(6.8-6.14) Current carrying wires + forces

What forms around a current carrying wire?

A magnetic field

How is an electromagnet made?

A current is passed through a coil of wire

Describe the magnetic field around a straight wire.

Concentric circles

that get further apart

as distance from the wire increase

Describe the magnetic field around a flat circular coil.

N→S in the centre of the coil

Field lines curve outward and loop around,

forming concentric circles around the wire

The circles get further apart further away from the wire

Describe the magnetic field around a solenoid.

Same as that of a bar magnet:

flowing N→S

uniform through it

N→S loops around it

How is a solenoid’s strength increased?

Increasing current

Increasing turns for a given length

Decreasing length and keeping the no. of turns

Adding an iron core through the centre

What is the right-hand rule used for?

To work out the direction of the magnetic field

How is the right hand rule used?

Thumb along direction of the current

Fingers loop to show direction of the field

What factors affect the strength of the field around a straight wire?

Higher current = larger field

Closer to wire = larger field

There is a force on a charged particle…

when it moves in a magnetic field

as long as the motion is not parallel to the field

When does the motor effect occur?

A current-carrying wire is placed in a magnetic field (normally between 2 magnets)

and experiences a force

What causes the motor effect?

The interaction between

• the field of the wire

• the field of the magnets

which results in a force on the wire

Describe the make-up of a simple D.C motor.

A coil of wire attached to a split ring commutator

which is connected to in a circuit with contacts to carbon brushes

all placed within a uniform magnetic field

When horizontal, the wire forms a complete circuit.

Describe the motion of a D.C. motor.

Force is exerted on the wire due to the interacting fields

they act in opposite directions on each side of the coil

• causing the coil to rotate

At 90 degrees, the split ring isn’t in contact with the brushes

• no forces and no current is flowing through the coil

Momentum of turn causes the coil to rotate slightly

• ring connects to brushes

• current remains in the same direction

• the forces act on different sides now

• coil spins in one direction continuously

What factors affect a D.C. motor?

Increase current/strength of field = increased speed

Reverse current/polarity = reverse direction

Increase current/strength of field/no. of turns = increased force supplied

How does a loudspeaker work?

A coil is wrapped around one pole of a permanent magnet

A/C current is passed through the coil

• creating an alternating field around the coil

The coil’s A/C field interacts with the magnet’s field

• exerting a force on the coil

• that is an alternating force

• causing oscillations

The vibrating coil = vibrating speaker cone

• causing the air to oscillate, creating sound waves

What are the finger/value pairs for Fleming’s Left Hand Rule?

F- field

M- magnetic field

C- current

What factors affect the force on the wire?

Stronger fields = stronger forces

Higher current/stronger magnets/perpendicular wire = stronger forces