P15

Question 1

What happens when a current flows through a conducting wire

Answer 1

When a current flows through a conducting wire a magnetic field is produced around the wire

Question 2

How to increase the strength of the magnetic field around the wire

Answer 2

The strength of the magnetic field depends on the current through the wire and the distance from the wire

Question 3

What does making the wire into a solenoid do and what is the magnetic field like in a solenoid

Answer 3

Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire.
The magnetic field inside a solenoid is strong and uniform

Question 4

How are solenoids similar to bar magnets

Answer 4

The magnetic field around a solenoid has a similar shape to that of a bar magnet.

Question 5

What does adding an iron core do to a solenoid and what does this make when this happens

Answer 5

Adding an iron core increases the strength of the magnetic field of a solenoid.
An electromagnet is a solenoid with an iron core

Question 6

What is the motor effect

Answer 6

When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other and causes the conductor to move. This is called the motor effect.

Question 7

What is the equation for a conductor at right angles to a magnetic field and carrying a current

Answer 7

force = magnetic flux density × current × length
F = B x I x L

Question 8

What is the basis for an electric motor

Answer 8

A coil of wire carrying a current in a magnetic field tends to rotate, has a split ring commutator

Question 9

What is magnetic flux density

Answer 9

How many field lines there are in a region which shows the strength of the magnetic field

Question 10

What is fleming’s left hand rule

Answer 10

Using your left hand:
First finger represents Field ( pointing finger )
Second finger represents the direction of the Current ( longest finger )
Thumb represents the motion

Question 11

How does an electric motor work

Answer 11

In a DC motor forces act on two side arms of a coil of wire thats carrying a current
The coil is on an axle and the forces act one up and one down, it rotates
The direction of the motor can be reversed either by swapping the polarity of the DC supply ( reversing current ) or swapping magnetic poles ( reversing field )
Has a split ring commutator

Question 12

How is the motor effect used in headphones and speakers

Answer 12

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves

Question 13

How does the generator create a potential difference and how is a current induced in the conductor

Answer 13

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
If the conductor is part of a complete circuit, a current is induced in the conductor

Question 14

What does an induced current do

Answer 14

Generates a magnetic field that opposes the original change

Either the movement of the conductor or the change in magnetic field

Question 15

How do you change the size of the induced potential difference

Answer 15

Increasinf the speed of the movement - cutting more magnetic fields in a given time
Increase the strength of the magnetic field

Question 16

How do alternators work

Answer 16

Coil spins, current is induced in the coil.
Current changes direction every half turn
Alternators have slip rings and brushes so contacts don’t swap every half turn
Produce an alternating potential difference

Question 17

How do dynamos work

Answer 17

Same as alternators
But they have a split ring commutator
This swaps the connection every half turn to keep the current flowing in the same direction

Question 18

How do microphones work

Answer 18

Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits

Question 19

How do loudspeakers work

Answer 19

Alternating current sent through a coil of wire attached to the base of a paper cone
Coil surrounds one pole of a permanent magnet and is surrounded by yhe other pole so the current causes a force on the coil
When the current reverses, the force acts in the opposite direction which causes the cone to move in the opposite direction too
Variations in current make cone vibrate which makes air around the cone vibrate and creates variations in pressure that makes a sound wave
The frequency of sound wave is same as frequency of the AC so by controlling the frequency of the AC you can alter the sound wave produced

Question 20

What do transformers do

Answer 20

Change the the size of the potential difference of an alternating current

Question 21

How do transformers work

Answer 21

They all have 2 coils of wire, the primary and secondary, joined by an iron coil
When an alternating potential difference is applied across the primary coil, the iron core magnetises and demagnetises quickly. These changes in the magnetic field induces an alternating PD in the secondary coil
If the second coil is part of a complete circuit, this causes a current to be induced
The ratio between the primary and secondary PDs is the same as the ratio between tbe number of turns on the primary and secondary coils

Question 22

Equation for voltage and number of coils for primary and secondary coils

Answer 22

vp/ vs = np/ ns
p= primary coil
s= secondary coil
V= voltage 
N= number of coils