GCSE Physics P4- Magnetism OCR Gateway

Magnetic material

iron, nickel, cobalt

What direction do magnetic field lines point?

NORTH TO SOUTH

Magnetic flux density

strength of magnetic field, measured in teslas (T)

The closer the field lines are...

..., the stronger the magnetic field at that point

What happens when you place the north and south of two permanent bar magnets near each other?

A uniform field forms between the poles

Describe how you could use iron fillings to see magnetic fields?

put a magnet under a piece of paper and scatter some iron fillings on top of the paper

Describe how you could use a compass to plot a magnetic field?

- Put a magnet on a piece of paper and place the compass on the paper next to magnet
-Compass will point in direction of field line
-Mark direction of field line in pencil on paper
-Move compass into different positions and mark their direction each time to get different field lines

What happens when a magnet is brought near a magnetic material?

The material has been induced and therefore acts as a magnet

Whats the different between permanent and induced magnetism?

Induced magnets usually lose their magnetism once the magnet has been moved away

How does current affect the magnetic flux density?

The larger the electric current, the stronger the magnetic field

Describe the magnetic field around a current- carrying wire

Concentric circles with the wire at the centre

How can you find the direction of the field around a current carrying wire?

- Right Hand Rule
- Using compasses

Describe the graph of magnetic flux density against distance from wire

-Non-linear
-At first as distance increases, strength quickly decreases however this slows down with the increasing distance

Describe the magnetic field around a flat circular coil

Concentric ellipses (stretched circles) of magnetic field lines around the coil

Describe the magnetic field around a solenoid?

Inside the solenoid it is strong and uniform however outside it looks like a bar magnet as it has a north and south pole

What type of magnet is a solenoid?

electromagnet

State why the magnetic effect at the ends of solenoid would increase?

If current is increased
The number of coils of wire is increased (length stays the same)
Cross-sectional area of solenoid is decreased
Length of solenoid is decreased
Iron core is added inside

What happens when a current carrying conductor is put between magnetic poles?

the two magnetic fields affect each other causing a force on the wire

What does a dot represent?

A wire carrying current out of the page

What does a cross represent?

Current going into the page

What happens if either the current or magnetic field are made stronger?

Resulting force increases

Describe Fleming's left hand rule

thumb = force
first finger = magnetic field
second finger = current

Why does the coil move in a simple d.c motor?

Current flows in different directions each side of coil, and each side of the coil is perpendicular to the magnetic field, each side experiences forces in opposite directions. As its on a spindle, it rotates.

What is the job of the split-ring commutator in the d.c motor?

It swaps the contacts every half turn to keep the motor rotating in the same direction

How can the direction of the d.c motor be reversed?

Swapping the magnetic poles over or reversing the current

How can you speed up the d.c motor?

Increase the current
Add more turns to the coil
Increase magnetic flux density

Describe how a loudspeaker works?

ac electrical signals fed to wire coil wrapped around cone base
The interaction between the current and the magnetic field forces the coil to move in one direction
As its a.c the current changes direction, forcing the coil back in the other direction.
As current alternates, coil moves back and fourth.
Movements make the coil vibrate which creates pressure variations in the air (sound)

Describe the inside of a loudspeaker?

It contains a coil of wire at the base of the speaker which is surrounded by 1 magnet

Electromagnetic induction

When a potential different is induced across a conductor which is experiencing a change in its external magnetic field. The change happens when it passes through magnetic field lines

When do you get electromagnetic induction?

1) When a electrical conductor and a magnetic field move relative to each other
2) The magnetic field through an electrical conductor changes

What does a generator do?

uses the relative motion of a conductor and magnetic field to generate (induce) a voltage and a current

If direction of rotation is reversed, what happens to the direction of the induced current/voltage?

It also reverses

How can you increase the current induced from a generator?

More turns in the coil
Increase magnetic flux density
Increase speed of rotation

When rotating a magnet in an alternator, how is alternating current produced?

Magnet spins, alternating voltage is generated across the coil ends.
Voltage changes direction every half turn because direction of the field changes as magnet rotates, this produces a.c

How can a.c be produced when rotating a coil in a magnetic field of an alternator?

Slip rings at the coil ends remain in contact with the brushes that connect to rest of circuit, therefore the contacts don't swap every half turn so a.c is produced.

How dynamos generate direct current?

Rotate a coil in a magnetic field; the output voltage and current changes direction every half turn producing a.c.
A split ring commutator is used to swap the connection every half turn to keep the flowing in the same direction, therefore it changes from a.c to d.c

How does a microphone work?

Sound waves cause the diaphragm to move back and fourth when hit by them.
As diaphragm moves, the coil of wire moves as well, generating a voltage at the ends of the coil of wire.
As the coil is a part of the circuit, the induced voltage means variations in current in the circuit

What do transformers do?

Change the size of the voltage of an alternating current.

Describe the parts of a transformer

They have 2 coils; primary and secondary, joined with an iron core

How do transformers work?

Alternating pd supplied across primary
iron core magnetises and demagnetises quickly
This changing magnetic field induces an alternating pd in secondary coil
If it is in a complete circuit, current is also induced

What do step down transformers do?

Decrease voltage
They have more turns on primary coil than secondary

What do step up transformers do?

Increase voltage
They have more turns on the secondary coil than primary