Hello again everyone! Time again for another guide on HSC Physics. This one is going to cover generators and electromagnetic induction. Quite a few concepts here, but nothing overly complicated, which is good news! Be sure to go check out my guide on
Motors to give this stuff some context.
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.First, electromagnetic induction and associated theory. There is a lot to cover here before we even look at Generators.
Faraday was the first to observe electromagnetic induction; he detected an induced current in a solenoid when it was exposed to a changing magnetic field. This is an experiment you are likely to have replicated; it is quite simple. Faraday discovered that the induced current was stronger when the rate of change of magnetic flux density (magnetic field strength) was greater.
Here, it is important to distinguish between magnetic flux and magnetic flux density. Magnetic Flux is simply the number of magnetic field lines passing through a given area. Magnetic flux density, however, is how we measure magnetic fields, and is found by dividing magnetic flux by the area.
Next to cover is
Lenz's Law . This law states that an induced emf (electromotive force, potential difference, all essentially the same) always generates a magnetic field which opposes the change that created it. This makes sense in terms of the Conservation of Energy; if the new magnetic field assisted the change, then energy would be created. In motors, we remember that the coil spins inside a magnetic field. Thus, the coil experiences a changing magnetic flux density, and thus, a current is induced. Lenz's law says that this new current/emf acts in the opposite direction, thus generating an opposing torque. This is called
back emf .
Back emf opposes the supply emf in a motor. Back emf increases as the motor spins faster; thus, a stationary motor contains much more current than a spinning one!
Eddy currents are a common source of confusion. Eddy currents are circular flows of current which occur in conductive materials exposed to a changing magnetic field. According to Lenz’s Law, this changing magnetic flux will induce an opposing emf and magnetic field in the conductor. We can use the right hand grip rule to determine the orientation of the resultant magnetic field, and indeed, this a very common question in HSC exams. Remember the rule; clench your right hand in a fist with your thumb pointed. Your fingers point in the direction of current, and your thumb points to the North Pole of the resultant field.
(Excuse the poor quality image editing on my behalf)
Let's look at a general induction question to practice:
Right, so we know that the induced current will oppose the change. Thus, the south pole will be at the bottom. So, the north pole is at the top; point your thumb upwards and clench your fist. Your fingers point anticlockwise. Now, inside the coil, the magnetic field lines go from South to North (the easiest way to remember this is just the opposite of what happens on the outside, it can be a little confusing). So, the answer is B!
Another common question will ask you to explain how induction is useful in induction stovetops or induction braking. I'll give a sample response to both.
Example One : Explain how induction cooktops heat food.
Induction cooktops use electromagnetic induction to heat food. Beneath the stovetop there are solenoids. When the stove is switched on, they are fed an AC current. This AC current causes a constantly changing magnetic flux in the saucepan above, inducing eddy currents (circular flows of current in a conductor). The resistance of the pan causes it to heat up when these currents flow, thus heating the food.
Example Two : Explain the role of eddy currents in electromagnetic braking.
Eddy currents are circular flows of current which occur in a conductor exposed to a changing magnetic field. These currents are utilised in electromagnetic braking. The moving carriage is fitted with super magnets or electromagnets. Copper sheets are placed where the carriage should stop. As the carriage approaches, eddy currents are induced in the copper, opposing the motion that created them according. This is due to Lenz's Law, which states that an induced emf will oppose the change which created it. Thus, the carriage is decelerated.
Questions on Lenz's Law, eddy currents, and induction used in these ways, are common. Be sure you are ready for them, and remember, diagrams are an awesome way to demonstrate your understanding.
Right, now to generators. Generators are essentially identical to a motor in structure (again, go see my other guide if you need a refresher). In a generator, instead of being attached to a load, the axle is attached to some kind of spinning energy source (EG- a windmill). This spinning energy source causes the coil to move in the magnetic field, thus inducing an electromotive force which can be fed to an external circuit. DC generators, like motors, use a split ring commutator to reverse the direction of current every half turn. AC Generators simply use
slip rings , maintaining the changing current for the external circuitry.
Look at the slip rings in this MC question from 2014.
The question says it rotates one rotation with the switch open. The slip rings maintain contact with the coil, but with the switch open, no current flows on this first rotation. The answer is immediately restricted to be B!
Comparisons between motors and generators are a common question in the HSC. Essentially, the structure is nearly identical, but the devices fulfil opposite functions.
There are several arguments regarding AC vs DC generators. AC Generators are easier to construct, experience less wear, and can use transformers (this will be covered later). However, they require extensive shielding and insulation to account for eddy currents and induction in surrounding metals. DC Generators also produce more energy per volt. AC Generators are traditionally used for more large scale power production and distribution, DC generators are useful for small scale power production (EG- in car batteries).
You are also asked to understand the AC vs DC argument which occurred between the respective systems of Westinghouse and Edison. This is one of those ones that a lot of students ignore, and therefore, one that BOSTES loves to ask. Be prepared!
Westinghouse was invested in AC current, Edison in DC.
Edison’s DC system:
• Recorded power losses of over one third with limited transmission distance
• Would require a great number of generators, causing higher costs and more pollution
• Could not promise efficiency
While Westinghouse’s AC system:
• Could transmit power over large distances through use of transformers, with less than one percent power loss
• Generators could be constructed near fuel source and power transmitted
• Was arguably more dangerous, but could be cordoned off and protected for small cost
Needless to say, the Westinghouse AC system prevailed and is what we use today.
There are a few more dot points to cover in terms of generators and power distribution, but I am leaving them for the next guide, which will be a big one covering all the
"impact on society" questions, transformers, and AC induction motors.
So, that's all for this guide! Stay tuned for more on the M&G topic, and then Ideas to Implementation! Be sure to register and ask questions, as many as you like, I am happy to help! Happy study! 
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