magnetism and electromagnetism Flashcards
magnetic materials
Nickel
Iron
Cobalt
Steel
can permanent magnets attract AND repel magnetic materials?
no, permanent magnets can only ATTRACT magnetic materials
can magnets attract and repel other magnets?
yes, magnets repel and attract other magnets
the strongest part of a magnet
its poles, the north and south pole
what is a magnetically hard material
a magnetically hard material keeps its magnetism once it has been magnetised
what is a magnetically soft material
a magnetically soft material loses its magnetism easily and are therefore useful as temporary magnets
e.g iron is a magnetically soft material and is therefore not suitable for a permanent magnet
magnetic field line
magnetic field lines are used to represent the strength, shape and direction of a magnetic field
magnetic field
around every magnet there is a volume of space where we can detect magnetism, this is known as a magnetic field
what happens when you place a magnetic material in a magnetic field?
when you place a magnetic material in a magnetic field, magnetism is induced and it becomes a temporary magnets. this is known as induced magnetism and they are called induced magnets
practical: investigate the magnetic field pattern for a permanent bar magnet and between two bar magnets
- place a permanent bar magnet between two books and place a sheet of paper on top
- sprinkle some iron filings onto the paper above the magnet
- tap the paper gently and the iron filings will move to reveal the magnetic field lines
or
- place a bar magnet on a piece of paper
- place a large number of small compasses on the paper near the magnet
- look carefully at the pattern shown by the needles of the compasses
repeat the experiment using two bar magnets, placing them 5cm apart
main features of magnetic field lines
magnetic field lines show:
- the shape of the magnetic field
- the direction of the magnetic force, the field lines ‘travel’ from north to south
- the strength of the magnetic field, the closer the field lines, the stronger the magnetic field
uniform magnetic field
the magnetic field created between the north pole of one magnet and the south pole of another magnet is shown as a series of evenly spaced lines. this is known as a uniform magnetic field, the strength and direction is the same everywhere
electromagnetism
when there is an electric current in a conductor a magnetic field is created around it, this is called electromagnetism.
the construction of an electromagnet
A soft iron core wrapped in wire. When current flows through the coil of wire it becomes magnetic.
draw magnetic field patterns for:
- a straight wire
- a flat circular coil
- and a solenoid when each is carrying a current
increasing the strength of the magnetic field in a current-carrying wire
- increase the current in the wire
- wrapping the wire into a coil/solenoid
increasing the strength of the field around a solenoid
- increase the current
- increase the number of turns in the coil
- wrapping the solenoid around a magnetically soft core e.g iron (electromagnet)
charged particles in magnetic fields
when a charged particle moves through a magnetic field it experiences a force, as long as its motion is not parallel to the field
why a force is exerted on a current-carrying wire in a magnetic field
when a current-carrying wire is put between magnetic poles or when a charged particle moves in a magnetic field, its magnetic field interacts with the magnetic field of the permanent magnets. the result is a force exerted on the wire. this can cause the wire to move
electric motors
- Current flows in the coil. This creates a magnetic field around the coil.
- This magnetic field interacts with the field from the permanent magnet.
- This exerts a force on the coil which causes it to move
- The split-ring commutator changes the direction of the current every 180 degrees. This reverses the direction of the forces, allowing the coil to continue rotating in the same direction
how to reverse the direction of the motor
- reverse the polarity of the d.c supply
- reverse the poles of the magnet
how to increase the speed of the motor
- add more turns to the coil
- increase the current
- increase the strength of the magnetic field
- add a soft iron core
loudspeakers
- An alternating current from an amplifier passes through the coils wrapped around the base of the cone
- this creates a magnetic field around the coil
- this field interacts with the magnetic field from the permanent magnets
- The current is constantly changing direction and magnitude
- this creates a constantly changing force on the coil which causes it to vibrate in and out, moving the cone
- The cone vibrates which we hear as sound waves.
electromagnetic induction
if we move a wire across a magnetic field at right angles, a voltage is induced in the wire. if the wire is part of a complete circuit then a current is produced
factors that affect the size of the induced voltage
- how fast you move the wire
- how strong the magnet is (stronger the magnet, more magnetic field lines are ‘cut’)
- wrapping the wire into a coil so that more pieces of wire ‘cut’ the magnetic field lines
generators
generators rotate a coil in a magnetic field or a magnet in a coil
- as the coil spins it ‘cuts’ the magnetic field lines which induces a voltage and if the circuit is complete, a current
- in a generator the kinetic energy is being converted into electrical energy
transformers
a transformer has two coils, the primary coil and the secondary coil joined with an iron core
- when an alternating voltage is applied across the primary coil, this causes an alternating current to flow so the magnetically soft iron core magnetises and de-magnetises quickly
- this induces an alternating voltage in the secondary coil because the wires cut the alternating magnetic field lines
- this causes an alternating current to flow in the secondary coil
step-up and step-down transformers
step-up transformers have more turns in the secondary coil than in the primary coil. this means that the voltage is being increased
step-down transformers have more turns in the primary coil than in the secondary coil. the voltage is being decreased
transformer voltage and turns formula
Vp/Vs=np/ns
input power=output power formula
Vp x Ip= Vs x Is
transformers and national grids
step-up and step-down transformers are used when transmitting electricity across the country:
from the power station to the national grid a step-up transformer is used to increase the voltage, therefore decreasing the current. this reduces energy loss in the wires since a higher current causes the wires to heat up
from the national grid to homes a step-down transformer is used to decrease the voltage and increase the current supplied to homes so it is more useful and safer to use
