Magnetism and electromagnetism Flashcards
Which way do magnetic field lines always go?
from the north to south poles
What will two of the same poles of a magnet do?
repel eachother
What will two different poles of a magnet do?
attract eachother
What do compasses show?
the direction of magnetic field lines
because compasses contain tiny magnets that always point north
What are permenant magnets?
magnets that produce their own magnetic field
What are induced magnets?
magnetic materials that turn into a magnet when they are put into a magnetic field (near a magnet)
What happens when a magnetic material is put near a permanent magnet?
it attracts to it
quickly gains its magnetism
What does the right hand thumb rule show?
thumb - current
fingers - direction of magnetic field lines
When a current flows through a wire, what happens to the wire?
a magnetic field is created around the wire
this field is made up of concentric circles perpendicular to the wire (with the wire in the centre)
What is a solenoid?
a coil of wire
that has a magnetic field
What are the magnetic field lines inside and outside a solenoid like?
inside - strong and uniform (same strength and direction)
outside - just like a bar magnet
What is an electromagnet?
a solenoid with an iron core
a magnet which has a magnetic field that can be turned on or off
What can electromagnets be used for?
in cranes in scrap yards (to pick up scrap metal)
as switches within other circuits
What is magnetic flux density?
how many field lines there are in a region
these lines represent how strong the magnet is
What is the motor effect?
When a current carrying wire is put between two magnetic poles, the magnetic field around the wire reacts with the magnetic field between the poles, which causes the wire and magnet to exert a force on each other. This can cause the wire to move.
What does Fleming’s left hand rule show?
the motor effect
thumb - movement of wire (the direction of the force)
first finger - direction of magnetic field
middle finger - current (wire)
What is the generator effect?
The induction of potential difference (and current if there is a complete circuit) in a wire which is experiencing a change in magnetic field.
How is the generator effect caused?
By changing the magnetic field lines in a wire.
For example by moving a magnet inside a coil of wire or by moving a conductor (wire) in a magnetic field.
(cutting field lines)
How can the size of the induced potential difference be changed in a generator?
- increasing the speed of movement (cutting more magnetic field lines in a given time)
- increasing the strength of a magnetic field (so that there are more field lines that can be cut)
What is an alternator?
a generator that generates an alternating current
What is a dynamo?
a generator that generates a direct current
What are the two types of generators and what makes them different?
alternators generate alternating current
dynamos generate direct current
alternators use slip rings and brushes so that the contacts don’t swap every half turn, meaning that every half turn the magnetic field reverses which means the current flows a different way each half turn (so is alternating)
dynamos use a split-ring commutator so that the connections are swapped each half turn to keep the current flowing in the same direction
How do loud speakers (and headphones) work?
using the motor effect
An alternating current goes through a coil of wire, which surrounds one pole of a permanent magnet and is surrounded by the other pole. This current causes a force (and therefore movement) on the coil, so that when the current reverses, the force acts in the opposite direction. The variations in the current make the cone vibrate so that the surrounding air vibrates, creating sound waves.
What are transformers?
devices that use the generator effect to change the potential difference for an alternating current
What are step up transformers?
transformers that increase the potential difference
have more turns on the secondary coil
What are step down transformers?
transformers that decrease the potential difference
have more turns on the primary coil
How do transformers work?
Transformers have two coils of wire (primary and secondary) joined by an iron core.
When an alternating current is applied to the primary coil, the iron core magnetises and demagnetises quickly so that there is a changing magnetic field, which induces an alternating potential difference in the secondary coil.
If the secondary coil is part of a complete circuit, a current will be induced too.
