Flashcards
Physics topic 7
Cards in this deck (31)
- 1Magnetic fieldA region where magnets or magnetic materials experience a force
- 2Magnetic field linesThe closer together the lines of the stronger the magnetic field. The further away from magnet you get the weaker the field is
The magnetic field is strongest at the poles of the magnet- also meaning the magnetic forces also strongest at the poles - 3Question TextAnswer Text
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- 14Motor effect forceMagnitude of the force increases with the strength of the magnetic field
The force also increases with the amount of current passing through the conductor - 15The force acting on the conductor in a magnetic field depends on three things:1) The magnetic flux density- how many field lines there are in the region- showing the strength of the magnetic field
2) The size of the current through the conductor
3) The length of the conductor that is in the magnetic field - 16When current is 90° to the magnetic field it is in the forces acting on it can be found using:F= BI1
Force= magnetic flux density✖️ current✖️length
Force - (N)
Magnetic flux density-(T,Tesla)
Current- (A)
Length-(m) - 17Fleming’s left hand ruleFirst finger= Field
SeCond finger= Current
ThuMb= Motion - 18The generator effect =The induction of a potential difference in a wire which is experiencing a change in magnetic field
- 19You can induce a potential difference in a conductor by:1) moving a magnet in a coil of wire
2) moving conductor in a magnetic field (“cutting” magnetic field lines) - 20How rotation can cause the generator effect1) i’ll see you two in the magnet the magnetic field through the coil changes. This change induces a potential difference which can make current flow in the wire.
2) every time the magnet moves through half a turn, the direction of the magnetic field through the coil reverses. When this happens the potential difference reversed so the current flows in the opposite direction around the coil of wire
3) if the magnet keeps turning in the same direction then the potential difference will keep reversing every half turn and you get an alternating current - 21Induced currents always opposed the change that made itThe magnetic field created by an induced current always acts against the change that made it.
- 22How do you change the size of the induced potential differenceIncreased by:
1) increasing the speed of the movement - cutting more magnetic field lines in a given time
2) increasing the strength of the magnetic field ( more field lines can be cut) - 23Alternators generate alternating currents by:1) rotate the coil in a magnetic field
2) construction is like a motor
3) As the coil spins , a current is induced in the coil changing the direction every half turn
4) have slip rings and brushes instead of split ring communicators so the contacts don’t swap every half turn
5) produce alternating pd - 24How dynamos generate direct currentDynamos work in the same way as alternators except they have a split ring commutator instead of slip rings
Swapping the connection every half turn to keep the current flowing in the same direction - 25Why you can use oscilloscope to see the generated pd1) show how pd generated in coil changes over time
2) for ac - line goes up and down crossing horizontal axis
3) for dc- line isn’t straight but stays above axis - pd always positive
4) height of line at given point is the generated pd at that time
5) increasing the frequency of revolutions increases the overall of but also creates more peaks too - 26How loudspeaker and headphones use electromagnets and work because of the motor effect1) alternating current is sent through coil of wire attached to base of a paper cone
2) The coils surrounds one pile of a permanent magnet and is surrounded by the other pole so the current causes a force on the coil (causing cone to move)
3) when the current reverses the force acts in the opposite direction which causes the cone to move in the opposite direction
4) variations in the current make the cone vibrate which makes air around the cone vibrate and creates variations in pressure that cause a sound wave
5) the frequency of the sound wave is the same as the frequency of the ac so by controlling the frequency of the ac you can alter the sound wave produced - 27How microphones generate current from sound waves1)Sound waves hit flexible diaphragm that is attatched to the coil of wire, wrapped around magnet
2) this causes the coil of wire to move in the magnetic field generating a current
3) the movement of coil depends on the properties of the sound wave
4) this is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit - 28How transformers can change the size of pd of an alternating current1) they all have two coils of wire ( primary and secondary) joined with an iron core
3) when alternating pd is applied across the primary coil the iron core magnetises and demagnetises quickly. This change in magnetic field induces an alternating pd in the secondary coil
4) if second coil is part of a complete circuit this causes current to be induced
5) the ratio between primary and secondary pd is the same as the ratio between the number of turns on primary and secondary coils - 29Step up transformers:Step the potential difference up. Have more turns on secondary coil than primary coil
- 30Step down transformers:Step the potential difference down. Have more turns in primary coil than secondary
- 31Transformer equation:Vp/Vs= Np/ Ns
Vp- input pd
Vs- output pd
Np- number of turns on primary coil
Ns - number of turns of secondary coil
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- Flashcards on Physics topic 7 , created by phoebe Garr on 08/06/2018.
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- phoebe Garr
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