Electromagnetism & The Motor Effect Flashcards

All 21 cards in this deck

What are the steps to use iron filings to show the shape of the magnetic field produced by a current?
e.g. a straight wire passing vertically through a horizontal card

1. Sprinkle iron filings over the card (before switching the current on).
2. Switch on the current and tap the card.
3. The pattern produced shows the shape of the field — circles around the wire.

What is the shape of the magnetic field produced by a current in a long straight conductor?

Concentric circles centred on the wire, with the circles getting further apart further from the wire.
Direction is given by the right-hand grip rule.

True or false? When a current flows in a straight wire, a nearby plotting compass needle points straight at the wire.

False. The needle lies tangential to the circular magnetic field lines around the wire.

What two things does the strength of the magnetic field around a long straight conductor depend on?

The size of the current (bigger current = stronger field) and the distance from the wire (further away = weaker field).
Stronger field is shown by field lines closer together.

Describe the magnetic field pattern of a current-carrying solenoid.

A very strong, almost uniform field along the inside (parallel, evenly spaced lines) and a much weaker field outside, shaped like that of a bar magnet.

Why is the field inside a solenoid very strong but weak outside?

Inside, the fields from the individual coils add together; outside, the fields from the coils cancel.

Give three ways to increase the strength of an electromagnet.

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

True or false? Reversing the direction of the current in a solenoid swaps its north and south poles.

True. The field direction reverses when the current direction reverses.

What is the motor effect?

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

What causes the force on a current-carrying wire placed near a magnet?

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

How does the force on the magnet compare with the force on the current-carrying wire when their fields interact?

The forces are equal in size and opposite in direction.

True or false? The force on a current-carrying wire is greatest when the wire is parallel to the magnetic field.

False. The force is greatest when the wire is at right angles to the field, and zero when it is parallel to the field.

What are the steps to use Fleming's left-hand rule to find the direction of the force on a wire?
e.g. a wire carrying current between the poles of a magnet

1. Hold the first finger, second finger and thumb of the left hand mutually perpendicular.
2. Point the first finger in the direction of the magnetic field (N to S) and the second finger in the direction of the current.
3. The thumb then points in the direction of the force/motion.

True or false? Fleming's left-hand rule can be used when the current is parallel to the magnetic field.

False. It only applies when the force, current and magnetic field are mutually perpendicular.

State two practical ways of reversing the direction of the force on a current-carrying wire in a magnetic field.

Reverse the current (e.g. swap the battery connections round) or reverse the poles (e.g. turn the magnet round).

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

False. The force acts in the same direction as before — the two reversals cancel out.

What is the equation for the force on a conductor at right angles to a magnetic field, with units?

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

True or false? In $F = B \times I \times l$ a length in mm and a current in mA can be substituted directly.

False. Convert to metres and amperes first, e.g. 30 mm = 0.030 m and 93.1 mA = 0.0931 A.

How do you use the right-hand grip rule to find the direction of the magnetic field around a straight wire?

Grip the wire with your right hand, thumb pointing in the direction of the current. Your curled fingers show the direction of the field lines.

What is an electromagnet?

A magnet made by passing a current through a coil of wire (e.g. a solenoid), so it is magnetic only while the current flows.

What does the magnetic flux density, $B$, of a field tell you?

How strong the magnetic field is, shown by how closely packed the field lines are.