Contextual Outline:

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Modern 9.3 MOTORS ANDis geared to using electricity.
industrialised society GENERATORS
Electricity has characteristics that have made it uniquely
appropriate for powering a highly technological society. There are many energy sources that can be readily converted into electricity. In Australia, most power plants burn a fuel, such as coal, or use the energy of falling water to generate electricity on a large scale. Electricity is also relatively easy to distribute. Electricity authorities use high-voltage transmission lines and transformers to distribute electricity to homes and industries around each state. Voltages can be as high as 5 × 105 volts from power stations but by the time this reaches homes, the electricity has been transformed to 240 volts. While it is relatively economical to generate electric power at a steady rate, there are both financial and environmental issues that should be considered when assessing the long-term impact of supplying commercial and household power.

The design of a motor for an electrical appliance requires
consideration of whether it will run at a set speed, how much power it must supply, whether it will be powered by AC or DC and what reliability is required. The essentials of an electric motor are the supply of electrical energy to a coil in a magnetic field causing it to rotate.

The generation of electrical power requires relative motion
between a magnetic field and a conductor. In a generator,
mechanical energy is converted into electrical energy while the opposite occurs in an electric motor.
The electricity produced by most generators is in the form of alternating current. In general AC generators, motors and other electrical equipment are simpler, cheaper and more reliable than their DC counterparts. AC electricity can be easily transformed into higher or lower voltages making it more versatile than DC

electricity.

9.3 MOTORS
AND
GENERATORS

This module increases students’ understanding of the applications and uses of physics and the implications of physics for society and the environment.
© Board of Studies NSW, Stage 6 Physics Syllabus

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9.3 MOTORS AND GENERATORS – 9.3.1

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1. Motors use the effect of forces on current-carrying conductors in magnetic fields
1. discuss the effect on the magnitude of the force on a current-carrying conductor of variations in:
– the strength of the magnetic field in which it is located ↑B = ↑F

– the magnitude of the current in the conductor
↑I = ↑F

– the length of the conductor in the external magnetic field ↑l = ↑F

– the angle between the direction of the external magnetic field and the direction of the length of the conductor
↑θ = ↑F
Use 𝐹 =

𝐵𝐼𝑙 sin 𝜃 to easily find the above.

2. describe qualitatively and quantitatively the force between long parallel current-carrying conductors:

II
F
k 1 2
l
d 
We know that a wire with a current flowing though it will create a magnetic field around it. And we know that if we have two wires with current flowing though them that are parallel to reach other then the magnetic fields will interact and create a force on the wires. If the current of the two wires is flowing in the same direction then they will attract. If the current is flowing in opposite directions they will repel. You can work this out by applying the right hand grip rule (see below) to the wires. If the current is in the same direction then the magnetic field will be in the same direction and the two wires will attract to create one big magnetic field. The opposite can be said when the current in the wires is in the opposite direction. The above formula only works for long wires.

current

Fig. 47
magnetic field

The right hand grip rule is used to
determine the direction of the magnetic
field around a current carrying conductor.

The diagrams below show how the magnetic field surrounding two wires with the current in the same direction result in the attraction of the wires, and when the current is in opposite direction they repel.

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9.3 MOTORS AND GENERATORS – 9.3.1

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Fig. 48

Diagram adapted from ‗Motors and generators‘ produced by Learning Materials Production, OTEN.

The force between these two wires is given by,

II
F
k 1 2
l
d
3. define torque as the turning moment of a force using:

  Fd
Torque is just a turning moment.
𝜏 = torque (Nm)
𝐹 = force (N)
𝑑 = perpendicular distance (m)
Consider the situation below, a force of 3 N is acting on a block of wood that is pivoted. To calculate the torque we can either use 𝜏 = 2 × 3 = 3 2 Nm, or 𝜏 = 2 × 3 cos 45° = 3 2 Nm. 2m

Fig. 49
2m

45°

3N

Torque is important when dealing with motors as it determines the turning moment of the motor.

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9.3 MOTORS AND GENERATORS – 9.3.1

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4. identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field
The motor effect is the action of a force experienced by a current carrying conductor in an external magnetic field.
The direction of the force can be determined by the
right hand push rule. Remembering that magnetic field
lines go from north to south. The magnitude of the
force can be determined by 𝐹 = 𝐵𝐼𝑙 sin 𝜃
current

The motor effect is the basis of a motor, in which two
opposite forces on opposite sides, create a torque,
creating a spinning motion used in motors.

force

magnetic
field

Fig. 50
Right hand push rule.

Fig. 51

5. describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces
If a current is flowing though the coil and an external magnetic field is present then there will be forces acting on the coil. That is, each side of the coil that is perpendicular to the magnetic field will experience a force due to the motor effect. By the right hand palm rule, the left side (in the diagram below) will experience a force down, and the right will experience a force up. Together these forces create a turning moment. The coil will experience maximum torque at this point.

Fig. 52 - In a motor, the two sides will experience a force that together creates a turning force, a maximum force occurs when the plane of the coil is parallel to the magnetic field.

When the coil has rotated to the position shown below, then the two forces for both sides of the motor will be acting in opposite directions along the same line of action and hence they will cancel each other out. However momentum pushes the coil past this point, along with the change in _____________________________________________________________________________________________________ HSC PHYSICS SYLLABUS NOTES 2007 – ANDREW HARVEY

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direction of the current due to the split ring commutator, the coil will start to experience a turning force again.

Fig. 53 - In a motor, there will be no net force on the coil when the plane of the coil is perpendicular to the magnetic field.

The diagrams below show the operation of a DC electric motor.

1

At this point the torque on the coil is a maximum.

3

At this point the torque is zero, as the
perpendicular distance is zero. Also at this point
the current changes direction, which is shown in
the next frame.

2

The torque is less than the frame before. This is
because torque is force multiplied by the
perpendicular distance. You can think of this as
either the distance becoming less, or just taking
the perpendicular component of the force, but
either way the torque is less.

4

The current has changed direction. Now although
the net force acting on the coil is zero,
momentum keeps it turning past this point.

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5

Now that the current has changed direction, the
torque keeps the coil turning in the same
direction.

6

… and we have reached the first frame again.

You will notice that the torque on the coil looks like this:

Fig. 54

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6. describe the main features of a DC electric motor and the role of each feature The DC electric motor uses the motor effect to create a continuous spinning motion. The main features of a DC electric motor are shown below.

coil
armature

Fig. 55

magnet

brush

split-ring commutator

A motor must have a magnetic field to work. This field is provided by either permanent magnets or electromagnets. Remember that magnetic field lines travel from north to south. If you apply the right hand push rule to the two wires currents perpendicular to the magnetic field lines you will find that the two side push in opposite directions, which act together to rotate the coil. However the force will act in such a way that the plane of the coil is kept perpendicular to the magnetic field lines. So to prevent this, the commutator is used. By changing the direction of the current at the right time thus changing the direction of the force, and thus it will spin continuously. Momentum will move the coil past this point when it is perpendicular to the magnetic field, as at this point the forces cancel out and the coil will not spin.

The brushes just keep the current flowing into the commutator, without sparking. They allow a sliding contact with the commutator, allowing them to be stationary while the commutator is spinning.
The armature is the thing that the coil is wrapped around. This is usually iron. It gives the coils mass, resulting in momentum, and also holds the coils in place. Without the armature if the force is too great then the coil will bend and not turn the thing that the motor is trying to turn. A current must also be present in the coil.

7. identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets
The magnetic field in a DC motor can be produced using either a permanent magnet, or an electromagnet (made using a current-carrying coil and an iron core...

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