Electromagnetism

Question 1

What are hard magnets

Answer 1

Retain their magnetic properties for some time

Eg. Steel

Question 2

What are soft materials

Answer 2

Gain/lose their magnetic properties immediately (used in electromagnets)
Eg. Iron

Question 3

What is magnetic field

Answer 3

Magnets exert forces without touching

Question 4

Can magnetism be induced in some material

Answer 4

Yes when they are placed in an external magnetic field

Question 5

Practical- investigate the magnetic field pattern for a bar magnet

Answer 5

1. Place magnet under paper and sprinkle with iron fillings, tap paper so fillings settle into field lines
2. Place magnet on paper and place test compass nearby - draw points and line for where compass points, and repeat in many places around the magnet (this shows field direction)

Question 6

Electric current in a conductor

Answer 6

Produces a magnetic field around it

Question 7

What is a uniform magnetic field

Answer 7

When two opposite poles are put next to each other (but still separated)
Parallel field lines, evenly spaced field lines

Question 8

What is an electromagnet

Answer 8

• a coil of wire carrying a current
• iron
• produces a magnetic field like a bar magnet

Question 9

How does a scrap yard crane work

Answer 9

• current flows through coil of wire
• wrapped around soft iron core
• magnetic field
• when current is turned off it loses magnetisation

Question 10

When is there a force on a charged particle

Answer 10

When it moves in a magnetic field as long as it’s motion is not parallel to the field

Question 11

How does a Loudspeaker work?

Answer 11

• current runs through coil and current alternates
• current produces magnetic field
• magnetic field constantly changes because current is changing
• interacts with second external permanent magnetic field
• applies force on magnet/washers
• cup vibrates air making longitudinal wave

Question 12

What is the motor effect

Answer 12

Original magnetic field + magnetic field from wire = the motor effect = force felt by current in magnetic field

Question 13

How does force on wire increase

Answer 13

• if strength of magnetic field increases
• current increases
• number of coils increases

Question 14

How does force on wire reverse

Answer 14

• if magnetic field reverses

- if current reverses

Question 15

What is a motor?

Answer 15

Current flowing around the coil feels oppositely directed forces on each side from a second external magnetic field
These forces cause the coil to move
Every half turn the split ring commutator causes the current to reverse in the coil
Keeps the coil spinning in one direction

Question 16

How do split ring commutators work

Answer 16

Brushes lose contact with the split ring commutator
Current no longer flows through the motor coil
The coil will continue to rotate clockwise due to its momentum
Brushes regain contact with split ring commutator
Current flows through the motor coil but in the opposite direction, forces exert a clockwise turning effect on the coil

Question 17

When do you use the left hand rule

Answer 17

to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field

Question 18

When is a voltage induced in a conductor or coil

Answer 18

Moving a magnetic field across a wire or moving a wire through a magnetic field

Question 19

How to increase the induced voltage

Answer 19

Stronger magnetic field 
Faster movement
More turns on a coil
Wider coils
Put iron core in the coil

Question 20

How does a generator work?

Answer 20

A coils rotates through a magnetic field so the field is constantly changing direction through the loop

Question 21

Structure of a transformer

Answer 21

Two unconnected coils wrapped around an iron core

Question 22

How does a transformer work

Answer 22

Current produces a magnetic field
AC current in 1st coil goes to alternating magnetic field
Changing magnetic field induces AC current in 2nd
Induced voltage affected by number of turns in coil

Question 23

Step up transformers

Answer 23

2nd coil voltage and coil turns > 1st coil voltage and coil turns

Question 24

Why do national grid transmit at high voltages

Answer 24

Resistance transforms electric energy to heat so they lose energy
High current produces heat, but as v goes up current goes down

Question 25

Why is voltage stepped down

Answer 25

Voltage is stepped down for different consumers, lowest level is 230V for domestic use
(Final step down transformer will be at a sub station within a few hundred metres of each group of houses)

Question 26

Relationship between input and output voltages and turn ratio is

Answer 26

Input voltage / output voltage = primary turns / secondary turns

Question 27

If a transformer is 100 percent efficient what is the power input

Answer 27

P1 = P2

So VI1 = VI2