Electromagnetic effects

Question 1

Explain - inducing an EMF

Answer 1

When a conductor (e.g. a metal wire) is moved through a magnetic field It cuts through the field lines which induces an EMF

Question 2

What happens when a magnet is pushed into or taken out of a coil

Answer 2

As the magnet moved through the coil, the field lines cut through the turns of the coil This induces an EMF in the coil

Question 3

What factors effect EMF

Answer 3

The size of induced EMF is proportional to the rate at which the field lines are cut If the field lines are cut at a faster rate, the EMF will increase

Question 4

How can you cut field lines at a faster rate

Answer 4

Moving the wire or magnet faster Using a stronger magnet (increasing number of field line) Adding more turns to a coil

Question 5

Explain - alternating current (a.c.)

Answer 5

A current that continuously changes direction Goes back and forth around a circuit

Question 6

Explain - direct current (d.c.)

Answer 6

A current that is steady, constantly going the same way around a circuit from positive to negative

Question 7

Example of alternating current

Answer 7

Mains electricity

Question 8

Example of direct current

Answer 8

Electric cells (batteries)

Question 9

Explain an A.C. generator

Answer 9

As the coil rotates it cuts through field lines which induces an EMF between the end of the coil which could then create a current

Question 10

How can the size of the EMF in an A.C. generator be increased

Answer 10

Turning the coil faster Increased strength of magnetic field Adding more turns to the coil

Question 11

What are the function of slip rings

Answer 11

Slip rings are attached to the ends of the coil These transfer the current to metal brushes whilst allowing the coil to rotate freely

Question 12

How does the output of the EMF vary

Answer 12

The EMF is greatest when the coil is horizontal , as in this position it cuts through the field at the fastest rate The EMF is smallest when the coil is vertical, as in this position it does not cut through field lines

Question 13

Definition - Transformer

Answer 13

An electrical device that can be used to increase or decrease the voltage of an alternating current It consists of two coils wrapped around a soft iron core

Question 14

What are the four different types of transformers? What do each of them do?

Answer 14

Step up transformer - increases the voltage of a power source; this has more turns on the secondary coil than the primary coil Step down transformer - decreases the voltage of a power source; this has more turns on the primary coil than the secondary coil

Question 15

How does a transformer work

Answer 15

When an alternating current is supplied to the primary coil, a changing magnetic force is the produced in the primary coil The field passes through a soft iron core and through he secondary coil The changing magnetic field in the secondary coil induces an EMF The EMF is also alternating and has the same frequency as the original current

Question 16

State the two equations for transformers

Answer 16

1.) Input (primary) voltage/ Output (secondary) voltage = primary turns/ secondary turns

Vp / V_P = N_p/N_s

2.) Input power = output power

I_pV_p = I_sV_s

Question 17

Point to learn on transformers

Answer 17

Although transformers can increase the voltage of a power source, due to the law of the conservation of energy, they cannot increase the power output

Hence, increasing voltage decreases current, and vice-versa

Question 18

In what condition is electricity transmitted along overhead cables

Answer 18

At high voltages

Question 19

What transformers are used to transmit electricity

Answer 19

Step-up transformer - to raise the potential difference before transmission

Step-down transformer - To step the potential difference back down to normal levels when it reaches its destination

Question 20

How does high voltage reduce power loss

Answer 20

When electricity is transmitted over large distances, the current in the wire heats them, resulting in energy loss

By raising the voltage at which the electricity is transmitted at, the same amount of power can be transmitted using a much smaller current

This results in less heat being produced in the wire and hence less energy loss

Question 22

What happens when there is a current in a wire?

Answer 22

A magnetic field is created around the wire

Question 23

Magnetic field strength and direction

Answer 23

The direction of the force on the north pole of a magnet placed at that point

The strength and drection depend on the size and direction of the current:

If the current is increased, the field will get stronger

If the direction of the current is changed, the direction is field changed

The closer the field lines of a magnet are together, the stronger the magnetic field

The field lines around a wire get further apart the further they are away from the magnet

Question 24

Explain the magnetic field around a coil

Answer 24

When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it

Question 25

What is a solenoid

Answer 25

A long coil

The magnetic field around a solenoid look the same as the magnetic field around a bar magnet

Question 26

Explain the strength and direction of a solenoid

Answer 26

Inside the solenoid, the field lines straighten up and are very close together - they form a strong uniform field

Question 27

Applications of a solenoid

Answer 27

Can be used as an electromagnet by adding a soft iron core

These electromagnets can be used in doorbells and electronic door locks

This work when the current passes through the coil, it attracts the switch, closing it which allows a current in the right-hand part of the circuit

Question 28

Sate the motor effect

Answer 28

When a current passes through a wire in a magnetic field a force is exerted on the wire

Question 30

What does the direction on the force in the motor effect depend on

Answer 30

The direction of the field

The direction of the current

Reversing either of these will reverse the direction of the force

Question 31

State the left hand rule

Answer 31

The force is always directed at 90* to both the field and the current

The direction of the force on a current-carrying wire can be worked out by using the left-hand rule

Question 32

How does a magnetic field deflect a charged particle

Answer 32

When a charged particle passes through a magnetic field, the field can exert a force on the particle, deflecting it

This happens because the moving charge forms a current

Question 33

How can this deflection be demonstrated

Answer 33

Using a cathode ray tube and a pair of magnets

Or by passing a collimated beam of beta particles between the poles of a horse shoe magnet

Question 34

What is a D.C. motor

Answer 34

It consists of a coil in a uniform magnetic field