Electromagnetism & The Motor Effect Flashcards

All 37 cards in this deck

What is the shape of the magnetic field around a long straight current-carrying wire?

Concentric circles centred on the wire, in planes at right angles to it.

What apparatus is used to show that a current in a long straight wire produces a magnetic field?

A vertical wire pushed through a horizontal card, with plotting compasses (or iron filings) on the card; when the current is switched on they show circular field lines.

What are the steps to find the direction of the magnetic field around a straight current-carrying wire?

e.g. conventional current flowing vertically upwards

1. Grip the wire with your right hand, thumb pointing along the conventional current: thumb points up.
2. Your curled fingers give the field direction: field circles the wire.
3. Viewed from above, the field goes anticlockwise.

What two things determine the strength of the magnetic field around a long straight wire?

The size of the current (bigger current, stronger field) and the distance from the wire (further away, weaker field).

Which way does a plotting compass needle point when placed on a card around a current-carrying wire?

Along the field line at that point, i.e. tangential to the circular field lines.

True or false? The magnetic field lines around a straight current-carrying wire run parallel to the wire.

False. They are concentric circles around the wire, at right angles to it.

What is a solenoid?

A long coil of wire that produces a magnetic field when a current flows in it (an example of an electromagnet).

Why is the magnetic field inside a solenoid strong and almost uniform?

The fields from the individual coils add together along the centre of the solenoid.

Why is the magnetic field outside a solenoid weaker than inside?

The fields from the individual coils largely cancel each other outside the solenoid.

What does the magnetic field pattern outside a solenoid look like?

The same as the field of a bar magnet, with a north pole at one end and a south pole at the other.

Name three ways to increase the strength of an electromagnet.

Increase the current, increase the number of turns on the coil, add an iron core.

What are the steps to investigate how current affects the strength of an electromagnet?

e.g. testing currents of 1 A, 2 A and 3 A

1. Change the current with a variable resistor and measure it with an ammeter: 1 A, 2 A, 3 A.
2. Measure the strength each time, e.g. count the paper clips picked up.
3. Keep the number of turns, the core and the coil the same (control variables).

What is the motor effect?

A current-carrying conductor placed in (near) a magnetic field experiences a force.

When a current-carrying wire near a magnet feels a force, what force acts on the magnet?

An equal and opposite force (the forces are equal in size and opposite in direction).

What causes the force on a current-carrying wire in a magnetic field?

The interaction between the magnetic field of the magnet and the magnetic field produced by the current in the wire.

How must a current-carrying wire be placed in a magnetic field for the force on it to be maximum, and when is the force zero?

Maximum when the wire is at right angles to the field; zero when the wire is parallel to the field.

What happens to the force on a current-carrying wire in a magnetic field if the current is reversed?

The force acts in the opposite direction (same size).

True or false? Reversing both the current and the magnetic field reverses the direction of the force on the wire.

False. The two reversals cancel, so the force acts in its original direction.

Which hand is used for Fleming's rule for the motor effect?

The left hand.

In Fleming's left-hand rule, what do the thumb, first finger and second finger represent?

Thumb = force (motion), First finger = Field (N to S), seCond finger = Current (conventional, + to −).

What are the steps to use Fleming's left-hand rule to find the direction of the force?

e.g. field pointing to the right, conventional current out of the page

1. First finger along the field (N to S): point it to the right.
2. Second finger along the conventional current: point it out of the page.
3. Thumb, at right angles to both, gives the force: upwards.

What condition must the force, current and magnetic field satisfy for Fleming's left-hand rule to be used?

They must be mutually perpendicular (all three at right angles to each other).

True or false? In Fleming's left-hand rule the second finger points in the direction of electron flow.

False. It points along the conventional current, from positive to negative.

True or false? In Fleming's left-hand rule the thumb shows the direction of the current.

False. The thumb shows the force (motion); the second finger shows the current.

What is the equation for the force on a current-carrying conductor at right angles to a magnetic field?

$F = B \times I \times l$ (force in N = magnetic flux density in T × current in A × length in m).

What are the units of magnetic flux density?

Tesla (T), also written as newton per ampere metre (N/A m).

What are the steps to calculate the current in a wire from the force on it?

e.g. $F = 0.06$ N, $B = 0.2$ T, $l = 0.5$ m

1. Write $F = B \times I \times l$.
2. Rearrange: $I = \dfrac{F}{B \times l}$.
3. Substitute: $I = \dfrac{0.06}{0.2 \times 0.5} = 0.6$ A.

Before using $F = B \times I \times l$, what must a length given in millimetres be converted to?

Metres.

e.g. $30$ mm $= 0.03$ m.

True or false? In $F = B \times I \times l$, $l$ is the total length of the wire.

False. It is only the length of wire inside the magnetic field.

For which orientation of the wire does the equation $F = B \times I \times l$ apply?

When the conductor is at right angles (perpendicular) to the magnetic field.

Why does a current-carrying coil in a magnetic field rotate in an electric motor?

The current flows in opposite directions on opposite sides of the coil, so the forces on those sides act in opposite directions, giving a turning effect.

What is the job of the split-ring commutator in a d.c. motor?

It reverses the current in the coil every half turn, so the coil keeps rotating in the same direction.

What is the job of the brushes in an electric motor?

They keep an electrical contact between the circuit and the rotating commutator, so current can flow in the spinning coil.

Name three ways to increase the turning effect of an electric motor.

Increase the current, use more turns on the coil, use a stronger magnetic field (or add an iron core).

How can the direction of rotation of a d.c. motor be reversed?

Reverse the current (swap the supply connections) or reverse the magnetic field (swap the magnet poles).

True or false? Without a split-ring commutator the coil of a d.c. motor would keep turning in the same direction.

False. It would stop or rock back and forth, because the forces would turn it back after half a turn.

True or false? The magnetic field around a long straight current-carrying wire gets weaker the further you move away from the wire.

True. Field strength decreases with distance from the wire (and increases with larger current).