tpic 7- magnetism and electromagnetism

What is a magnet

Any material or object that produces a magnetic field

What is a magnetic field

A region where magnets, magnetic material and wire carrying currents experience a force acting on them

The more dense the field lines are…

…the stronger the magnetic field is in that area.

How do you find north and South Pole of a Magnet using a compass

Needle of a compass is a tiny bar magnet that is attracted to South Pole of any other magnet it is near, so compass points in direction of magnetic field it’s in. You can move a compass around a magnet and trace its position on some paper to build up a picture of what the magnetic field looks like

Permanent magnet…

…produce their own magnetic fields

Induced magnets…

…magnetic materials that turn into magnets when they’re in a magnetic field

What are the main magnetic materials

nickel, iron, cobalt, steel

What is electromagnetism

The phenomenon whereby electric currents produce their own magnetic fields

What happens when a current flows through a wire

A magnetic field is created around the wire

Describe the magnetic field of a current carrying wire

Field made up of concentric circles perpendicular to the wire, with the wire in the centre

Right hand rule

Using your right hand point your thumb in the direction of the current and curl your fingers. The direction of your fingers is the direction of the field

What causes the strength of a magnetic field produced to change in a current carrying wire

Current, distance from wire

What is a solenoid

A coil of wire

Describe the magnetic field of a solenoid

Magnetic field inside is strong and uniform, outside the coil the field is same as one round a bar magnet

How can you increase the strength of the magnetic field of a solenoid

Increase current, increase number of coils (keeping length the same), decrease length of coils (keeping number of coils the same), iron core inside of solenoid

Why does adding an iron core inside a solenoid increase the strength of a magnetic field

Iron core becomes an induced magnet when current is flowing. The magnetic field of the core and coil combine to make a stronger magnet overall

What happens when you stop the current of a solenoid

Magnetic field disappears

How can you reverse the direction of the magnetic field in solenoid

Reversing direction of current or voltage

What are uses of electromagnets

Cranes, relay switches

What is the motor effect

When a current carrying wire in a magnetic field experiences a force

What does the wire have to be to experience the full force

90 degrees to magnetic field

What happens if the wire was running in the same direction as the field

It would feel no force

Flemmings left hand rule

Using your left hand, point your first finger in direction of the field, the second finger in the direction of the current and stick the thumb out so it’s at 90 degrees to the other two fingers. The thumb shows the direction of the force

Equation for the size of the force

Force(N) = magnetic flux density(T) x current(A) x length(m)

How does an electric motor work

When a current flows through the loop, the two side arms, which are at 90° to the field, each experience a force due to the motor effect. They experience forces in opposite directions because the direction of the current in each arm is opposite. The loop will start to rotate around its axis because the forces act one up and one down. When the wire loop reaches a vertical position the forces will still be acting one up and one down so the loop gets stuck.

What do you use to keep the motor rotating in the same direction

Split commutator

What does a split commutator do

Swaps the positive and negative connections every half turn so force acting on coil will always be in same direction

How does a loudspeaker work

An alternating current is sent through a coil of wire attached to the base of a paper cone. The coil surrounds one pole of a permanent magnet and is surrounded by the other pole so the current causes a force on the coil causing it to move. When the current reverses the direction of the force reverses causing the cone to move in the opposite direction. The variations in the current makes the cone vibrate which makes the air around the cone vibrate and creates variations in air pressure that form a sound wave.

How do you increase the speed of an electric motor

Increase current, add more turns to coil, increase magnetic flux density with stronger magnets

What is the generator effect

The induction of a potential difference (and current of circuit is complete) across a wire experiencing a change in an external magnetic field

How to increase the size of induced potential difference

Increasing strength of magnetic field, move wire or magnets more quickly, add more turns to wire/coil

Magnetic field created by induced current always…

…acts against the change that made it

What type of current does an alternator generate

Alternating current

How does an alternator use the generator effect to generate an alternating current

As the coil spins a current is induced in the coil. Every half turn this current changes direction. Alternators use slip rings and brushes so that the contacts don’t swap every half turn. This maintains the alternating potential difference generating an alternating current

What type of current do dynamos generate

Direct current

How does a dynamo generate a direct current

Similar to alternators but they use a split ring commutator as their electrical contact. This swaps the connection every half turn to keep the current flowing in the same direction and generate direct current

What happens when you increase the frequency of rotations in the generator

Overall potential difference increases

How does a microphone work

Sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet. This causes the coil of wire to move back and forth in the magnetic field which generates a current. The movement of the coil depends on the properties of the sound wave

What are transformers

Devices that can change the potential difference of an electrical supply using the generator effect (only works on a.c)

What is the structure of a transformer

Two coils, primary and secondary, wrapped around an iron core

Why is iron core used in a transformer

Can magnetise and demagnetise quickly

How does a transformer work

1) An alternating potential difference is applied across the primary coil which causes a current to flow and generates a magnetic field around the coil

2) The alternating magnetic field in primary coil induces an alternating magnetic field in the iron core

3) The iron cores magnetic field induces a potential difference across the secondary coil and as it’s part of a complete circuit causes a current to flow around the secondary coil

What do step up and step down transformers do to an electricity supply

Changes it so that the size of the output potential difference is not the same as the input potential difference

Describe the structure of a step up transformer

The number of turns on the secondary coil and the size of the pd across it are greater than across the primary coil

Describe the structure of a step down transformer

The number of turns on the secondary coil and the size of the pd across it are smaller than across the primary coil

Transformer equation

Potential difference across primary coil/ potential difference across secondary coil = number of turns on primary coil/ number of turns on secondary coil

Transformer equation efficiency

Vs x Is = Vp x Ip