physics - magnetism & the motor effect (12.1 - 12.14)

12.1 what do unlike magnetic poles do?

attract

12.1 what do like magnetic poles do?

repel

12.2 examples of permanent & temporary magnetic materials (not specified)

cobalt

steel

nickel

iron

12.2 uses of permanent & temporary magnetic materials

electric motors

generators

loudspeakers

other electrical devices

door latches

knife holders

12.3 permanent magnets

always magnetic

attracts magnetic materials

magnetic field definition

space around magnetic where it attracts magnetic materials

12.3 induced magnets

piece of magnetic material in magnetic field - becomes magnet (induced magnet)

taken out of field - stops being magnetic

12.4 shape & direction of magnetic field - bar magnets

magnetic field all around bar magnet

field lines go from north → south

field lines closer together = magnetic field stronger

12.4 shape & direction of magnetic field - uniform field

2 flat magnets produce uniform magnetic field between them

field lines go from north → south

field lines closer together = magnetic field stronger

12.5 what does plotting compass show?

shape & direction of field of magnet/Earth’s magnetic field

12.5 how to use plotting compasses?

1. place magnet on piece of plain paper

2. place compass near one of magnet’s poles

3. draw small dot where needle of compass is pointing

4. move compass so tail of needle is aligned with dot

5. draw another small dot where needle is pointing

6. repeat steps 3-5 until reach other pole of magnet/ edge of paper

7. draw lines connecting dots

8. add arrows to lines from north → south to show direction of magnetic field lines

9. repeat steps 1-7 starting at diff. points on magnet

12.6 how is behaviour of magnetic compass related to evidence that earth’s core must be magnetic?

needle of compass always points to position near earth’s north pole

shows earth generates own magnetic field - core must be magnetic

12.7 how to show current can create magnetic effect around long straight conductor?

wire passing through piece of card

current flows through wire - iron filings on card make circular patterns

iron filings line up with direction of magnetic field - shows current causes magnetic field

direction of magnetic field depends on direction of current

12.8 what does strength of field depend on?

size of current

distance from long straight conductor

12.8 how does current affect strength of field?

higher current = stronger field

12.8 how does distance from long straight conductor affect strength of field?

closer to long straight conductor = stronger field

12.9 what is a solenoid an example of?

an electromagnet

12.9 what do fields from individual coils inside solenoid do?

add together to form very strong almost uniform field along centre of solenoid

cancel out to give weaker field outside solenoid

how to make magnetic field of electromagnet stronger?

put piece of iron (iron core) inside coil

12.10 what happens to current carrying conductor when it is placed near magnet?

it experiences a force

12.10 what happens to magnet when current carrying conductor placed near it?

equal & opposite force acts on magnet

12.11 what are magnetic forces due to?

interactions between magnetic fields

12.11 how are magnetic forces due to interactions between magnetic fields?

wire carrying current placed between 2 magnets - experiences force

current in wire creates magnetic field around wire

magnetic field around wire interacts with magnetic field between magnets

when is force greatest? (wire carrying current between 2 magnets)

when wire at right angles to magnetic field produced by magnets

when is force 0? (wire carrying current between 2 magnets)

when wire in same direction as magnetic field produced by magnets

12.12 what does Fleming’s left-hand rule show?

relative directions of:

• force

• current

• magnetic field

12.12 Fleming’s left-hand rule

12.13 force on conductor at right angles to magnetic field carrying current equation

force on conductor at right angles to magnetic field carrying current (N) = magnetic flux density (T) x current (A) x length (m)

F = BIl (BIL)

12.14 how is force on conductor in magnetic field used to cause rotation in electric motors?

force on each part of wire carrying current in magnetic field

split-ring commutator: current changes direction every half turn - ensures force on coil always turns it same direction

carbon brushes: make electrical contacts between circuit & motor

what does using coil with many turns of wire do? (in electric motor)

increases total force turning coil