7.2 The Motor Effect

What happens when current flows through a conducting wire?

Magnetic field produced around wire

• When a current flows through a conducting wire a magnetic field is produced around the wire.

  • What does the strength of the magnetic field depend on?

• Current through the wire

• Distance from the wire

What does shaping a wire to form a solenoid do?

Increases strength of magnetic field created by current through wire

How do you describe the magnetic field inside a solenoid? (2)

• Strong

• Uniform

How do you describe the magnetic field outside a solenoid?

Similar shape to a bar magnet

What does adding a iron core to a solenoid do? (2)

• Creates electromagnet

• Increases strength of magnetic field

What is an electromagnet?

Solenoid with iron core

Why does the iron core of an electromagnet increase the strength of the magnetic field?

Magnetic field (created by solenoid) magnetises iron bar

The magnetic field around a wire with current flowing through it is a different pattern to the magnetic field caused by a permanent magnet — it is __________ ______.

concentric circles

How can you use the ‘right hand grip rule’ to know the direction of the current and magnetic field around a wire with current flowing through it?

• Thumb = direction of current (+ to -)

• Other fingers = direction of magnetic field (concentric circles)

How do you demonstrate a magnetic field?

• Thread wire through card

• Place plotting compasses on card — around wire

• Plotting compasses make a circular pattern

What is a solenoid?

• Long coil of insulated wire

• Current flowing through it

• The magnetic field inside the solenoid is strong and uniform.

  • What does uniform mean in this context?

• Magnetic field lines all in the same direction

  • Parallel to the axis of the solenoid

Where is the south pole on a solenoid?

End where the direction of the current is clockwise

What is a circuit breaker?

Switch in series with an electromagnet

How does a circuit breaker work?

• Current flows through iron parts of circuit — includes coil

• Coil is wrapped around an iron core — creates electromagnet

• Current flowing through electromagnet creates magnetic field

• If current is too high — magnetic field becomes strong enough to attract iron rocker — held on a plastic pivot

  • Breaks circuit = current stops flowing

• Ratchet/spring stops iron rocker from returning to normal position (until button is pressed)

  • Otherwise, current flows again

What would happen if the ratchet/spring in a circuit breaker wasn’t there?

• Rocker moves = no current flow

• No current flowing through the coil — electromagnet stops attracting rocker

• Rocker returns to normal position — closing the circuit

• Current would flow again — process repeats

What is a relay?

Any switch operated by a magnet or electromagnet

What is the motor effect?

• A conductor carrying a current is placed in a magnetic field

• Magnet producing the field and conductor exert a force on each other

What is the equation for a conductor at right angles to a magnetic field and carrying a current? State the units.

force = magnetic flux density × current × length

$$F=BIl$$

• force — newtons (N)

• magnetic flux density — tesla (T)

• current — amps (A)

• length — metres (m)

Higher magnetic flux density = _________ magnetic field

stronger

What are the units for magnetic flux density?

Tesla (T)

• The size of the force created by placing a conductor inside a magnetic field also depends on the angle between the wire and magnetic field.

When is the force greatest and weakest?

• Greatest — wire perpendicular to magnetic field

• Weakest — wire parallel to magnetic field

In Fleming’s Left Hand Rule, what does your thumb represent?

Direction of force

In Fleming’s Left Hand Rule, what does your first finger represent?

Magnetic field (N → S)

In Fleming’s Left Hand Rule, what does your second finger represent?

Current (+ to -)

Electrons are __________ and so they move:

• from the ___________ end of the cell because _____ charges ______.

• towards the _________ end of the cell because the ________ charges ________.

• negative

• negative

• like

• repel

• positive

• opposite

• attract

Conventional current flows from __________ to __________.

• positive

• negative

What are the four factors that affect the size of the force on a conductor in a magnetic field?

• Strength of magnetic field

• Amount of current flowing through wire

• Length of conductor inside magnetic field

• Angle between wire and magnetic field

What is the basis of an electric motor?

A coil of wire carrying a current in a magnetic field tends to rotate.

How does an electric motor work?

• Current flows through coil — creates a magnetic field

• Magnetic field interacts with magnetic field produced by permanent magnet

  • Causes a force on coil

  • Force causes coil to rotate

• Split ring commutator reverses current every half turn — keeps coil spinning in same direction

• You can make an electric motor rotate faster by increasing the force.

  • How can you do this? (4)

• Increase current

• Increase strength of magnetic field

• More coils

• Reduce distance between coils and magnets

• You can reverse the direction of the motor by reversing the direction of the force.

  • How can you do this? (2)

• Reverse direction of current

• Swap magnets over

  • Magnetic field runs in opposite direction

Why would an electric motor built without a split ring commutator not do a full turn?

• Forces on the coil depend on direction of the current

• After half-turn — sides of coil have swapped positions

• Without reversing current — forces would act in opposite direction to rotation

• Would just turn 180° one way and then flip and turn 180° back

Why are metal or carbon brushes used in a split ring commutator?

Allows current to flow through to the commutator — still allows commutator to move freely

How does the idea of moments explain why a coil rotates in a magnetic field?

• Forces are in different directions — both act in a clockwise direction

• Clockwise moment > anticlockwise moment (= 0) so object rotates

Loudspeakers and headphones use the ______ effect to convert variations in ________ in __________ ______ to the ___________ ___________ in ________ ________.

• motor

• current

• electrical circuits

• pressure variations

• sound waves

In a loudspeaker, what is the frequency of the alternating current equal to? (2)

• Frequency cone vibrates at

• Pitch of sound

How does a loudspeaker work?

• Current passes through coil

• Current creates magnetic field

• Magnetic field interacts with magnetic field formed by permanent magnet — causes a force on the coil

• Current is alternating and changes direction

  • This changes direction of magnetic field and force

• Changing force vibrates cone — causes air particles to vibrate and creates sound

The magnetic flux density 10 cm from the wire is 4 microtesla.

Which of the following is the same as 4 microtesla? [1 mark]

4 × 10^-6 T

• Figure 19 shows a simple electric motor.

• When there is a current in the coil, the coil rotates continuously.

Explain why. [4 marks]

• Current creates a magnetic field around the coil

• Magnetic field interacts with magnetic field — produced by permanent magnet

  • Causes a force on the coil

  • Force causes coil to rotate

• Two halves of the commutator swap from one brush to the other

• Commutator reverses the current in the coil

A student investigated the strength of an electromagnet.

The student investigated how the strength depended on:

• the current in the wire

• the number of turns of wire around the iron core

Figure 7 shows the equipment used.

The student measured the strength of the electromagnet as the maximum weight the electromagnet could hold.

Table 1 shows the results.

Current in amps	Number of turns of wire	Maximum weight in newtons
1.0	30	6.5
1.5	20	6.4
2.0	10	3.7

Explain why the method used by the student is not valid for this investigation. [2 marks]

• Changed two independent variables at the same time

• Not possible to know the effect of one variable or the other