p4 Flashcards
How does a split ring commutator work
It is a split metal ring
It is connected to conducting brushes
The brushes allow the electric current to pass onto the ring
So the current switches direction after the wire being pushed up at 90 degrees
The forces still act in the same direction, so it rotates in the same direction
Describe how a magnet and a current carrying conductor exert a force on one another
When you put 2 fields that are in the same direction they add up, but if they are in opposite directions they will cancel out
So an overall force is produced
To experience the full force, the wire has to be at 90 degrees to the mag field.
explain how the force exerted from a magnet and a current carrying conductor is used to cause rotation in electric motors
1) In a simple d.c. motor, a current-carrying coil sits between two opposite poles of a magnet.
2) Because the current is flowing in different directions on each side of the coil, and each side of the coil is
perpendicular to the magnetic field, each side will experience forces in opposite directions.
3) Because the coil is on a spindle, and the forces act in opposite directions on each side, it rotates.
4) The split-ring commutator is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction.
5) The direction of the motor can be reversed either by swapping the polarity of the d.c. supply (reversing the current) or swapping the magnetic poles over (reversing the field).
What can change in a magnetic field cause?
An induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change
How does an alternator work to generate a.c
Some alternators rotate a magnet in a coil of wire
As the magnet spins, an alternating p.d is induced across the ends of the coil. Thr p.d changes direction every half turn because of the direction of the field changes as the magnet rotates
This produces an a.c if the coil is part of a complete circuit
You can also generate a.c by rotating a coil in a mag field
Slip rings at the ends of the the coil remain in contact with brushes that are connected to the rest of the circuit. This means the contacts don’t swap every half turn, so they also produced an a.c
How does a dynamo work and generate d.c
Dynamos rotates a coil in a mag field
The output p.d and current change direction with every half rotation of the coil, producing a.c. The coil is part of a complete circuit
A split ring commutator swaps the connection every half turn to keep the current flowing in the same direction- so it changes from a.c to d.c
explain how the effect of an alternating current in one circuit, in inducing a current in another, is used in transformers
When an alternating p.d is applied across the primary coil, it produces an alternating magnetic field
As iron is a magnetic material the core also becomes magnetised. Because the coil is producing an alternating magnetic field, the magnetisation in the core also alternates
A changing magnetic field induces a p.d in the secondary coil
explain how the ratio of the potential differences across the two coils in a transformer depends on the ratio of the numbers of turns in each
The number of turns in a coil affects the size of the induced p.d across it. The ratio between the p.d across the primary and secondary coils is the same as the ratio between the number of turns in the primary and secondary coil
Transformers are nearly 100% efficient so power in primary coil= power in secondary coil
This means that if the secondary coil has more turns, the p.d Increases and the current decreases
How do dynamic microphones work using electromagnetic induction
GENERATOR
A dynamic microphone structure is like a loudspeaker , but the cone is replaced by a diaphragm
Sound waves cause the diaphragm to move back and forth when hit my them
As the diaphragm moves, the coil of wire moves, inducing a alternating p.d across the ends of the coil of wire
The coil is part of a circuit, so the induced p.d means variations in current in the electrical circuit
How do loudspeakers work
MOTOR
2) A loudspeaker contains a coil of wire which is surrounded by one magnet. Another magnet is inside the coil.
3) A.c. (alternating current) electrical signals are fed to the coil of wire, which is wrapped around the base of a cone.
4) The interaction between the magnetic field and the current in the coil forces the coil to move in one direction. As it’s an alternating current. the current changes direction, forcing the coil back in the other direction. As the current continues to alternate, the coil moves back and forth.
5) These movements make the cone vibrate. This creates pressure variations in the air, i.e. sound.
Difference between motors and generators
Generators use relative motion of a conductor and magnetic field to induce a p.d and current
Motors uses an induced p.d and current to generate motion
Uses of step down transformer
Power circuits for home
Uses of step up transformers
Power stations
Give the 3 magnetic materials
Iron, nickel, cobalt
What is magnetic flux density
The strength of the magnetic field, measured in Teslas
What are 4 ways u can increase the strength of a solenoid
Increase current in wire
Increase number of coils (length stays same)
Decrease length of solenoid (coils stay the same)
Add an iron core (it becomes magnetised so increase the strength)
How could you use a horseshoe magnet to demonstrate forces between current carrying conductor and mag field
Apply a current to a set of rails in a horseshoe magnet
A bar is placed on the rails, which completes the circuit
This generates a force that rolls the bar along the rails
If you put a magnet in and out of a solenoid will you get a.c or d.c ?
A.C
As u move the magnet in, the changing mag field induces a p.d and therefore current in the solenoid in one direction
As u move it out, it induces a p.d, and therefore current in the opposite direction
