HSC Physics Question Thread

[itssona]

hi could someone please tell me why its not anticlockwise, and instead, its clockwise?
I used the grip rule and got anticlockwise. How do I use the right hand palm rule?

[Dragomistress]

Hey, what would be an appropriate response to, "Assess the role of split ring commutator in allowing DC motors to spin continuously in one direction"

[jamonwindeyer]

hi could someone please tell me why its not anticlockwise, and instead, its clockwise?
I used the grip rule and got anticlockwise. How do I use the right hand palm rule?

Grip rule gives me clockwise! We are introducing a field out of the page, so we want a field into the page to compensate. Point your right thumb into the page, and your right fingers should be wrapping clockwise!

Hey, what would be an appropriate response to, "Assess the role of split ring commutator in allowing DC motors to spin continuously in one direction"

Hey! In a sentence, "It reverses current direction every half turn to maintain a constant direction of torque." Essentially, if we didn't have the split ring commutator, the current direction would not switch. This means the forces on each wire would stay in the same direction, which means after every half turn, they'd start pushing the coil back in the other direction. The split ring commutator makes that switch to keep the torque generated in the same direction

[clovvy]

Hi,
I was reading through ATAR Notes physics ((Jamon's notes)...  One of the syllabus heading says 'conduct an investigation to verify the effects of distance, magnetic field strength, and relative motion, on electromagnetic induction in a coil by a moving magnet'...  I liked the explanations on this but I could not find this anywhere in the syllabus...  Can anyone explain? Thanks

[blasonduo]

Hi,
I was reading through ATAR Notes physics ((Jamon's notes)...  One of the syllabus heading says 'conduct an investigation to verify the effects of distance, magnetic field strength, and relative motion, on electromagnetic induction in a coil by a moving magnet'...  I liked the explanations on this but I could not find this anywhere in the syllabus...  Can anyone explain? Thanks

Hello! He has just simplified this dot point:

plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when:

– the distance between the coil and magnet is varied
– the strength of the magnet is varied
– the relative motion between the coil and the magnet is varied

[radnan11]

Hi, AN
Do the brushes in a DC motor necessarily need to be made out of graphite(carbon) or can they be other materials eg. copper?

Thanks

[jamonwindeyer]

Hi, AN
Do the brushes in a DC motor necessarily need to be made out of graphite(carbon) or can they be other materials eg. copper?

Thanks

Hey there radnan11! Can definitely be made of anything that conducts electricity - And it depends on the application as well. Copper brushes tend to have better conductance, so sometimes they'll be used. Usually though, as you say, it's a carbon brush (usually with some copper powder in there as well, to make it slightly less resistive)

Lots of reasons for that preference. The physical composition of these is better for brushes as it is a little softer (will wear away itself instead of wearing away the commutator), has a higher melting point (the arcs that occur in brushed motors get hoooooot, need to make sure yo stuff don't melt), and tends not to have as much friction at the contact. The slightly higher resistance of these brushes is also good for dealing with the large currents that exist while the motor is starting

^ None of that is assessable knowledge, but since you are asking about material which is already sort of beyond the syllabus, thought I'd give you some background

[stels]

Hi, I'm writing up a practical for the dot point:
"Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when the distance between the coil and magnet is varied, the strength of the magnet is varied and the relative motion between the coil and the magnet is varied"

For the hypothesis, I need 3 predictions with 3 correct explanations, so basically 3 sentences.

The theory is:
At smaller distances, the magnetic field is denser, meaning more flux lines will be cut and so a greater EMF will be induced.

So can I just add the words "It is predicted" in front of the statement above?
"It is predicted that at smaller distances, the magnetic field is denser, meaning more flux lines will be cut and so a greater EMF will be induced."
...or is that too limited for a hypothesis since I didn't say anything about larger distances, should I edit anything?

For the other two hypothesis:
It is predicted that through increasing the number of identical magnets, a stronger magnetic field will be created, meaning greater amount of flux is cut (is there another word for this since I don't entirely get what it means when a flux is cut), and because of this, it induces a greater EMF.
It is predicted that as the relative motion becomes greater, there will be a greater induced EMF because there is a faster change in flux.

ALSO, I need 2 correctly identified risks and two minimisation strategies. I feel like this experiment is fairly safe, no safety glasses were needed or anything.

