetism e.g horseshoe magnet

Question 8

Temporary magnets

Answer 8

Become magnetised in the presence of a magnetic field e.g paperclips

Question 9

Electromagnets

Answer 9

Created by a current in a wire - the magnetism disappears when the current is turned off e.g solenoid

Question 10

Common magnetic materials

Answer 10

Nickel, Iron, Cobalt and steel (‘kNICkerS’)

Question 11

Field lines go from…

Answer 11

North to south pole. They never cross - closer the lines the stronger the force

Question 12

Magnetic flux

Answer 12

• lines of a magnetic field are called magnetic flux
• How close they are together is called the magnetic field strength or magnetic flux density
• Symbol is B and is measured in Tesla (T)

Question 13

Detecting magnetic field

Answer 13

Done with iron filings sprinkled on paper above magnet or done using a plotting compass

Question 14

Bigger current (I) =

Answer 14

Stronger field (B) = lines closer together

Question 15

Right hand grip rule

Answer 15

• Determines direction of field. Thumb = current, other fingers = direction of field
• can also find direction of the field from a coil or solenoid.

Question 16

Plotting compass with wire

Answer 16

Can go through hole in middle of the page and plotting compass placed around it on a flat piece of card.

Question 17

Wire - Blob

Answer 17

current out of the page

Question 18

Wire - cross

Answer 18

Current into page

Question 19

Motors

Answer 19

• Use currents and permanent magnets to create motion
• a current carrying coil is placed in the magnetic field. Flemmings left hand rule tells us that the one side experiences an upward force and the other a downward force. The coil rotates.
• to keep the motor going, a split ring commutator reverses the current direction in the coil every 180°

Question 20

Electromagnet induction

Answer 20

Moving a wire through a magnetic field induces a pd in the wire. If the wire is connected to a circuit a current flows. It doesn’t matter what moves: the wire through the field or the magnet past a (coil of) wire

Question 21

To increase the induced pd

Answer 21

• Move the wire faster
• Use a stronger field
• Have more wire (more turns on a coil)

Question 22

Lenz’z Law

Answer 22

The current generated produces its own magnetic field which is in the opposite direction to the field producing it. We see this effect when a magnet is dropped down a copper pipe (it falls very slowly)

Question 23

Generators

Answer 23

A coil is turned in a magnetic field and a pd is induced - this can be used to generate an electric current

Question 24

Alternators

Answer 24

Use slip rings to generate an ac (alternating current)

Question 25

Dynamos

Answer 25

Use a split ring commutator to produce a direct current

Question 26

Transformers

Answer 26

Only work with ac’s

Look at picture equation!!

Question 27

Microphones

Answer 27

• The compressions and rarefaction cause the diaphragm to move
• The diaphragm is connected to a coil of wire, so the coil moves
• Inside the coil is a permanent magnet, so motion of coil induces a pd in the coil
• The induced pd across the ends of the wire is the electrical signal produced

Question 28

Loudspeaker

Answer 28

• Input wires are connected to a pd varying at the required frequency
• This pd produces a changing current in the coil
• The coil is in a magnetic field, so the coil will move in and out at the input frequency
• The coil is attached to a cone, so the cone moves in and out
• The motion of the cone causes sound waves to be produced in the air around