Hello everyone! Welcome to another HSC Physics guide; this one is going to cover the remaining parts of the Motors and Generators topic. This means transformers, AC induction motors, power distribution, and the role of electricity and transformers in society. It’s a fair bit of stuff, but thankfully, nothing too difficult to grasp! So sit back, relax, and let’s do some revision.
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.Okay, so let’s begin with transformers. Transformers are used to change the voltage of electricity in AC circuits. The voltage can be increased or decreased as required. You actually don’t have to know how these work! But it is handy to know the basic premise.
All transformers consist of two coils wrapped around a ferromagnetic core. AC current fed to the primary coil generates a changing magnetic flux, which induces an EMF output in the secondary coil. The core amplifies the effects of the changing flux.
Now comes the bit you only need to be able to describe, not explain why it happens. The size of the emf (and thus, the voltage) in a coil is dependent on the number of turns in that coil. Thus, step up transformers have more coils in the secondary coil, and allow the voltage to be increased. Step down transformers have less coils in the secondary coil, allowing the voltage to be decreased.
This is a step up transformer. There is more coils in the secondary coil, so the emf (voltage) increases.
The formula which describes this is below. The ratio of voltages in primary/secondary is equal to the ratio of turns.
This a common exam question!
Example One: The primary winding of a transformer contains 2000 turns. The primary AC voltage is 23 000 volts and the output voltage is 660 000 volts. Calculate the number of turns in the secondary winding.
Easy. Sub and go:
The important thing to remember about transformers, is that no energy is created or destroyed. The electrical energy in each core must be equal (assuming 100% efficiency), so
.
Of course, no transformer is 100% efficient. Where does the energy go? The answer is in the core. The ferromagnetic core is super important to allow the changing flux to actually have a noticeable effect on the secondary coil. Without it, transformers would barely work. However, the core (usually made of iron) also experiences this changing flux, thus inducing
eddy currents. . Be sure to check out my guide on induction if you need a refresher here.
These eddy currents waste energy in the form of heat which builds up in the core. This is obviously a bad thing, and to minimise it,
laminated cores are used. Insulated laminations are used to divide the core into smaller sections, significantly reducing the size of eddy currents and thus reducing the energy loss due to heat.
Transformers are absolutely vital to modern power distribution networks. Let’s look at why.
First, they minimise power loss. Power losses in transmission wires occur due to heating, according to the formula
. This energy loss is costly and can cause damage to infrastructure. Transformers allow this power loss to be minimised, by increasing voltage and thus minimising current. This becomes increasingly important over large distances, and so is ultimately the reason AC is used for power distribution. Electricity for Australian homes is normally generated with a voltage of 23 kV, then stepped up to 330 kV for long range transmission, then gradually stepped down to the 240 V we use in household appliances.
In addition, many household appliances contain miniature transformers. Appliances are designed to operate on a specific voltage, AC or DC. This is particularly true for battery operated devices, which run on low voltage DC. Not operating at the correct voltage can cause overheating and endanger the user. Transformers and rectifiers (converts AC to DC) are used to moderate the power supply to the appliance, or even the specific part in use. For example, the computer I am typing this on is likely using a step-up transformer to power my big screen, while stepping down the voltage to turn on my little webcam light. This allows appliances to be designed for the most efficient voltage, and the mains supply can just be transformed to suit this.
Analysing the role of transformers in electrical power in some way is a common question. The above paragraphs would fit nicely into an extended response on this subject. Even more common is
evaluating the impact of transformers. The important point is that transformers
ARE the reason we use electricity so much today. So, is this good or bad?
It could definitely be considered a good thing, because:
• Shift from DC to AC for mass power production, resulting in lower costs, and less urban pollution.
• Minimising energy wastage has reduced pollution in the atmosphere, reduced the cost of electricity, and maximized the efficiency of electrical systems
• Allows power stations to be located far from urban centres
• Allows development of appliances which use different voltages from 240V outlets
• Minimised costs of production, thus reducing costs of products for consumers, and improving quality of life
• Less reliance on steam power created a greener/cleaner urban scape, thus improving general health
Negative impacts surround health issues due to pollution, massive increases in obesity rates and other lifestyle related illnesses, and increased levels of unemployment due to replacing manual labour with machinery. Use your imagination! There is lots to argue.
However, impacts on the environment are almost completely negative. Increased reliance on electrical power increases reliance on fossil fuels, in turn contributing to global warming and increasing temperatures, raising sea levels, thus reducing available agricultural land, increasing food costs and cost of living.
Absolutely, in these questions I recommend doing an argument similar to:
"While the development of the AC Generator/transformer (whichever is asked) has improved our quality of life in many ways, the negative impacts on the environment cannot be ignored" . Put your own spin on it! You might argue that the environmental damage is worth it, you might argue that it isn't. Make it your own and express yourself clearly, and it is hard to lose marks!
Two last little bits of theory to cover. The first, AC induction motors! These can be a little confusing to understand, but questions will not go into too much depth. Essentially, these motors work based on Lenz's Law. Instead of a constant magnetic field, AC induction motors have a rotating magnetic field. This is done with an arrangement of electromagnets; think a hula hoop covered in Christmas lights or something, and the lights turn on one by one, in order! It's essentially the same idea.
This rotating field exposes the rotor to a changing flux (note, induction motors do not have a coil!). The rotor is shaped like one of those wheels that hamsters run on. Eddy currents flow in response to the changing field, and according to Lenz's Law, these currents will oppose the change which created them. The net result? The rotor
chases the magnetic field, like a mouse chasing the Christmas light! That's as much detail as you need, but try and think back to your practical to get a better picture.
Another terribly edited image, this one of a squirrel cage rotor!
Finally, we look at energy transformations in the home and industry. You just need a few examples here, to answer common questions like:
Identify three energy transformations which make use of electrical energy in the home. You should specify what new energy is created and what purpose it serves. Easy marks, just say things like this, and then add what each appliance you choose is used for!
• Electrical energy is transformed into radiant (electromagnetic) energy in toasters, microwaves, radios, and light globes
• Electrical energy is converted into kinetic energy (movement) in blenders, speakers, and electric tooth- brushes
• Electrical energy is converted into chemical potential energy in rechargeable batteries
For industry, it might be harder to think of examples. Here are a couple:
Electrical energy is transformed into electromagnetic waves as X Rays in medical imaging, light in laser printing, heat in large industrial ovens, and radio waves for communication
• Electrical energy is transformed into kinetic energy in cars, trucks and plans, as well as in essentially any kind of manufacturing process
And, that's it! You can cross Motors and Generators off your list! Many would say it is the easiest topic in the course, and I'd agree, though there are some slightly difficult concepts. With that in mind, if anything in this or any of the other guides has proven complicated, please feel free to register and ask me any questions you had! Or leave some feedback, say hey, share photos of your pet kitten, anything! Stay tuned for more guides, but until next time, happy study! 
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