AQA P3 - Magnetism and Stars

The second half of P3, Magnetism and Stars, in a powerpoint. 

Based on the CGP Physics revision guide, but more condensed.

Think this covers everything you'll need to know, check out the powerpoint I made of the first half of P3 if you haven't already! :) Let me know if this helps!

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· Magnetic field = region where magnetic materials and wires carrying
currents experience a force acting on them.
· Field lines go from north pole to south pole.
· Current carrying wire = magnetic field made up of concentric circles
with wire in centre.
· Magnetic field = strong, uniform.
· Outside the coil = like a bar magnet (ends of solenoid act like north and
south pole.
· Direction of current reversed = N and S poles reversed.
· Increase the strength of magnetic field by adding `soft' iron core ­
makes an electromagnet.
· Magnetically soft = magnetises and demagnetises easily/ doesn't stay
· Iron, steel and nickel = magnetic.…read more

Slide 3

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· A current in a magnetic field experiences a force.
· Force = increased if current or magnetic field increased.
· Force on wire = 90° to wire and magnetic field.
· Wire must be at 90° to magnetic field for full force.
· If wire runs along magnetic field, no force will be experienced.
· At angles between = some force.
· DC supply used.
· Factors which speed up =
1. More current.
2. More turns on the coil.
3. Stronger magnetic field.
4. Soft iron core.
· Forces on spindle = one up, one down.
· Split ring commutator = swaps contacts every half turn, keep motor
rotating in same direction.
· Direction of motor reversed = swap polarity of DC supply, swap
magnetic poles over.…read more

Slide 4

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· Electromagnetic induction = creation of voltage (maybe current) in a
wire experiencing a change in magnetic field.
· Done by moving a magnet in a coil of wire ­ cutting magnetic field
lines. Moving magnet from side to side = small current created.
· Moving magnet in opposite direction = voltage/current reversed.
· Polarity of magnet reversed = voltage/current reversed.
· Move magnet or coil backwards and forwards = alternating (AC)
· Turning magnet end to end in coil = current. How generators work.
· Turn magnet = magnetic field through coil changes, induces a voltage,
makes current flow in wire.
· After half a turn, direction of magnetic field reverses = voltage and
current reversed.
· Keep turning magnet in same direction = AC current.…read more

Slide 5

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· Generators rotate a coil in a magnetic field.
· As coil spins, current is induced, and changes direction every half turn.
· Slip rings and brushes used so contacts don't swap every half turn.
· Produce AC voltage.
· Bigger voltage =
1. Stronger magnet.
2. Increase turns on coil.
3. Increase area of coil.
4. Increase speed of movement.
· Rotate magnet, not coil.
· Field through coil swaps every half turn = output the same as generator.
· Used to power bike lights.…read more

Slide 6

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· Step voltage up.
· More turns on secondary coil than primary coil.
· Step voltage down.
· More turns on primary coil than secondary coil.
1. Primary coil produces magnetic field within iron core ­ nearly all passes
through secondary coil.
2. As alternating current in primary coil, field in iron core is also
3. Secondary coil cuts field lines in the core.
4. Field in core induces alternating voltage in secondary coil with same
frequency as alternating current in primary coil = electromagnetic
5. Number of turns determines whether voltage induced in secondary =
greater or less than in primary.
· Only works with AC. A DC supply would still cause a magnetic field in the
core, but it wouldn't be alternating so no induction in secondary coil.
· Iron core carries magnetic field, not current.…read more

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