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What is a magnetic field
A region where magnets, magnetic material and also wires carrying currents experience a force acting on them.
What two things does the strength of a magnetic field around a wire depend on?
The distance from the wire and the current thru the wire
Why is the magnetic field of a solenoid stronger than a normal wire?
The magnetic field lines are much closer as a result of the coiled shape, and the closer the field lines, the stronger the magnetic field is.
How do you increase the strength of a wire? (3)
Shaping it in a solenoid: Magnetic fields are closer
Adding an iron core: increases the strength and becomes magnetised
Adding more turns in the solenoid coil: increases the density of magnetic fields, strengthening the magnetic field of the magnet overall.
4 types of magnetic materials
Iron, steel, cobalt, nickel
What is the motor effect?
A current carrying wire in a mag field experiencing a force.
Describe how a simple DC electric motor works.
A loop of wire that’s free to rotate about an axis is places in a mag field
When DC flows thru loop, the two side arms of the wire experience a force due to the motor effect as they are 90 degrees to the magnetic field of the magnets.
The force causes moments that act in the same direction, causing the coil to rotate
The loop will rotate as the current in each arm is opposite
The loop rotates as one force pushes on arm up and the other down
Until the loop gets stuck in a vertical position, where the forces are up and down
The direction of current must be reversed to allow the direction of the force to reverse to let the wire finish a turn.
A split ring commutator swaps the contacts of the loop every half a turn, reversing the current
Each half revolution, the 2 halves of the rotating commutator swap from one carbon brush to the other
Each half of the commutator is linked to the contacts of the loop.
The commutator reverses the current in the coil, keeping the forces in the same direction.
Give three ways to increase the speed of the simple electric motor
Increasing current
Increasing strength of mag field
Using a coil of wire instead of a single loop
Explain how loudspeakers work.
An AC current, which is varying, is sent through a coil of wire attached to the base of a paper cone.
The coil surrounds one pole of a permanent magnet and is surrounded by the other
The AC current creates a magnetic field around the coil
Which interacts with the permanent magnet field
Producing a resultant force due to the motor effect
Causing the coil to move
The varying current causes the coil to vibrate
Causing the cone to vibrate
When the direction of the current reverses, the direction of the resultant force reverses
The vibrating cone produces pressure variations in the air needed to form a sound wave, forming rarefactions and compressions.
What is the generator effect?
The induction of a pd (and current if complete circuit) across a conductor which is experiencing a change in an external magnetic field.
How does the generator effect happen?
Moving the electrical conductor in a magnetic field
Moving or changing a magnetic field to the electrical conductor
How do we get an alternating current in a coil from a magnet? (2)
Moving the magnet in and out of the coil
Switching the polarity of the magnet
How do you change the size of the induced PD?
Increasing the speed of the movement - cutting more magnetic field lines in a given time
Increasing the strength of the magnetic field - so that more field lines can be cut
Shaping the wire into a coil - more magnetic field lines can be cut
What are the 3 ways to increase the size of PD in an alternating current?
Increase strength of mag field
Increase number of coils
Increase area of the coil
How does a microphone work?
Sound waves hit a flexible diaphragm that is attached to a solenoid. This diaphragm vibrates.
This causes the coil of wire around the magnet to vibrate in the magnetic field, generating a current
The coil cuts through mag field lines of the magnet.
A potential difference is induced across the ends of the coil
The movement of the coil depends on the properties of the sound wave
The microphones convert the pressure variations of a sound wave into variations in current in a circuit
How does a transformer work?
An alternating current is passed through the primary coil producing a magnetic field around it.
When the alternating current flows through the primary coil in a transformer it produces a magnetic field, magnetising the iron core.
the ac current in the primary coil means the mag field in the iron core is alternating too
This constantly changing mag field cuts in through the secondary coil.
The changing field creates a changing alternating PD across the ends of the secondary coil as a result of the generator effect
The circuit connected secondary coil translates the current inputted in the primary coil exactly out, meaning both coils have the same frequency of ac current flowing in it.
How are is the PD changed on the second coil?
The size of the PD in the primary coil
The number of turns on each coil
Why would the transformer not work with dc current?
Mag field wouldn’t be constantly changing
So no induction in the secondary coil as a changing field is required to induce a pd
How is an ac current produced by an alternator?
The coil cuts through mag field lines
A pd is induced across the coil
there is a complete circuit so a current is induced in the coil
Every half turn, the current changes direction as the mag field changes direction due to flemings right hand rule
