20.5 Applications of magnetic forces

paper cone

coil

magnet

Moving-coil loudspeakers Moving-coil loudspeakers A moving-coil loudspeaker contains a coil

– free to move in the magnetic field of a permanent magnet

Varying current passes through the coil.

The coil experiences varying magnetic force.

varying I

20.5 Applications of magnetic forces

The coil moves inwards and outwards.

Sound is produced.

The paper cone attached vibrates.

20.5 Applications of magnetic forces

side CD

When a current passes through the coil...

currentA

BC

D

magnetic fielddownward force

upward force

side AB

side BCno force

The coil rotates.

Turning effect on a current-Turning effect on a current-carrying coil in a magnetic field carrying coil in a magnetic field

20.5 Applications of magnetic forces

The turning effect on the coil can be increased by:increasing the currentincreasing the strength of magnetic field winding more turns of wireincreasing the area of the coil

20.5 Applications of magnetic forces

Moving-coil galvanometers Moving-coil galvanometers

A moving-coil galvanometer can measure a small

current. makes use of the

magnetic turning effect on a current-carrying coil.

20.5 Applications of magnetic forces

Structure of a moving-coil galvanometer

scalepointer

cylindrical magnet

aluminium frame

soft iron core hair spring

coil

20.5 Applications of magnetic forces

I

The current flows through the coil.

The coil rotates.

Magnetic turning effect balances restoring turning effect of the hair springs.

The coil stop rotating.

current deflection

Simulation: How moving-coil galvanometers work

20.5 Applications of magnetic forces

Simple d.c. motor Simple d.c. motor

The coil rotates clockwise.

A B

CD

observer’s eye

1A

B

CD

observer’s eye

2

Why can’t a galvanometer rotate continuously in one

direction?

20.5 Applications of magnetic forces

There is no magnetic turning effect.

The coil keeps turning due to inertia.

A

B

C

D

observer’s eye observer’s eye

A

B

C

D

The turning effect is reversed.

The coil rotates anti-clockwise.

When the coil is at a vertical position...

3 When the coil passes the vertical position...

4

20.5 Applications of magnetic forces

A

B

C

D

observer’s eye

5

The coil comes to rest at vertical position eventually.

20.5 Applications of magnetic forces

+

–

A d.c motor can rotate in one direction continuously.

commutator

brush

coilmagnet

Two half-rings are attached to coil.

connects the coil to the battery.

20.5 Applications of magnetic forces

The coil rotates as the current flows.

The coil keeps turning due to inertia.

At the start...1 When the coil is at the vertical position...2

+–

AB

DC

+–

A

B

D

C

20.5 Applications of magnetic forces

The contacts between the half-rings and the brushes interchanges.

When the coil passes the vertical position...3

AB

DC

Current in the coil is reversed.

Magnetic forces acting on the coil is reversed.The coil keeps on rotating

in the same direction.Simulation: How d.c. motors work

20.5 Applications of magnetic forces

It contains several coils spaced at an angle to each other.

Its wires are wound around a soft iron core.

magnetic field strength

A practical motor

Electromagnets are used instead of permanent magnets.

magnetic field strength

20.5 Applications of magnetic forces

That’s the end of Section 20.5

Check Point

Key Ideas

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