P7-electromagnetism Flashcards
What is produced around a wire when current flows through a conducting wire?
a magnetic field
what two things affect the strength of a magnetic field?
the current through the wire and the distance from the wire
what is an electromagnet?
a magnet which can be switched on and off
Give an example of an electromagnet:
a solenoid with an iron core
What is a solenoid and why are they used?
it is a wire wrapped into a coil. it increases the strength of the magnetic field
what is the magnetic field inside a solenoid like?
it is strong and uniform, has same strength and direction at every point.
what is the magnetic field outside a solenoid like?
like the one around a bar magnet
how can you increase the strength of a solenoid even more?
by adding a block of iron in the centre (iron becomes an induced magnet when current is flowing)
state two uses of electromagnets:
1-for cranes to pick up and drop materials
2-act as switches in circuits
Name the effect: When a potential difference is induced in a wire experiencing a change in magnetic field
the generator effect
what is induced when a coil is moved through a magnetic field?
a potential difference
what happens when you stop moving the wire between the two magnets?
the potential difference disappears. this is because the pd is created by the change in magnetic field
what happens to the PD each time the direction of the wire changes?
the direction of the PD changes
what changes when a complete circuit is used instead of a wire?
both potential difference AND current are produced
what happens if you move the bar magnet alongside the magnetic field lines?
no potential difference is created.
name three things which can increase the size of the PD:
1-increase magnet strength
2-move wire faster
3-shape wire into coil, and the more turns in the coil the higher the PD
name two ways of decreasing the PD:
1-decrease magnet strength
2-move wire slower
what is another way of inducing potential difference?
moving a magnet in and out of a coil
what happens to the current when the direction of movement of the magnet changes?
current direction changes
what does the induced current create?
its own magnetic field
the magnetic field _______ the movement of the magnet.
meaning we are doing ____ to move the magnet.
opposes, work. when N pole in coil, that end of coil becomes N so repels magnet. When N pole pulled out, that end becomes S pole, attracting magnet.
explain how alternators work:
generators rotate coil in magnetic field, coil spins and current is induced. every half turn, current swaps direction. Slip rings and brushes used so contacts don’t swap every half turn. AC comes out here
In an alternator, when is generated PD highest?
when the coil is horizontal
what happens to the PD when the coil is vertical?
the PD goes to zero
why does the PD reverse direction?
because the two sides of the coil are moving in a different direction to before
what’s different about dynamos compared to alternators?
dynamos have split ring commutators. this swaps the connection every turn to keep current flowing in same direction.
describe 4 ways to increase induced current:
1-faster rotation
2-stronger magnetic field
3-more turns on coil
4-coil has a larger area
how do microphones work?
sound wave hits diaphragm, coil of wire moves in a magnetic field (generating current) . Movement of coil depends on sound wave’s properties. Microphones convert pressure variations of sound wave into variations in current in an electrical current
what does a basic transformer consist of?
a primary coil and a secondary coil wound around an iron core
why is iron used for transformers?
it is easily magnetised
why are transformers used?
to increase or decrease the size of alternating PD
what is the difference between a step-up transformer and a step-down transformer?
step up- increases PD
step down- decreases PD
why do transformers only work with a.c?
because a changing magnetic field is required to induce a.c in the secondary coil.
why is a low current used in the national grid?
so less energy is wasted heating wires and national grid is more efficient.
