Electromagnetic Effects Flashcards
Electromagnetic induction
EMF will be induced in a conductor if there is movement between the conductor and the magnetic field
It will also be induced if the conductor is stationary in a changing magnetic field, for example moving a magnet in and out solenoid
When the conductor is moved through the magnetic field it cuts through the lines of the magnetic field, this forces the charges to move inside creating voltage - electricity (inducing EMF)
The amount of voltage depend on how you move the wire, if you move the wire up and down which is perpendicular to he magnetic field you will create maximum voltage, if you move the wire but if you move it left and down which is inline with the magnetic field you will create zero voltage
Lenz law
If a magnet is pushed north end first into a coil of wire then the end of the coil closest to the magnet will become a North Pole
If a magnet is now pulled away from the coil of wire then the end of the coil closest to the magnet will become a South Pole
This is because The induced potential difference always opposes the change that produces it
This means that any magnetic field created by the EMF will act so that it tries to stop the wire or magnet from moving
Lenz law - direction of the induced EM/current opposes the change causing it
Right hand dynamo rule
When moving a wire through a magnetic field, the direction of the induced EMF can be worked out by using the Right-Hand Dynamo rule
Do a gun position with your hands and then point your middle finger to the left so it perpendicular to the pointing finger
Thumb = thrust (motion)
First finger (pointing one) = field
second finger (middle finger = current
Factors affecting EM induction
The magnitude (size) of the induced EMF is determined by:
- The speed at which the wire, coil or magnet is moved
Increasing the speed will increase the rate at which the magnetic field lines are cut
This will increase the induced potential difference
- The number of turns on the coils of wire
- The size of the coils
Increasing the area and number of turns in the wire will increase the potential difference because there will be more wire to cut through - The strength of the magnetic field
Increasing the strength of the magnetic field will increase the potential difference induced
Magnetic fields around wires
When a current flows through a wire a magnetic field is produced around the wire
The magnetic field is made up of concentric circles (two or more rings that have the same centre point)
As the distance from the wire increases the circles get further apart
Increasing the amount of current flowing through the wire will increase the strength of the magnetic field
(Field lines come closer together)
The right-hand thumb rule can be used to work out the direction of the magnetic field
Put a thumbs up sign and the thumb should be pointing along the direction of the current and the other fingers is the direction of the field
A circle with a dot in the centre shows the current is flowing out of the plant
A circle with a cross in the centres shows that current is flowing into the plane (like a dart)
Magnetic field around solenoids
Solenoid - a wire thats been coiled
The magnetic field around the solenoid is similar to that of a bar magnet (strong and uniform)
If the current is travelling around in a clockwise direction then it is the south pole
If the current is travelling around in an anticlockwise direction then it is the North Pole
If there is no current flowing through the wire then there will be no magnetic field
Magnetic effects of changing current
A solenoid can be used as an electromagnet by adding a soft iron core
The iron core will become an induced magnet when current is flowing through the coils
The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall
It can be switched on and off
Changing the direction of the current also changes the direction of the magnetic field produced by the iron core
Factor affecting magnetic field strength and solenoids
The strength of the magnetic field produced around a solenoid can be increased by:
- Increasing the size of the current which is flowing through the wire
- Increasing the number of coils
- Adding an iron core through the centre of the coils
The strength of an electromagnet can be changed by:
Increasing the current will increase the magnetic field produced around the electromagnet
Application of the magnetic effect
Relays are switches that open and close via the action of an electromagnet this is when you use a low voltage to turn on a high voltage circuit
When the coil is induced it attracts the switch closer causing it to close
Electric bells also utilise relay circuits to function
A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet
An alternating current passes through the coil of the loudspeaker
This creates a changing magnetic field around the coil
the force exerted on the cone will constantly change direction (vibrate)
This makes the cone oscillate
This makes the air oscillate, creating sound waves
Alternating vs direct current
In a direct current the current only flows in one direction whereas in the current continuously changes directon
AC generator
An AC generator consists of a coil of wire between two permanent magnets this produces a force onto the wire making it go up/down
They generate AC current because a slip ring commentator is used to reverse the current every half cycle so it keeps rotating in the same direction (turning effect will be the same direction)
They also use carbon brushes to transmit the current from the battery to the coils without breaking the wires
EMF can be increased by:
Increasing the number turns on the coil
Increasing the area of coil using a stronger magnet
Increasing the speed of rotation
DC motors are almost identical however in DC motors we provide the electricity for the wire to move up and down to create motion
AC generator we provide motion to create electricity
Flemings left hand rule, force on a current carrying conductor
If you pass a current through a wire and its between a magnetic field the wire will move
This is because both of the magnetic fields from the wire and the magnets interact with each other pushing each other out
A force acts on a current carrying conduction in a magnetic field Flemings hand rule shows the relative direction of the force, field and current
Force - thumb
Mag field - first finger (remember its from NORTH TO SOUTH)
Current - second finger
Alternating current (a.c)
The alternating current looks like a normal longitudinal wave
The alternating current split in half is the one with a dioxide which means that it is forward bias
If you move a wire through a magnetic field in a circle then it produces an AC
Transformer
Transformer is a device that increases or decreases the voltage of an ac supply without losing power
- AC voltage goes through the primary coil and it produces a magnetic field
- The iron core gets induced and it cuts through the secondary coil so electromagnetic induction occurs and AC comes out
If the primary coil has more coils than the secondary coil the voltage will go down (step down transformer)
If the primary coil has less coils than the secondary coil the voltage will go up (step up transformer)
