HSC Physics Question Thread

[jamonwindeyer]

A motor is connected in series with an ammeter and a 12V power supply. The internal resistance of the motor is 1.5 Ohms. When the motor is running at full speed, the ammeter reads 3.0 A. Account for this.

Hey! So this is a Back EMF thing - If you use Ohm's Law with \(V=12\) and \(R=1.5\) -> You get \(I=8\)A! But when the motor is running, you get Back EMF induced, which will subtract from the supply current - In this case, it has to be \(I_\text{Back}=5\)A, since that is what the ammeter measures as the total current when it is running

There's a bit of explanation on Back EMF here, in an old guide I wrote many a year ago

[clovvy]

Hey! So this is a Back EMF thing - If you use Ohm's Law with \(V=12\) and \(R=1.5\) -> You get \(I=8\)A! But when the motor is running, you get Back EMF induced, which will subtract from the supply current - In this case, it has to be \(I_\text{Back}=5\)A, since that is what the ammeter measures as the total current when it is running

There's a bit of explanation on Back EMF here, in an old guide I wrote many a year ago

That's weird, the answer for this is 7.5 and where did 5A come from?

[Fizzycyst]

5A would be the back current, to find back EMF multiply by 1.5 which gives 7.5V as you state.

5A came from the expected current (12/1.5) minus the measured current (3A).

It’s worthwile to note the formula EMFnet = EMFsupply - EMFback and note this can be used also with current, which is essentially what Jamon did.

That's weird, the answer for this is 7.5 and where did 5A come from?

[jamonwindeyer]

That's weird, the answer for this is 7.5 and where did 5A come from?

Weird indeed - Not sure where they'd get 7.5A from:



Edit: Ahh, they want the EMF not the current - Fair enough!! It's a written response so they'd be happy with either or there, they are more after your account for why it is happening in the first place

[clovvy]

Weird indeed - Not sure where they'd get 7.5A from:



Edit: Ahh, they want the EMF not the current - Fair enough!! It's a written response so they'd be happy with either or there, they are more after your account for why it is happening in the first place

It's in volts not amps

[jamonwindeyer]

It's in volts not amps

Yep, clicked as soon as I saw Fizzycyst's response, see my edit above - Make sure you can explain it that's the main thing they'll be looking for! Since it is an account question they want the reasons, the maths should only be there as a support

[jasn9776]

Is it true that when the current stops suddenly or changes direction, a back-emf is induced?

So the exam question asked: Why when the load of DC motor is increased, it is forced to slow down, the torque it applies onto the load increases.

Is it enough to say the back emf is reduced because of slowed turning, therefore slow change in magnetic flux, thus lower emf produced. Vnet=Vsupply - Vback. V=IR as Vnet goes up I goes up. torque=BIAcostheta.

But the marking guidelines said the reversal of current causes back emf every-time. As the DC commutator reverses direction of current, back-emf is produced. When under load, spins slower therefore reversal of current direction is less frequent, hence actual voltage is greater.

Which one is better?

[danaodish]

Hi!
Just quickly, my teacher said that we'd be given the 2019 Data sheet for this years (2018) HSC... I don't think that's true but does anyone know if it is? 

[RuiAce]

Hi!
Just quickly, my teacher said that we'd be given the 2019 Data sheet for this years (2018) HSC... I don't think that's true but does anyone know if it is? 

That seems absurd since it literally says that it's the data sheet for 2019, but here's the link.

(Pending someone more qualified to re-answer your question though.)

[jamonwindeyer]

Hi!
Just quickly, my teacher said that we'd be given the 2019 Data sheet for this years (2018) HSC... I don't think that's true but does anyone know if it is? 

I am also skeptical that this is true! Either way, it has all the values you'll need (and more). You'll definitely get the current formula sheet since there are formulas on the old one that aren't on the new one

But yeah, I'd guess there's been a misunderstanding somewhere along the line

[jamonwindeyer]

Is it true that when the current stops suddenly or changes direction, a back-emf is induced?

So the exam question asked: Why when the load of DC motor is increased, it is forced to slow down, the torque it applies onto the load increases.

Is it enough to say the back emf is reduced because of slowed turning, therefore slow change in magnetic flux, thus lower emf produced. Vnet=Vsupply - Vback. V=IR as Vnet goes up I goes up. torque=BIAcostheta.

But the marking guidelines said the reversal of current causes back emf every-time. As the DC commutator reverses direction of current, back-emf is produced. When under load, spins slower therefore reversal of current direction is less frequent, hence actual voltage is greater.

Which one is better?

Hiya! I like the first version with the formulas waaaay better. A current changing direction doesn't really generate back emf. There would be induced current flow in conductive parts of the motor because of that switch, yes. But like...

Yeah, I'm clutching at straws here trying to give that answer some credit, but I don't really get it. The first one seems far better to me where's this marking criteria from?

[jasn9776]

Hiya! I like the first version with the formulas waaaay better. A current changing direction doesn't really generate back emf. There would be induced current flow in conductive parts of the motor because of that switch, yes. But like...

Yeah, I'm clutching at straws here trying to give that answer some credit, but I don't really get it. The first one seems far better to me where's this marking criteria from?

From QATs 2015
Describe the change in current direction through the coils of a DC motor
Relates this to reduced operating voltage or current due to back-emf.
Relates this slowing down of the motor to increased current or torque.
Part of sample answer
"The reversal of direction of current through the coil causes back-emf every time, meaning that the operating voltage is always less than the applied voltage. When a load forces the motor to slow down the reversal of current direction through the coils is less frequent, hence the actual voltage - and current - is greater, hence torque is increases."

Also is a universal motor using AC unable to increase torque? The next question says the high rate of current reversal when AC is applied means that the slower rate of coils turning has little effect on back-emf experienced in the coils.

[jamonwindeyer]

From QATs 2015
Describe the change in current direction through the coils of a DC motor
Relates this to reduced operating voltage or current due to back-emf.
Relates this slowing down of the motor to increased current or torque.
Part of sample answer
"The reversal of direction of current through the coil causes back-emf every time, meaning that the operating voltage is always less than the applied voltage. When a load forces the motor to slow down the reversal of current direction through the coils is less frequent, hence the actual voltage - and current - is greater, hence torque is increases."

Interesting! Yeah, as I said, not a fan - Maybe I'm just not clicking with what it means but I'd be putting red crosses on that if I were marking it, especially next to the other answer you gave which I think makes more sense (and is actually correct, aha).

Also is a universal motor using AC unable to increase torque? The next question says the high rate of current reversal when AC is applied means that the slower rate of coils turning has little effect on back-emf experienced in the coils.

Sorry my friend, I've literally got no idea what the heck this exam is going on about 

(Universal motors can vary their torque, but I'd say you'd be hard pressed to find that asked anywhere near a HSC exam)

[itssona]

can anyone explain why in 2009 Q21 (a) the answer is X?? how do we know which way to current is going in order to see that the force on X is up? thanks
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-physics.pdf

[Floatzel98]

can anyone explain why in 2009 Q21 (a) the answer is X?? how do we know which way to current is going in order to see that the force on X is up? thanks
https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-physics.pdf

Look at the orientation of the battery.