Physics - 15 Electromagnetism - 15.4 The Motor Effect & 15.5 The Generator Effect & 15.6 The Alternating-Current Generator

Motor effect

force exerted on a current-carrying conductor due to a magnetic field

How can the force exerted on a current-carrying conductor in a magnetic field be increased? [2]

- increasing the current

- increasing the strength of the magnetic field

How does the angle of the conductor to the magnetic field affect the force on the conductor? [2]

- strongest when perpendicular

- zero when parallel

Conventional current

current that flows from the positive side of the battery to the negative side; used in diagrams

Fleming's left-hand rule

method of using the left hand to find one of the force, magnetic field or current acting on a wire

Fleming's left-hand rule: force

thumb

Fleming's left-hand rule: magnetic field

index finger

Fleming's left-hand rule: current

middle finger

What will an alternating current cause a wire in a magnetic field to do?

vibrate

Magnetic flux density

representation of the magnitude and direction of the magnetic field

What is the unit of magnetic flux density?

Tesla (T)

What is the symbol of magnetic flux density?

B

Word equation for force on a wire

force = magnetic flux density x current x length

Symbol equation for force on a wire

F = BIl

How does an electric motor work? [3]

- current-carrying coil of wire is placed in a uniform magnetic field

- opposite force is exerted on each side of the coil, causing it to rotate

- the direction of current is changed every half-turn by the split-ring commutator

Split-ring commutator

rotates with the coil in an electric motor and reverses the direction of the current every half turn

Why is the split-ring commutator necessary? [2]

- if a coil undergoes a half-turn, the forces on the wires will no longer point the correct direction to keep rotating in the same direction

- to maintain the same rotation, the split-ring commutator reverses the direction of the current

Brush (electric motor)

conductors which maintain a connection between the rotating coil and the circuit

Rotor/armature

rotating member of an electrical machine.

Generator effect

when a magnetic field moves relative to a conductor, a potential difference is induced across the conductor, driving a current in a complete circuit

Induce

to cause or produce

Electromagnetic induction

process of creating a current in a circuit by changing a magnetic field

Conservation of energy with a generator

generated induced current is a result of work done to change the magnetic field relative to the conductor

How can the current induced in a generator be increased? [4]

- increasing speed of movement

- increasing strength of magnetic field

- increasing number of turns in coil

- increasing surface area of coil

Solenoid rule

When the pole of a magnet is moved towards a solenoid, an (opposite/like) pole is induced in the solenoid which (repels/attracts) the magnet

like, repels

When the pole of a magnet is moved away from a solenoid, an (opposite/like) pole is induced in the solenoid which (repels/attracts) the magnet

opposite, attracts

What does reversing the direction of movement do to the induced current?

reverses the flow of current

Galvanometer

ammeter which can detect very small electrical currents

Why is the current induced by a generator an alternating current? [3]

- the coil rotates

- the circuit and slip rings are stationary

- so, the side of the coil connected to the circuit changes with each half-turn

What does increasing the speed of the rotation in a generator do? [2]

- increase the frequency of the alternating current

- increase the peak potential difference of the current

Alternating current graph

resembles a sine graph, with waves

What position does the peak of an alternating-current graph correlate to in a generator?

sides of the coil crossing directly through the magnetic field

What position does the x-intersection of an alternating-current graph correlate to in a generator?

sides of the coil not crossing through the magnetic field

Slip ring

metal rings connected to the rotation coil in a generator; they make electrical contact with an external circuit

Dynamo

generator which generates a direct current

How does a dynamo generate a direct current? [3]

- dynamo works like an ordinary generator

- has a split-ring commutator, which switches the direction of current each half-turn

- since rotation causes a reverse, the commutator keeps it constant

What is the nature of the current from a dynamo? [2]

- goes from minimum to maximum value twice each cycle

- never changes polarity

Microphone

device for converting sound waves into electrical energy

Loudspeaker

device that converts pulses of electricity into sound waves

How does a microphone work? [4]

- sound waves cause a flexible membrane to vibrate

- this causes a permanent magnet to vibrate

- this induces an alternating current in a coil of wire

- current alternates with the same frequency as the sound wave

Why is the current from a microphone an alternating current?

the magnet moves in both directions relative to the coil

How does a loudspeaker work? [5]

- an alternating current is supplied to a coil of wire

- the coil becomes an electromagnet due to the current

- the magnetic field interacts with a permanent magnet, causing the coil to move back and forth

- coil is attached to diaphragm; generates sound waves

- sound waves have same frequency as supplied current

Why is the current supplied to a loudspeaker an alternating current?

the diaphragm needs to move both back and forth to generate sound waves