Topic 8: Electric and Magnetic Fields

Question 1

What happens when you rub two insulated materials together?

Answer 1

• When insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped onto the other
• As the materials are insulators, the electrons are not free to move, the build up of this charge is static electricity –> the materials become electrically charged, with a positive static charge on the one that has lost electrons and an equal negative static on the other
• The way the electrons are transferred depends on the two materials involved, the negative electrons always move
• e.g. polythene and acetate rods being rubbed with a cloth cluster

Question 2

What’s static electricity?

Answer 2

The build up of charge on insulating materials

Question 3

How do electrically charged objects attract uncharged objects? - e.g. balloon sticks to wall

Answer 3

• When you run a balloon in your hair or clothes, it transfers electrons to the balloon leaving it with a negative charge
• The balloon will then stick to a wall because the charges on the surface of the wall can move a little so the negative charges on the balloon repel the negative charges on the wall’s surface
• This leaves a positive charge on the surface which attracts the negatively charged balloon
• Called ATTRACTION BY INDUCTION

Question 4

How are sparks caused?

Answer 4

• As an electric charge builds on an object, the potential difference between the object and the earth (earth is 0V) increases
• If the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth (this is the spark)
• They can also jump to any earth conductor that is nearby (which is why you can get static shocks from clothes)

Question 5

What are 3 uses for static electricity?

Answer 5

Photocopiers - use static electricity to copy images onto a charged plate before printing them

Reduce the dust and smoke that rises out of industrial chimneys

Electrostatic sprayers - Apply a fine, even coat of whatever’s being sprayed (e.g. paint sprayer): the spray gun is charged which charges up the small drops of paint, they all repel each other because they have the same charge which gives a fine even coat. The object to be painted has an opposite charge to the gun so it attracts the paint droplets

Question 6

What are 3 dangers of static electricity?

Answer 6

Refuelling cars - as fuel flows out of a filler pipe, static can build up and can lead to a spark which may cause an explosion in dusty places e.g. at a petrol station

Static on aeroplanes - friction between air and plane causes the plane to become charged. This build up of static charge can interfere with communication equipment

Lightning - Inside storm clouds raindrops and ice bump together which makes the top positively charged and the bottom negatively charged which creates a huge voltage and a big spark

Question 7

How can you reduce the dangers of static electricity?

Answer 7

By EARTHING charged objects - connect charged object to ground using a conductor (e.g. a copper wire) EARTHING
This provides an easy route for static charges to travel to the ground so no charge can build up to make a spark
Electrons flow down the conductor the the ground if the charge is negative, and flow up from the ground if its positive
e.g. fuel tankers must be earthed to prevent sparks that may cause the fuel to explode

Question 8

What’s an electric field?

Answer 8

The region around a charged object where, if a second charged object was placed inside it, a force would be exerted on both of the charges

Question 9

Which way do electric field lines go?

Answer 9

From positive to negative

Question 10

What happens when a charged object is placed into an electric field?

Answer 10

It feels a force, the force is caused by the electric fields around the two charged objects interacting

Question 11

What’s a magnetic field?

Answer 11

A region where other magnets or magnetic materials experience a force

Question 12

What are the main 3 magnetic elements?

Answer 12

Iron, Nickel and Cobalt

Question 13

Is steel magnetic?

Answer 13

Steel is magnetic because it contains iron

Question 14

What’s a permanent magnet?

Answer 14

A magnet that produces its own magnetic field all the time (e.g. bar magnet)

Question 15

What’s an induced/temporary magnet?

Answer 15

A magnet that only produces a magnetic field when it’s in another magnetic field

Question 16

What happens when magnetically ‘soft’ materials are induced? (once they leave the magnetic field)

Answer 16

e.g. pure iron and nickel-iron alloys lost their magnetism very quickly

Question 17

What happens when magnetically ‘hard’ materials are induced? (once they leave the magnetic field)

Answer 17

e.g. steel loses its magnetism more slowly

Question 18

What are permanent magnets made of?

Answer 18

Magnetically hard materials

Question 19

What are 5 uses for magnetic materials?

