Magnetism Flashcards
Where are magnetic forces strongest at?
The poles of the magnet
What happens when you bring two magnets close together?
They exert a force on each other
What force is exerted when you bring 2 like poles close?
Force of repulsion
What happens when you bring 2 different poles close?
Force of attraction
What type of force are attraction and repulsion?
Non contact
Because the magnets don’t touch to exert the force
Permanent magnet
Produces its own magnetic field
Examples of permanent magnets
Bar magnets
Induced magnets
An object that becomes a magnet only when placed in a magnetic field
Examples of induced magnetism
2 non magnetic objects placed near a permanent magnet
Thus causing and induced magnet
What do induced magnets cause?
A force or attraction between the induced magnet and permanent magnet
What happens if you take away the permanent magnet from an induced magnet?
Loses most/all of its magnetism quickly
4 types of magnetic material
Iron
Cobalt
Nickel
Steel (alloy of iron)
Can be made into a permanent or induced magnet
How to remember the 4 types of magnetic material?
COINS
CO balt
I ron
N ickel
S teel
Magnetic field
Region around a magnet where a force acts on another magnet or a magnetic material
What happens if you place a magnetic material in a magnetic field?
Experiences a force of attraction towards the permanent magnet
Thus becomes an induced magnet
What does the strength of a magnetic field depend on?
How far away the object is from the magnet
Further away = weaker field
Close = stronger field
Also if it’s at the poles
What can we used to find the direction of a magnetic field and why?
A compass
Because it contains a small permanent bar magnet
Method to find the magnetic field using a compass
Place the compass near the North Pole of the bar magnet
Draw a cross at the North Pole of the compass
Move it so the South Pole of compass is at the cross then another cross at the North Pole
Continue until complete magnetic field line that’s joined up
Show direction using arrow from north to south
How do we know where the magnetic field is strongest?
There are field-lines closer together
Which is at the poles
How do we know the earth has a magnetic field?
Because holding a compass it points in the north south direction as the compass contains a small bar magnet
What causes the earths magnetic field?
The core
What happens when a current flows through a conducting wire?
It produces a magnetic field around the wire
Like multiple circles around the wire
How do we prove a current causes a magnetic field?
Because if there’s no current the compass lines up with earths magnetic field
But turning it on the compass needle deflects
What determines the strength of the magnetic field around a wire?
The size of the current
Larger current = stronger field
How close the field is too the wire
Moving away from the wire = weaker field
What determines the direction of a magnetic field around a wire?
The direction the current flows through the wire
Changing direction of current reverses direction of magnetic field
How can we prove changing the current direction changes field direction?
Placing a compass near the field
The needle will deflect in the other direction than it did before
How do we find direction of the magnetic field produces by the wire?
Using the right hand grip rule:
Make a thumbs up using right hand
Make the thumb point in the direction of conventional current
The other fingers point in direction of magnetic field
How can we increase the strength of a magnetic field produced by a current?
Coil the wire (SOLENOID)
Increase current
How do solenoids work and make a stronger field?
The magnetic field produced when current goes through the solenoid is uniform and straight through the coils
This has a similar shape to a bar magnets field lines
How to find direction of the field produced by a solenoid?
Use the right hand grip rule again
Make a thumbs up with right hand
The other fingers (not thumb) points in direction of conventional current in the solenoid
And your thumb = North Pole
How to increase the strength of a magnetic field produced by a solenoid
Increase the size of the current
Increase number of turns in the coil
Place a piece of iron inside the solenoid (iron core)
What is a solenoid containing an iron core called?
Electromagnet
Why are electromagnets useful?
We can change its magnetic field strength by changing size of the current
We can turn it on and off
What appliances use electromagnets?
A relay
An electric doorbell
What are the purpose of relays?
To turn off high voltage circuits
Because if you try and turn off the switch yourself it could cause sparking or electrocution
How do relays work?
Another low voltage circuit is near a high voltage one
The low one has an electromagnet
The high one is separated into 2 by contacts that are controlled by a spring and iron block
So when field is turned on the iron is attracted that completes the circuit
Relays when they are turned on
A current flows through low voltage circuit
Thus a magnetic field is produced through the electromagnet (straight lines)
The iron block in high voltage circuit is attracted due to the field
So it moves down and closes the contacts so they touch thus the high voltage circuit is complete =turned on
Relays when they are turned off
No current flows through low voltage circuit nor electromagnet
Thus no field is produced
The high voltage circuit remains off because no field = no iron attraction = contacts remain open
What do electromagnetic doorbell circuits contain?
An electromagnet
With an iron contact that is opposite to it
Which is attached to a ‘clapper’
That is next to a bell
How do electromagnetic bells work?
