Hello again everyone! Time once again for another Physics guide. We are done with the Space part of the course, so if you have any questions there, feel free to post it in the forums and either myself, another moderator, or one of your fellow HSC’ers will be sure to help you out. Now we move on to motors and generators. The first will cover motors; a little shorter than the next two guides, but it forms the basis of knowledge in some key areas. I’m going to assume a basic understanding of current, resistance and voltage; there are heaps of resources available if you need a refresh!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.So, firstly, we need to define a very vital effect; the
motor effect . The motor effect is the force experienced by a current carrying conductor in a magnetic field. We look at this in more detail in Ideas to Implementation, but essentially, a
moving electric charge (ie, a current) will experience a force in a magnetic field. This causes a force on the conductor.
The force on the conductor due to a motor effect is dependent on the strength of the magnetic field
, the current
, the length of conductor in the field
, and the angle
between the conductor and the magnetic field lines.
This formula makes it clear that the force is maximised when the conductor is perpendicular to the field, and is zero if it is parallel.
Before we look at motors, one more thing. Parallel conductors carrying a current will experience a force between them. If the current is in the same direction, this force is attractive. Otherwise, it is repulsive. This is a common mathematical question in the HSC, which normally makes for easy marks. Just use the formula, where
:
The motor effect is the basis for the operation of motors. The simple DC motor studied in the course looks like this. In terms of the motor effect, current is shown in blue, magnetic field in purple, force in green. All the elements are perpendicular. This is the right hand slap rule! Hold up your right hand like you are waving, with your thumb pointed out. If the magnetic field lines are your fingers, and the current is your thumb, you will slap in the direction of the force
It consists of two main parts.
The
stator creates the required magnetic field. This may be provided by permanent magnets, or by electromagnets (current carrying coils). It needs only to produce a field of the required strength.
The
rotor is the part of the motor that spins. This normally consists of an axle which is attached to some external load (EG- a wheel), and the armature. The armature consists of the coils, usually wrapped around an iron core. A current flows through this coil, connected to a power source through brushes which maintain contact with the circuitry, as the motor spins.
The coils are what makes the motor turn. According to the motor effect, the two sides of the coil perpendicular to the field will experience a force. The current flows in opposite directions on either side, so this force is in opposite directions. This causes the motor to spin.
We don’t normally consider the individual forces on the sides of the coil. We consider the resultant
torque. Torque is, put simply, turning force. Forces act linearly, torque acts rotationally. The simple formula for torque we need for this course is
, where
is a force applied at a distance
from the rotation point. We also have a formula for the torque experienced by a coil in a motor, which is dependent on magnetic field
, the area of the coil
, the current
, the number of turns
in the coil, and the angle with the field.
What we notice about this formula is that the torque on the coil will change direction every half turn (180 degrees). Obviously, for a DC motor, we want the torque to be in a single direction. This is where the role of the
split ring commutator proves vital. In DC motors, this commutator is connected to both sides of the coil, with a split between each section. This commutator spins as the axle spins, and the brushes come into contact with the opposite side of the commutator every half turn. This reverses the direction of current through the coil every half turn, thus maintaining a constant direction of torque.
Now, all of this is quite hard to picture, and very easily confusing. I found the easiest way to understand how it all worked was video content. There are lots of resources available, with animations or active diagrams, which will prove very useful if you are having trouble. Thinking back to your practicals will undoubtedly help too. Just try a whole bunch of things until you wrap your head around it, and of course, if you need more detailed explanations, just ask! Happy to provide if they are needed.
Let’s look at a very common mathematical question:
Part One : Determine the force needed to lift the mass.
This ties back into our Space topic. To lift the mass, we need a force equal to the weight force of the object. Therefore,
Part Two : Calculate the minimum current required in the coil to lift the mass.
This questions asks us to come up with the torque required to generate this required force. We equate our two formulae for torque to find the required current. I’ll leave out the rearranging at the end:
Explaining the operation of a DC motor is a VERY common question. Always draw a diagram, label the key parts, and explain what they do. Set your information out clearly; there is a lot to cover, and the markers don’t want to see a big mess of scribbles and extra dot points in a confusing layout. Even if you add stuff, keep everything neat and as logical as possible.
Just as common, if not more so, is explaining how the motor effect is applied in either a loudspeaker or a galvanometer. These can be doozy questions. The trick with these is to make sure you explain how it works! Marks are lost by describing the part and their function, but not explaining how it contributes to the function. For example, consider the difference between these two sample responses:
A galvanometer uses a radial magnetic field created by a permanent magnet. A coil is attached to the needle, and anchored to a spring as shown. When a current goes through the coil, the needle moves to match the scale.
... and ...
A galvanometer utilises the motor effect for its operation. It uses a radial magnetic field created by a permanent magnet; this means that the torque experienced by the coil is constant at all points of its rotation. The torque experienced can therefore be defined as 
, where B, A and n are constant. Thus, torque is directly proportional to current. A spring provides an opposing torque, so that when a current flows through the coil, it rotates to the appropriate scale on the meter for the user.
Neither are perfect, but this a 2 mark versus a 4 mark response, roughly speaking. In the exam you would be better off using dot points for these. Keep this in mind! So, information on a galvanometer and a loudspeaker. Be sure you have a simple diagram in mind to draw (some good ones are shown):
Electric meters utilise the motor effect to measure current. Such meters utilise a radial magnetic field, meaning an identical torque is experienced at any angle within the housing. Thus, if the same coil is used, torque is directly proportional to the current flowing through the meter. The armature is designed to spin and stretch a spring; a greater current is required to generate enough torque to spin the armature against this opposing spring torque. This setup is calibrated so that the current changes produce appropriate changes in the position of a needle.
The loudspeaker also works due to the motor effect. An electromagnet is fed an Alternating Current, causing it to experience a force in interaction with the permanent magnetic field. The electromagnet vibrates with the same frequency as the input AC voltage, vibrating a sound cone which produces sound waves.
Besides this, some other miscellaneous questions which could be asked:
- Getting the direction of rotation from given information. Use the right hand slap rule for each side of the coil to check. Remember, magnetic field lines go from N to S, and conventional current flows from + to -
- Describing the function of particular parts of the motor. Easy marks if you know your stuff
- Comparisons between motors and generators
For this last part, you'll have to read the next guide on Generators, stay tuned!
Thanks for reading! If you have any questions, take 30 seconds to register and ask any questions you have below! I am happy to help, and it's a great chance to get some help and advice from the community. Happy study! 
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