Electromagnetism

Question 1

Force on wires

Answer 1

• when a current carrying wire is placed in a magnetic field at right angles, the two magnetic fields combine.
• where the field directions are the same, the field strength increases.
• where the field directions are opposite, the field strength decreases.
• causes the wire to experience a force in the direction of the weaker field.

Question 2

Right hand grip rule

Answer 2

• thumb indicates direction of current flow

- fingers indicate direction of field

Question 3

Transformer equation

Answer 3

Vs Ns
— = —-
Vp Np

Question 4

Electric power

Answer 4

Vp x Ip = Vs x Is

Question 5

Fleming’s left hand rule

Answer 5

ThuMb - direction of force (Motion)
First finger - direction of field
SeCond finger - direction of Current

Question 6

A loud speaker

Answer 6

• A current is fed into a coil of wire which is wrapped around a permanent magnets at the base of the cone
• this creates a force on the coil causing it to move back-and-forth which makes the cone vibrate

The higher the frequency of the input signal, the higher the note heard

The higher the input signal (amplitude) , the greater the amplitude and so the volume of the sound

Question 7

Another name for a coil of wire

Answer 7

Solenoid

Question 8

Induced voltage (dynamo effect)

Answer 8

• moving a wire/coil in and out of a magnetic field causes a voltage in the wire creating current
• The faster you move the wire through the flux, the more voltage is induced
• The direction of the voltage depends on the direction of the movement of the wire and on the polarity of the magnet and the direction of movement

Question 9

Electromagnets and uses

Answer 9

• made by winding wire around a soft iron core

- uses: doorbell, fire alarm, relay, circuit breaker

Question 10

How to increase the strength of an electromagnet

Answer 10

• increase current flowing
• increase the number of turns
• use a soft iron core

Question 11

How to increase the size of an induced voltage

Answer 11

• increase the number of windings on a coil
• increase the speed of movement of magnet or wire
• increase field strength of magnet
• Wind coils on a soft iron core

Question 12

How to increase output of generators

Answer 12

• increase windings on coil
• increase area of coil
• increase field strength
• spin it faster but this also change the frequency

Question 13

How do microphones work

Answer 13

• as the sound waves hit the diaphragm they cause it to move in and out
• this movement causes the coil to move over the fixed magnet
• this induces a voltage in the coil
• The voltage can be amplified and then played over loud speaker

Question 14

How to alternators work (AC Generators)

Answer 14

• as the coil rotates the magnetic field lines are cut
• this induces a voltage in the coil
• once the coil reaches vertical position, the voltage changes polarity (shape of graph tells you this)
• every 180° degrees the output switches from positive to negative

Question 15

How do dynamos work (DC Generators)

Answer 15

• as the coil rotates, the magnetic field lines are cut
• this induces a voltage in the coil
• once coil reaches vertical position, voltage changes polarity (shape of the graph tell you this)
• The split rings causes the voltage to reverse and so the output always stays positive.

Question 16

Transformers

Answer 16

ONLY WORKS FOR AC (voltage has to be changing)
- either a step up or step down
How they work:
- two coils are wound on opposite sides of a soft iron core
- alternating primary voltage is applied to the primary coil
- this creates an alternating electromagnetic field in the core
- the alternating magnetic field induces a voltage across the secondary coil

Question 17

Efficiency of transformers

Answer 17

• in theory, they should be 100% efficient since there are no moving parts
• Transformers lose power through: heat losses in core, Eddie currents in core.
• this is minimised by laminating iron core

Question 18

Charged particle in magnetic field

Answer 18

• they experience a force
• positive and negative particles experience opposite forces
• determined by fleming’s LH rule

Question 19

Why are step down transformers used between powerlines and peoples houses

Answer 19

They go from 200KV -> 240V because the voltage in power lines is too much for the average person to need in a house and it can get dangerous

Question 20

National grid

Answer 20

• The grid make sure that there are a number of ways electricity can flow to where it is needed so we are very unlikely to have a power outage
• they can use a National Grid to track how much energy is used every day in different areas (peak times)
• this way it’s not just down to one power station for one town

Question 21

Motor effect

Answer 21

• each side of the loop will experience a force in the different direction which causes it to spin
• when it reaches vertical position, the split ring commutator allows it to continue spinning and the momentum of the coil carries it past the small gap in the commutator

Question 22

Why is electricity transported in high voltages

Answer 22

• The higher the voltage lower the current and therefore less power is lost
• power lost = I2R
• The resistance in the cable causes power to be lost but the higher the voltage the less it occurs