Electromagnetism Flashcards
Soft magnetic materials
Easy to magnetise, easily lose their magnetism
e.g. iron
electromagnets are made out of magnetically soft materials
Hard magnetic materials
Difficult to magnetise, do not easily lose their magnetism (permanently magnetised)
e.g. steel
Field line properties
North to South
Closer field lines = stronger magnetic field and vice versa
Two magnetic field lines must never touch or cross other field lines
Uniform magnetic field
All field lines are parallel
All field lines are equal distances apart (field strength is the same)
Induced magnets
When a magnetic material is placed in a magnetic field, it can temporarily turn into a magnet
One end becomes a north pole, the other a south pole
Magnetic materials will always be attracted to a permanent magnet
Investigating magnetic field pattern method
Place magnet on top of a piece of paper
Scatter iron filings on the paper and shake to form the shape of the field lines
Use a compass from the north end to find the direction of the field lines
Electromagnetism of a wire
When a current flows through a wire a magnetic field is produced
The magnetic field is made up of concentric circles (no poles)
Right hand thumb rule shows direction of magnetic field
Increasing field strength of an solenoid
Increase current
Add more turns to the coil
Wrap solenoid around a soft iron core (easy to magnetise)
Fleming’s left-hand rule
F(Force)
B(Magnetic Field Line)
I(Current)
The Motor Effect
There is a CURRENT (in the wire)
The coil has a MAGNETIC FIELD, which INTERACTS with the field of the permanent magnet
This induces a FORCE on the wire
Increasing magnetic force
Increase current
Use stronger magnets
Place wire perpendicular to the direction of field lines between the poles of the magnet (maximum interaction between two magnetic fields)
!If the two magnetic fields are parallel, there is no interaction between them and so no force is produced
D.C. motor
The CURRENT in the coil produces a MAGNETIC FIELD
It INTERACTS with the uniform field and a FORCE is exerted on the wire
Forces act in opposite directions on each side of the coil, causing it to rotate
Once the coil has rotated 180°, the split ring commutator allows the direction of current to stay the same as it brushes against the circuit
The force/turning effect remains in the same direction
Loudspeakers
Consists of a coil of wire wrapped around one pole of a permanent magnet
An alternating current passes through the coil creating a changing magnetic field
Motor effect ………. The force constantly changes direction, oscillating
This causes the speaker cone and the air to oscillate, creating sound waves
The generator effect
A voltage is induced in a conductor or coil when it moves through a magnetic field or when a magnetic field changes through it, as it cuts the field lines
Faster relative speed of magnet/coil, stronger magnet, more turns = greater voltage
A.C. Transformer
Made up of turns of primary coil and secondary coil wrapped around iron core
An alternating current is supplied to the primary coil
The current is constantly changing direction, producing a changing magnetic field around the primary coil
The iron core is easily magnetised, passing the changing magnetic field through it to the secondary coil
The changing field cuts through the secondary coil and and induces a potential difference
Step-up transformer
Increases potential difference of a power source
More turns on secondary coil than primary coil
Used to reduce energy loss from high current in wires (V = IR)
Step-down transformer
Decreases potential difference of a power source
Fewer turns on the secondary coil than primary coil
Used for domestic purposes of electricity, needing lower voltage
Sequence of electricity transmission
Power station (25kV), STEP-UP -> Power lines (132kV), STEP-DOWN -> Buildings (230V)
Transformer Equation
primary potential diff/secondary potential diff = primary turns/secondary turns
Vp/Vs = Np/Ns
Equation can be flipped upside down as well
Ideal transformer equation
If a transformer is 100% efficient: Input power = Output power (P = IV)
Vp x Ip = Vs x Is