Answer 19

fridge doors - strip to keep it closed (permanent)
cranes - attract and move magnetic materials (induced)
Doorbells - electromagnets
MRI machines - magnetic fields

Question 20

What happens when a current flows through a wire?

Answer 20

A magnetic field is created around it

Question 21

What’s the motor effect?

Answer 21

A current in a magnetic field experiences a force

Question 22

What happens when a wire is put between magnetic poles?

Answer 22

The two magnetic fields interact - the result is a force on the wire

Question 23

How does the wire have to be to experience a full force in a magnetic field?

Answer 23

At 90 degrees to the magnetic field (if it runs along the magnetic field, it won’t experience any force at all)

Question 24

What does the magnetic flux density show?

Answer 24

The strength of the magnetic field

Question 25

How does a motor work?

Answer 25

• Forces act on two side arms of a coil of wire that carries a current
• The forces are unusual and act in opposite directions on each side, so the coil rotates
• Split ring commutator swaps the contacts every half turn so that the motor rotates in the same direction

Question 26

How can the direction of a motor be reversed?

Answer 26

By swapping the poles (reversing the field)
or
By swapping the polarity of the d.c. supply (reversing the current)

Question 27

What’s a solenoid?

Answer 27

A long coil of wire

Question 28

How is the magnetic fields within/outside a solenoid?

Answer 28

Within - strong, uniform
Outside - Weak (apart from the ends) because overlapping field lines cancel each other out

Question 29

What’s an electromagnet?

Answer 29

A magnet with a magnetic field that can be turned on and off using an electric current

Question 30

What are 4 ways to increase the strength of an electromagnet/solenoid?

Answer 30

Increase number of turns in coil (keeping length the same)
Add an iron core - becomes an induced magnet
Increase the current
Decrease the length

Question 31

What’s electromagnetic induction?

Answer 31

The induction of a potential difference in a wire which is experiencing a change in magnetic field

Question 32

In what situations do you get electromagnetic induction?

Answer 32

If an electrical conductor and a magnetic field move relative to each other:
- Do this by rotating/moving either a magnet or coil of wire or conductor in a magnetic field
- When you do this, the current will be reversed bv
When the magnetic field through an electrical conductor changes (happens in transformers)

Question 33

What do step up/step down transformers do?

Answer 33

Step up - increase potential difference (more turns on the secondary coil than primary)
Step down - decrease potential difference (more turns on primary coil than secondary)

Question 34

What do transformers do?

Answer 34

Transformers use induction to change the size of the potential difference of an alternating current

Question 35

How does a transformer work?

Answer 35

• 2 coils of wire, primary and secondary, joined with an iron core
• Alternating potential difference is applied across the primary coil which produces an alternating magnetic field
• Core is producing an alternating magnetic field, therefore magnetisation of the core also alternates
• This changing magnetic field induces a potential difference in the secondary coil

Question 36

What do dynamos generate?

Answer 36

Direct current
Split ring commutator swaps connection every half turn to keep current flowing in same direction (SIMILAR TO MOTORS)

Question 37

What do alternators generate?

Answer 37

Alternating current
Slip rings

Question 38

How do microphones work?

Answer 38

• Sound waves hit a flexible diaphragm that’s attached to a coil of wire, coil surrounds one pole of a permanent magnet and is surrounded by another pole
• Diaphragm and so the coil moves and a current is generated in the coil
• The movement of the coil depends on the properties of the sound wave (louder sounds make diaphragm move further)
• This is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit

Question 39

How do loudspeakers work?

Answer 39

• Coil is wrapped around one pole of a permanent magnet, so the a.c. signal causes a force on the coil which moves the cone.
• When current is reversed, the force acts in the opposite direction
• These movements make the cone vibrate which makes the air around the cone vibrate and creates the variations in pressure that causes a sound wave

Question 40

How does a power station work?

Answer 40

1) Burning fuel is used to head water and convert it to steam which turns a turbine
2) Turbine is connected to a powerful magnet (usually an electromagnet) inside a generator
3) As turbine spins, magnet spins creating a large p.d and alternating current in the coils
4) Coils are join together in parallel to produce a single output from the generator