Edexcel IGCSE Physics : 6 Magnetism and Electromagnetism

Describe the behaviour of magnets

magnets are objects capable of exerting a magnetic force which repel and attract other magnets/magnetic substances

What are the properties of magnetically hard materials?

materials which when magnetised permanently keeps its magnetism

e.g. steel

What are the properties of magnetically soft materials?

magnetically soft materials are materials which lose their magnetism easily and are temporary magnets

e.g. iron

Define the term magnetic field line

magnetic field lines are lines which represent the magnetic force on a north pole at a given point, they show the direction and magnitude of the source

field lines go from north to south

How can magnetism be induced in a material ?

when an object made from a magnetic material is placed inside a magnetic field it induces a magnetism in the material making it a magnet

practical : investigate the magnetic field pattern for a permanent bar magnet and between two bar magnets

How do you produce a uniform magnetic field pattern using two permanent magnets ?

there is a uniform magnetic field when opposite poles of two magnets are placed close together

as the field lines are parallel and evenly spaced

(you can also use a u shaped magnet)

What is the effect of electric current in a conductor?

when an electric current passes through a conductor a magnetic field is produced around it

How are electromagnets constructed ?

electromagnets are made by wrapping a coil of wire around a magnetically soft core

Therefore the magnet can be turned on and off

Draw a magnetic field pattern for a straight wire

Draw a magnetic field pattern for a flat circular coil

Draw a magnetic field pattern for a solenoid

What happens to a charged particles as it goes through a magnetic field?

when a charged particle moves through a magnetic field it experiences a force

as long as the motion is not parallel to the field

Why is a force exerted on a current carrying wire in a magnetic field?

1. the magnetic field of the magnet and the magnetic field of the wire overlap
2. in parts of the wire both magnetic field are in the same direction so there is a strong magnetic field
3. in other parts of the wire both magnetic fields are in different directions causing a weak magnetic field
4. due to the difference in strength of magnetic fields a force is felt by the wire pushing it from the stronger part of the wire to the weaker part

How does a simple d.c. electric motor work?

1. When there is a current in the wire one of the sides will experience a force pushing it upwards , the other side will experience a force pushing it downwards and the loop will begin to rotate
2. When the loop reaches a vertical position momentum pushes it past the vertical and so to keep it rotating the forces on the wires must reverse so the wire at the top is pushed down and the wire at the bottom is pushed up
3. This is done by using a split ring commutator to connect the loop of wire to the electrical supply , so that each time the loop of wire passes a vertical position the connections change and the direction of current changes
Now the loop will rotate continuously

How does a loudspeaker work ?

ac flows through the coil

magnetic field lines overlap , causing a force to act on the coil pushing it forwards

the ac current changes direction causing the field to change direction and the force to act in the opposite direction

this happens very quickly causing it to vibrate

the coil is connected to a paper cone which vibrates causing vibrations in the air particles producing sound

What is the left hand rule ?

How does the force on a current carrying conductor in a magnetic field change with magnitude and direction of the field and current ?

higher the magnitude/strength of the magnetic field the stronger the force

if the magnetic field changes direction the force acting on the conductor changes direction

if the current in the conductor increases the force applied increases

if the current in the conductor changes direction the force applied on conductor changes direction

How can a voltage be induced in a conductor or a coil ?

when there is movement between a wire and magnetic field

(the wire cuts the magnetic field lines)
or
when a magnetic field changes through a coil

(the wire cuts the magnetic field lines)

a voltage is induced in the wire or coil (which drives a current)

Describe the generation of electricity?

when a magnet rotates within a coil
or
when a coil rotates in a magnet

the magnetic field through the coil changes

which induces a voltage in the coil

which drives a current in the coil

Which factors affect the size of an induced voltage ?

1. if the wire is moved more quickly, a larger voltage is induced

2. if there is a stronger magnetic field , a larger voltage is induced as more field lines are cut

3. wrapping the wire into a coil/adding a coil , increases the size of the voltage induced as more pieces of wire move through the magnetic field and more field lines are cut

4. if the magnet is moved more quickly a larger voltage is induced

5. if the magnetic field changes more quickly, a larger voltage is induced

6. (for generation) increasing the length of wire inside the magnetic field

Describe the structure of a transformer?

a transformer consists of two coils wrapped around a soft iron core

a transformer is used to change the size of a voltage

How does a transformer work ?

an alternating voltage produces an alternating current in the primary coil

this creates a changing magnetic field which induces an alternating voltage in the secondary coil

the size of voltage induced is based on the number of turns on each coil

pturns>sturns

secondary voltage is smaller

sturns>pturns

secondary voltage is larger

What is the use of step-up and step-down transformers ?

electricity is generated and then goes into a step up transformer

this steps up the voltage , this reduces the current and therefore reduces power loss in the transmission cables

the transmission cables then go into a step down transformer so it can be used in homes

it steps down the voltage , increasing the current so it can be used in devices

State the relationship between : input voltage, output voltage, primary turns, secondar turns

Input Voltage/output voltage = num turns on primary coil / num turns on secondary coil

Vp/Vs = np/ns

State the relationship between input power and output power for 100% efficiency

Input power = output power

primary coil power = secondary coil power

VpIp = VsIs

for 100% efficiency