Physics IGCSE Magnets (edexcel claude)

Permanent magnet

A material that produces its own magnetic field all the time, without needing a current

Induced magnet

A material that only becomes magnetic when placed in a magnetic field, and loses most or all of its magnetism when removed from that field

Example of induced magnetism

A paperclip touching a magnet becomes magnetic itself and can pick up other paperclips, but drops them once the magnet is taken away

Magnetically hard material

difficult to magnetise but once magnetised, it retains its magnetism well, used to make permanent magnets

Magnetically soft material

A material that is easy to magnetise but loses its magnetism quickly once the field is removed, used to make electromagnets

Magnetic field line

A line showing the direction a north pole would move if placed at that point in the field, the arrows always point from north to south

Rules for drawing field lines

Lines never cross, they go from N to S outside the magnet, closer lines mean a stronger field

Uniform magnetic field

A field with the same strength and direction at every point, shown by evenly spaced, parallel field lines, produced between two opposite poles facing each other

How to produce a uniform field

Place a north pole and a south pole facing each other, close together, the field between them is uniform

Like poles

Repel each other

Unlike poles

Attract each other

How to test if something is a magnet

Bring it near a known magnet, if it can be repelled as well as attracted it is a magnet, if it is only ever attracted it is just a magnetic material

Electromagnet

A magnet created using an electric current, usually a solenoid wound around a soft iron core, which can be switched on and off

Why does a current carrying wire produce a magnetic field

Moving charge always generates a magnetic field around it

Magnetic field pattern of a straight wire

Concentric circles around the wire, direction found using the right hand grip rule

Magnetic field pattern of a flat circular coil

Circular field lines around each part of the wire, combines to form one larger loop shaped field through the centre

Solenoid

A coil of wire wound into a tube shape

Magnetic field pattern of a solenoid

Similar to a bar magnet

How to increase the strength of an electromagnet

Increase the current, increase the number of turns of wire, add an iron core inside the coil, wind the coils closer together

Why use a soft iron core in an electromagnet

magnetises easily when current flows, demagnetises when current stops, can be turned on and off

Motor effect

The effect where a current carrying wire placed in a magnetic field experiences a force, causing motion

Why is there a force on a current carrying wire in a magnetic field

wire’s magnetic field interacts with the external magnetic field, creating a force that push or pulls

Left hand rule

A way to predict the direction of the force on a current carrying wire in a magnetic field, thumb is motion or force, first finger is field, second finger is current

How to increase the force on a current carrying wire

Increase the current

How does force direction change with current or field direction

Reversing either the current direction or the magnetic field direction reverses the direction of the force, reversing both leaves the force direction unchanged

Force on a moving charged particle in a magnetic field

A charged particle moving through a magnetic field experiences a force, unless it is moving parallel to the field lines, in which case there is no force

D.C. electric motor

A device using the motor effect, a current carrying coil in a magnetic field experiences forces that make it rotate, converting electrical energy into kinetic energy

How to increase the speed or turning effect of a d.c. motor

Increase the current, increase the number of turns on the coil, use a stronger magnetic field

Loudspeaker

Uses a coil carrying a varying current in a magnetic field, the varying force this produces makes the coil vibrate, creating sound waves

Electromagnetic induction

The process of generating a voltage in a conductor by changing the magnetic field around it

How to induce a potential difference

Move a conductor or coil through a magnetic field, or change the magnetic field passing through a stationary conductor or coil

Factors that increase the size of an induced voltage

Moving the magnet or coil faster, using a stronger magnet, increasing the number of turns on the coil

Generator

A device that generates electricity by rotating a magnet inside a coil of wire, or rotating a coil of wire inside a magnetic field, inducing an alternating voltage

Factors affecting the size of voltage generated by a generator

Speed of rotation, strength of the magnetic field, number of turns on the coil

Transformer

A device consisting of a primary coil and a secondary coil wound around a shared iron core, used to change the size of an alternating voltage

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Step up transformer

Has more turns on the secondary coil than the primary, increases the voltage

Step down transformer

Has more turns on the primary coil than the secondary, decreases the voltage

Why are step up and step down transformers used in the National Grid

Step up transformers raise voltage and lower current for efficient long distance transmission, reducing energy loss as heat, step down transformers then reduce the voltage to safe levels for homes and businesses

Transformer equation

Primary voltage divided by secondary voltage equals primary turns divided by secondary turns

Transformer power equation

Input power equals output power, primary voltage times primary current equals secondary voltage times secondary current

Why is a 100 percent efficient transformer only theoretical

Real transformers lose some energy as heat due to resistance in the coils and magnetisation of the core, so output power is always slightly less than input power