P7 Electromagnetism Flashcards
What is a magnetic field
The region around a magnet where a force acts on another magnet or magnetic material.
What is an induced magnet?
A material that becomes magnetic when it is placed in a magnetic field. When removed it is no longer magnetic
What are permanent magnets?
Made from nickel, cobalt, some steels
The earths magnetic field
The geographical North Pole is actually the magnetic South Pole
The magnetic effect of a current
When a current flows through a wire it creates a magnetic field around the wire.
Flemings right hand grip rule
The thumb is the current direction
The fingers show the field direction
Magnetic field in a loop of wire
The magnetic field is strongest in the middle
Magnetic field in a solenoid
Similar to a bar magnet.
Strongest in the middle and at poles than than the outside.
The motor effect
When a wire with a current passing through it is placed between magnets the resultant force is stronger in one area and weaker in another. A force is produced on the wire causing it to move
Flemings left hand rule
thuMb- Motion
First finger- Field
seCond finger- Current
How can u increase the force on the wire
Increase:
Size of the magnetic field
Current passing through the wire
The length of the wire that is is the magnetic field
What us an electromagnet?
A copper solenoid around an iron core. This magnetises the core creating an electromagnet which is 1000x stronger than the coil by its self. It can be switched on and off as it is a temporary magnet
The electric bell
It’s circuit contains an electromagnet which attracts the hammer and iron contact this actions makes the bell ring but also breaks the circuit. The magnet is switched off as the circuit is broken. Once the hammer reaches the original position the circuit works again and the cycle continues.
How do relays work?
A relay is a magnetic switch. It uses a small current to switch on a much larger current.
Magnetic flux density (B)
The strength of a magnetic field measured in tesla (T)
How do D.C. motors work?
When a current passes through a coil placed between the poles of a magnet there is a force acting on each side.left hand rule shows that each side of the coil will move.one side up other side down. It has a split ring commutator which allows the motor to spin.
When the coil is vertical the circuit is broken but the coil has gained enough momentum move to other side. The commutator is now in connection with the circuit and the current can flow.
How do A.C. Motors work?
They work similar to a D.C. Motor accept they have slip rings. As the coil rotates, the direction of the current passing through the coil is reverses direction. When the coil has gone through half a turn, the direction of the current has reversed.
Producing sounds
A series of oscillations or vibrations that travel in the same direction as the direction of the oscillations (longitudinal waves)
Vibrations travel through the air as pressure waves and transfers energy
A moving coil loudspeaker
Has a permanent magnet and a coil of wire. The magnet is fixed but the coil can move forwards and backwards. Coil is attached to the cone. Outer edge of cone doesn’t move(fixed)
How does a moving coil loudspeaker work?
Cylindrical magnet produces a strong radial magnetic field at normal to the coil. A signal (current) varies flow through coil. A change in current changes magnetic force on coil. Changing force makes coil move in and out. As current varies, coil moves. Coil attached to cone makes cone move setting up vibrations in the air.
How do earphones work?
They work the same as loudspeakers except they are smaller and sound doesn’t have to travel as far
What is the generator effect
When a wire is stationary between a magnet there is no current in the wire. If the wire moves it cuts the magnetic field lines. A potential difference is induced across the end of the wire and if the wire is part of a complete circuit an induced current passes along the wire.
Inducing a potential difference
When a magnetic field changes near a conductor, even if the conductor does not move. Moving near a conductor changes the magnetic field around the conductor.
The induced pd can be increased by:
Moving the wire or magnet faster
Using a stronger magnet
Replacing the wire with a coil
What is the direction of the induced pd?
It is always such to oppose the motion or change producing it.
Energy is transferred from the movement of the magnet into the coil to a flow of current.
Moving coil microphone
Act like a loudspeaker in reverse. Energy is transferred from the moving air into the coil, causing a current to flow.
Coil is connected to a diaphragm which is fixed. The coil surrounds a permanent magnet. Sound waves make to diaphragm vibrate causing the coil to move forward and backwards in the gap, inducing a pd and causing a varying current to flow
Dynamo
Generates a direct current by moving the coil in a magnetic field. When it turns it cut the magnetic field lines and induces a potential difference between the end of the coil. Causing an induced current to pass through the coil and through the circuit the coil is part of.
An induced current can be increased by:
Increasing number of turns on the coil
Using more powerful magnets
Turning the coil faster
What does a transformer do?
Changes the size of an alternating pd.
What is a transformer?
2 coils of wire wound separately on an iron core. An alternating pd is applied to primary coil producing an alternating current in the primary coil. Producing alternating magnetic field in the iron core. The strength of magnetic field increases as iron is easily magnetised. Alternating magnetic field in iron core induces pd in the secondary coil. This induces an alternating current in the circuit connected to the secondary coil.
Step up transformer
Increases pd
More turns on secondary coil than on primary coil
Step down transformers
Decreases pd
More turns on primary coil than on secondary coil