Press the doorbell = switch closes
So a current flows through the electromagnet and produces a field
The field attracts the iron contact which pulls towards it
This also pulls the clapper to hit the bell and make a noise
In doing so the iron contact breaks the circuit so there’s no magnetic field
So it springs back into position and the process repeats
The motor effect
Placing a wire carrying an electric current in a magnetic field
Will cause a force making the wire move in a certain direction
What causes the motor effect?
The magnetic field around the wire is caused by a current flowing through it
Placing it in a magnetic field causes both fields to interact
Thus the wire experiences a force and makes it move
How do we find the size of the force in the motor effect?
Force = magnetic flux density (T) x current (A) x length of wire (m)
What is the magnetic flux density?
The strength of the magnetic field
Measured in T, teslas
What does the equation to find the force strength in the motor effect apply to?
ONLY IF the wire is as right angles to the other magnetic field
What does the size of the force in the motor effect depend on?
The strength of the magnetic field (magnetic flux density)
Current
Length of the wire.
How to find direction of the force caused by the motor effect?
Use Flemings LEFT HAND rule:
Place thumb, index and middle finger so they are at right angles
Index finger points in direction of magnetic field (NORTH TO SOUTH)
Middle finger is direction of conventional current
Direction of thumb = motion and force
What will happen motor effect wise if the wire with the current is placed parallel to the magnetic field?
The wire will experience no force nor move
How to remember Flemings left hand rule?
F = thumb = force
B = index = magnetic field
I = middle = current
In an electric motor, what happens when it rotates to 90°?
It will stay in place because the forces will make the top wire act upwards and the bottom diwnwards
How do we make the electric motor keep on rotating?
Use split rung commutator to break the current momentarily then be reserved so according to Flemings left hand rule will continue to rotate
How does the motor effect work in loudspeakers/ headphones?
A cone that has a coil of wire on one end
Which is connected to AC supply
A permanent wire is inside the coil of wire
Thus a magnetic field is produced through the coils
That interact with the bar magnets magnetic field
A resultant force is generated and the cone moves
As the current alternates it moves in and out = production of sound waves
How can we produce a higher pitch sound in a loudspeaker?
Use a higher frequency on the AC supply
How can we produce a lower pitch frequency in a loudspeaker?
Use a lower frequency on the AC supply
How to increase the volume of the sound produced by the loudspeaker?
Increase size of the current increases amplitude of the waves
Thus louder
How to decrease the volume of the sound produced by loudspeakers?
Decrease current
Decreases amplitude of waves
Thus quieter
Induced potential
Moving a loop of wire perpendicular up through a magnetic field induces a potential difference at its ends
Moving it back down through reverses it’s potential difference
However when it stops moving it loses its V
The generator effect
If you induce the potential difference by the wire cutting the magnetic field lines
And it’s a complete circuit it will generate a current
The direction of current also switches as you move it up and down through the field
But when it stops = current stops
If you move the field through the wire and keep the wire still will it still induce a voltage and maybe a current?
Yes
As long as the wire is directly cutting the field lines and moving
How do we get a larger induced potential difference and current?
Use a stronger magnetic field
Move wire more rapidly
Coil the wire (more turns = higher induced V and A)
Can we induce a current or voltage if we move a bar magnet in and out of a solenoid?
Yes
It works just like a single wire cutting magnetic field lines
So the direction of the current changes as you change the movement of the bar magnet
How else can we change the direction of current in a solenoid undergoing the generator effect?
Change the poles around = changes direction of a current
As a bar magnet moves in an out of a solenoid what does it produce?
A magnetic field that goes straight through the coils
Which opposes the movement of a magnet moving in and out
Alternator
Produces an AC
Microphone
Same set up as loudspeaker
When sound waves hit diaphragm it vibrates
Causing coil of wire to move in and of magnetic field = voltage induced
Passed through amplifier then into moving coil loudspeaker to increase volume
Microphone voltage pattern
AC
As coil moves back and forwards through field
Frequency of changing voltage is same as frequency of sound waves
Transformers
Step up = increases voltage
Step down = decreases voltage
How do transformers work?
Current flows through primary coil
Generates changing magnetic field direction along the iron core through secondary coil
Induces voltage
Step up transformer
More turns in the secondary coil than primary one
As it increases the voltage and steps it up
Step down transformer
More turns in the primary coil than secondary
As it decreases the voltage and steps it down
Power in transformers
Power in primary coil = primary in secondary coil
Power, potential difference and current in transformers
V in primary x I in primary = V in secondary x I in secondary
How to transmit a large amount of power across the national grid?
Either use a large current or voltage
Using large current will cause a large amount of power to be lost
So use a large voltage
