Electromagnetism Flashcards
Domain
Area inside a magnetic metal that can be thought of as a mini magnet within the structure
Flux density
Strength of magnetic field in a volume of space
Induced magnetism
Magnetism produced in a magnetic material by a permanent magnet or electromagnet
Induction
A potential difference produced when the flux passing through a conductor changes
Magnetic field
Area around a magnet where it will produce a force on another magnet or magnetic material. Points from north to south
Motor effect
Force produced on a conductor carrying a current placed at right angles to a magnetic field
Solenoid
A coil of wire carrying a current. Produces a magnetic field with the same shape as a bar magnet
Tesla
The unit used to measure flux density
Magnetic fields
Lines point from north to south around a bar magnet, strongest near the poles
Attract where the lines join up, repel when they don’t
Field around the wire
Has a circular magnetic field around it
Use right hand grip rule where thumb points in direction of current
Magnetic metals
Iron
Cobalt
Nickel
Force on a conductor
F=BIL F= force in Newtons B= flux density in Tesla I= current in Amperes L= length in metres
Motor effect
Two permanent magnets with a coiled wire between them carrying a current makes the wire spin
Fleming’s left hand rule
Thumb= F
Finger 1= B
Finger 2= I
All at right angles
Factors which increase induction
Increase flux density
More coils
Closer to the wire
Increased current
Speakers
Pass an alternating current through a coil in a permanent magnetic field. As current changes direction, coil attracts/repels
Microphones
Sound waves make diaphragm and coil vibrate backwards and forwards in a magnetic field. Induces a potential difference causing an alternating current
DC Motors
Current flows through coil
Motor effect produced a force
Motor has a split ring commutator which means current flows the same way relative to magnetic field
DC Dynamo
Coil is spun in the magnetic field
Induces potential difference
Coil has a split connection so the current will be a varying direct current
Maximum every 108 degrees
AC Alternator
Coil is spun in the magnetic field
Induces potential difference
As the connections to either side of the coil are permanent the current changes direction with every 180 degree turn
What do transformers work on?
Alternating current
Transformers
Laminated core made from iron, 2 coils on opposite sides
Step-up have more turns on secondary than primary, step-down the opposite
Changes size of alternating voltage
Step up turns power stations to grid, step down grid to home (230V)
Summary
Faster movement increases induced p.d.
Increasing length of wire increases induced p.d.
Stronger field increases induced p.d.
Changing direction of field reverses induced p.d. direction
Generator Effect
When moving magnetic field lines pass through a conductor (or a moving conductor passes through a magnetic field) a p.d. is induced in the conductor
2x speed=
2x p.d.
What happens as you go up the electromagnetic spectrum?
Increased frequency and wavelength
Decreased energy
How to answer motor effect questions
- Current flows around the circuit
- Creates a temporary magnetic field
- Interacts with the permanent magnetic field of the magnet
- Creates a force which turns the coil
- In the direction shown by fleming’s left hand rule
Brushes
Conduct current between stationary wires and rotating wire of motor
Commutator
Reverses the current every half turn to ensure motor turns continuously
Relay switch
Press the switch causing a current to flow, turn on very large currents safely
Circuit breaker
When the current is too high, electromagnet becomes magnetised and stops the flow to a section of the circuit
Transformers exam questions
- Either steps-up or steps-down the voltage
- Induces a magnetic field
- Current is alternating, causing changing magnetic field in the iron core
- Core strengthens the magnetic field
- Field lines interact with secondary coil, inducing a voltage or secondary cell
Magnetically soft
The magnet can be switched on and off
Magnetically hard
A permanent magnet
Plotting compass bar
Place the compass at one pole and draw where it points continuing until you reach the other pole, repeat going the other direction (top/bottom)
Plotting compass solenoid
Place the compass at one point on the wire
Turn the power supply on and off
Move it further along and turn on and off again
Move further away to see the weakening
