MAGNETISM AND ELECTROMAGNETISM Flashcards
What is a magnetic field?
A region where other magnets or magnetic materials experience a force
What direction do magnetic field lines go in?
They go from north to south (come out of north pole and into south pole)
What does it mean if magnetic field lines are closer together?
The force is stronger
What happens when two poles of a magnet are put near each other?
They will exert a force on each other either attractive or repulsive
What happens when a magnet and magnetic material are put near each other?
There is an attractive force between the two
Why does a compass point in the direction of the magnetic field it is in?
Because the north pole of the magnet is attracted to the south pole of any magnet it is in
Why does a compass always point north if it isn’t near a magnet?
Because Earth generates its own magnetic field which shows the inside core of the Earth is magnetic
What are permanent magnets?
They produce their own magnetic field
What are induced magnets?
Magnetic materials that turn into a magnet when they’re put into a magnetic field
What is the force between permanent and induced magnets always going to be?
Attractive
What happens when you take away the magnetic field from an induced magnet?
It quickly loses its magnetism and stops producing a magnetic field
What happens when a current flows through a wire?
A magnetic field is created around the wire
What rule shows you the direction the magnetic field is going around a wire (which has a current)?
The right hand thumb rule
What creates a stronger magnetic field around a wire (with a current)?
Larger current
Closer to the wire you are
What is a solenoid?
A coil
How can you increase the strength of a magnetic field that a wire produces?
By wrapping the wire into a coil called a solenoid
Why does a solenoid produce a stronger magnetic field?
Because the field lines around each loop of wire line up with each other
Therefore lots of field lines point in the same direction lose to each other
What is the magnetic field like inside a solenoid?
Strong and uniform (same strength and direction at every point)
What is the magnetic field like outside a solenoid?
Same as one round a bar magnet
How can you increase the field strength of solenoid even more (not adding more coils)?
A put a block of iron in the centre of the coil
What happens to the iron core in the middle of a solenoid?
It becomes an induced magnet
What happens when you stop the current going through a solenoid?
The magnetic field disappears
What is an electromagnet?
A solenoid with an iron core
The magnetic field can be turned on and off with an electric current
What can electromagnets be used for?
Cranes to attract and pick up things
Within circuits to act as switches
What is the motor effect?
When a conductor (wire) is put between magnetic poles
The magnetic field around the wire interacts with the magnetic field it has been placed in
The magnet and wire exert a force on each other
The wire moves
When will a wire experience the full force of a magnetic field?
When it is it at 90 degrees to the magnetic field (not parallel)
What happens to the magnitude of a force as the magnetic field strengthens?
The magnitude of the force increases
What increases the current passing through a conductor?
The force increasing
What does the force acting on a conductor in a magnetic field depend on?
Magnetic flux density (how many field lines there are in a region)
Size of current
Length of conductor
What is fleming’s left hand rule?
Motion (thumb)
Field
Current
How does a basic dc motor work?
Forces act on two side arms of a coil of wire thats carrying a current
Coil is on a spindle so the forces act one up and on down so it rotates
What is a split ring commutator?
Swaps the contacts every half term in a basic dc motor to keep the motor rotating in the same direction
Which direction does current always flow?
From positive to negative
Which direction does a magnetic field always flow?
From north to south
How do loudspeakers work?
AC current sent through coil of wire attached to base of paper cone
Coil surrounds one pole (S) of permanent magnet and is surrounded by the other pole (N)
Current causes force to act on coil which causes cone to move
When current reverses cone moves in opposite direction
Variation in current causes cone to vibrate and creates variations in pressure which cause a sound wave
How can you alter the frequency of a sound wave?
By altering the frequency of the AC
What is the generator effect?
The induction of a potential difference in a wire which is moving relative to a magnetic field or experiencing change in a magnetic field
What does the generator effect do?
It creates a potential difference in a conductor
Also creates a current if he conductor is part of a complete circuit
How do you create a generator effect?
By moving a magnet in a coil of wire
OR
Moving a conductor in a magnetic field
What usually is a conductor?
Wire (made into a solenoid sometimes)
What can the generator effect also be known as?
An electromagnetic induction
What happens when you shift the magnet side to side (generator effect)?
Creates a little ‘blip’ of the current if the conductor is part of a complete circuit
What happens when you move the magnet in the opposite direction (generator effect)?
Potential difference/current will be reversed (same if polarity of magnet is reversed)
What happens if you keep the magnet moving backwards and forwards (generator effect)?
You produce a potential difference that keeps swapping direction (an alternating current)
How do generators produce an alternating current?
Turn magnet which changes the magnetic field through the coil
This induces a pd and a current flows through wire
Half turn means direction of magnetic field reverses and so does pd and current
Turning magnet in same direction will keep reversing pd and current each half turn
What happens when you change the magnetic field around a wire?
A current is induced
What happens when you change the magnetic field around a wire and a current is induced?
The magnetic field created by an induced current acts against the change that made it
How can you increase the size of the potential difference by changing the magnetic field (generator effect)?
Increase speed of movement of magnetic field
Increase the strength of the magnetic field
How do alternators work?
Rotate a coil in a magnetic field
Coil spins so current is induced which changes direction every half turn
Produce alternating potential difference
What do slip rings and brushes do in alternators?
Instead of slip ring commutator so the contacts don’t swap every half turn
What current do alternators generate?
Alternating current
What current do dynamos generate?
Direct current
How do dynamos work?
Same as alternators but have a split ring commutator which swaps the connection every half turn to keep the current flowing in the same direction
What do oscilloscopes show?
How the potential difference generate in the coil changes over time
What do oscilloscopes look like for alternating currents?
Goes up and down crossing the horizontal axis (positive and negative)
What do oscilloscopes look like for direct currents?
Goes up and down but stays above the horizontal axis (stays positive)
What are mircophones?
Loudspeakers but in reverse
How do microphones work?
Sound waves hit flexible diaphragm attached to coil of wire wrapped around magnet
Coil of wire moves in magnetic field which generates current
Convert pressure variations of a sound wave into a current in circuit
What does the movement of coil depend on in a microphone?
The properties of the sound wave
e.g. louder sound wave makes the diaphragm move further
What do transformers do?
They change the size of the potential difference of an alternating current
What do transformers consist of?
Two coils of wire (primary and secondary) joined with an iron core
How do transformers work?
AC applied across primary coil
Iron core magnetises and demagnetises quickly
Changing magnetic field induces alternating potential difference in secondary coil
Causes current to be induced in secondary coil (if circuit)
What is the ratio between the primary and secondary potential differences the same as?
The ratio between the number of turns on the primary and secondary coils
What do step up transformers do?
Increase the potential difference
They have more turns on the secondary coil than primary coil
What do step down transformers do?
They decrease the potential difference
They have more turns on their primary coil than secondary coil