[Fizzycyst]

For all your 'theory' statements, make sure you link to the rate at which flux lines are cut, not just the cutting of flux lines. So, more flux lines cut per second or per unit time and the dot point wants current, not EMF.

I have always taught that hypotheses are just a possible answer to the aim, it is a statement, no need to write it is predicted, but different teachers do different things. Just say "as the distance between the magnet and the coil decreases..." tbh, you shouldnt even need to mention about rate of change of flux as it is not an explanation, it is legit just "what would happen", that is that the current should increase.

Relative motion refers to both speed and direction, you have identified speed, but what if the direction of relative motion was changed?

Without knowing how you will be conducting the prac, its pree impossible to help with hazard minimisation -- how do you intend on undertaking the prac? It's not an easy prac to do well! You need to keep in mind appropriate strategies to control variables which you do not want to change, for example for one set of results you want to change strength of magnet (pretty easy), but how will you keep the other things (distance and relative motion) controlled for this?

[kaustubh.patel]

Hwy a question about AC induction motor. If the main reason for the induction motor to work is due to the induced eddy currents creating a magnetic which interacts with the rotating magnetic field then why are there laminated discs in the squirrel cage. To enhance the eddy currents there shouldn't be laminated discs so the magnetic field those large eddy current produce will have a greater strength and thus have greater force output.

[jamonwindeyer]

Awesome first post (and username) from Fizzycyst! Want to emphasise this bit especially:

tbh, you shouldnt even need to mention about rate of change of flux as it is not an explanation, it is legit just "what would happen", that is that the current should increase.

I was taught the same for a hypothesis, that it doesn't need to explain what you will observe, just to go through the observations themselves but roll with your teacher since they will mark!
As some ideas for risks/prevention strategies:

- You will be using electricity in this experiment, which presents the risk of electrocution. Ensure that apparatus is not connected directly to mains power, and if possible, has circuit breakers/protection devices built in (otherwise, ensure the mains supply has these). Ensure there is no water near the experiment.

- You will be moving magnets quickly in this experiment, which means you might need to protect yourself against projectiles if you slip. Safety goggles

[jamonwindeyer]

Hwy a question about AC induction motor. If the main reason for the induction motor to work is due to the induced eddy currents creating a magnetic which interacts with the rotating magnetic field then why are there laminated discs in the squirrel cage. To enhance the eddy currents there shouldn't be laminated discs so the magnetic field those large eddy current produce will have a greater strength and thus have greater force output.

Hey! Great question - The HSC simplifies the construction of a squirrel cage rotor a bit which is why this gets a little tough. So:

- A squirrel cage rotor has a core made up of laminations (the bit you are talking about), but the actual squirrel cage itself is made of conductive bars that aren't laminated.
- The purpose of that core is to direct the magnetic field efficiently through the conductive bars of the squirrel cage, thus inducing a current and... You know the rest.
- We want all of that energy (or as much as possible) going to the conductive bars to be useful, meaning we need to minimise any induced currents in the core. Exactly the same as a transformer. This is why the rotor has laminations.

tl;dr - The laminations are only in specific places on the squirrel cage rotor that stop currents flowing unless they will directly assist with rotation

[justwannawish]

Hey,

Was just wondering what angle the theta in torque=NBIAcos theta was referring to? I know torque is max when theta=0 but when does that happen? And is it the opposite angle to F=BILsintheta?

Also I'm a bit confused with how to interpret coil diagrams in terms of torque? E.g. Questions like the attached photo don't make sense!!

[Fizzycyst]

In the torque equation theta refers to the angle between the plane of the coil and the magnetic field, which would be the theta as shown in the question.

The theta in the force equation, is the angle between the 'current carrying conductor' or flow of charged particles and the external magnetic field. In the diagram, the conductor itself is going into / out of the page and the field is going across the page, these two directions are perpendicular and hence theta = 90. When you do your right hand palm rule it is the angle made between your thumb and your index finger.

Basically, as a rule of thumb (Yes, pun intended), in motors the angle in the force equation is 90 degrees all the time through the rotation of the coil in the field.

[jamonwindeyer]

In the torque equation theta refers to the angle between the plane of the coil and the magnetic field, which would be the theta as shown in the question.

Yep! But careful justwannawish, that \(\theta\) angle in the image isn't what we need. It's the other part of that 90 degree angle, so the angle formed between the coil and the magnetic field, as I've marked in green down here

(Love your work, Fizzycyst )