VCE Physics Question Thread!

[odeaa]

Just came back from a revision lecture today. Yes, you do need to draw from point of application (something I've been avoiding all year LOL).
If they give you an actual diagram (i.e. car) -
 - FOUR normal forces, from the GROUND/PLANE on the TYRES
 - ONE weight force from CENTRE OF MASS.
If you get a block
 - Draw everything from centre of mass, this is a free body diagram.

Unlike Spesh

So the normal reaction for a block is drawn from the middle?

[zsteve]

Oops sorry: normal force for block still drawn from surface IF it is a force diagram. For free-body diagram, from centre of mass.
My mistake

[odeaa]

Oops sorry: normal force for block still drawn from surface IF it is a force diagram. For free-body diagram, from centre of mass.
My mistake

Allg, cheers for clearing that up

[stockstamp]

In Light, when you get questions on the photoelectric effect

If you are asked to find the Max Kinetic Energy, Stopping Voltage, Stopping potential, energy of the electron - do you consider this the same thing?

And if so are the units always the same or can they change?

[odeaa]

In Light, when you get questions on the photoelectric effect

If you are asked to find the Max Kinetic Energy, Stopping Voltage, Stopping potential, energy of the electron - do you consider this the same thing?

And if so are the units always the same or can they change?

Energy is in joules or eV, stopping voltage/potential is in volts
But yes they will be the same value

[zsteve]

How do the VicPhysics explanations solutions work? Do we lose a mark straight if we don't use the same points? Or are they just 'suggested solutions', other valid points may still be credited?
Also would they reflect VCAA's opinion

[zsteve]

Also do we need to know the different merits/disadvantages of photonic devices, e.g. extremely fast response time for photodiode, etc. for the exam? SD is not too clear on this

[knightrider]

Can anyone please explain why in this image attached.

Angle a is equal to angle b

[TheAspiringDoc]

Can anyone please explain why in this image attached.

Angle a is equal to angle b

Because they are alternating angles, within two parallel lines.
I'd recommend googling 

[odeaa]

Also do we need to know the different merits/disadvantages of photonic devices, e.g. extremely fast response time for photodiode, etc. for the exam? SD is not too clear on this

Yes, I've had a question on why you would use a photo diode and the solutions said because a fast response time was needed. Doesn't hurt to chuck it on your cheatsheet anyway

[odeaa]

How do you go about this question? (attached)
cheers

[Calculus]

Hi, I was wondering in the question below why it is incorrect to refer to Lenz's law instead of Faraday's law in your explanation. This question is from 2013 VCAA, question 15. I don't really understand the statement given in the examiner's report.

Question 15

Students are experimenting with an ideal transformer. The circuit is shown in Figure 19.

The primary coil has 1000 turns; the secondary coil has 6000 turns. There is a 1200 Ω resistor in the
secondary circuit. A 3.0 VRMS AC power supply is connected across the primary coil.

The students now modify the circuit, and connect a 3.0 V DC battery and a switch in the primary circuit,
as shown in Figure 20.

d. The students have been asked to observe the current in the resistor as the switch is closed. Before the
switch is closed, there is no current in the resistor. This does not surprise them. When the switch is
closed, there is a very short pulse of current in the resistor. When the switch remains closed, there is no
current in the resistor.

Explain why there is a short pulse of current as the switch is closed and why there is no current in the
resistor as the switch remains closed. No numbers are required in your answer, but you should refer to
the relevant law of physics. 3 marks

Examiner's Report Comment:

When the switch was closed there was a sudden change (increase) in the current. This resulted in a change in the flux. By applying Faraday’s law this flux change induced a voltage and thus current in the secondary coil. When the switch remained closed there was no further change in the current and thus no change in the flux. This resulted in no more voltage or current in the secondary coil.

Some students referred to the Physics principle involved as Lenz’s law instead of Faraday’s law. It was important to refer to a change in flux rather than a change in magnetic field. Many students described how a transformer worked with AC, but this did not address the question.

Figures 19 and 20 are attached.

Thanks very much in advance. 

[odeaa]

Hi, I was wondering in the question below why it is incorrect to state Lenz's law instead than Faraday's law. This question is from 2013 VCAA, question 15. I don't really understand the statement given in the examiner's report.

Question 15

Students are experimenting with an ideal transformer. The circuit is shown in Figure 19.

The primary coil has 1000 turns; the secondary coil has 6000 turns. There is a 1200 Ω resistor in the
secondary circuit. A 3.0 VRMS AC power supply is connected across the primary coil.

The students now modify the circuit, and connect a 3.0 V DC battery and a switch in the primary circuit,
as shown in Figure 20.

d. The students have been asked to observe the current in the resistor as the switch is closed. Before the
switch is closed, there is no current in the resistor. This does not surprise them. When the switch is
closed, there is a very short pulse of current in the resistor. When the switch remains closed, there is no
current in the resistor.

Explain why there is a short pulse of current as the switch is closed and why there is no current in the
resistor as the switch remains closed. No numbers are required in your answer, but you should refer to
the relevant law of physics. 3 marks

Examiner's Report Comment:

When the switch was closed there was a sudden change (increase) in the current. This resulted in a change in the flux. By applying Faraday’s law this flux change induced a voltage and thus current in the secondary coil. When the switch remained closed there was no further change in the current and thus no change in the flux. This resulted in no more voltage or current in the secondary coil.

Some students referred to the Physics principle involved as Lenz’s law instead of Faraday’s law. It was important to refer to a change in flux rather than a change in magnetic field. Many students described how a transformer worked with AC, but this did not address the question.

Figures 19 and 20 are attached.

Thanks very much in advance. 

Lenz law only deals with the direction of the induced current, while faradays law is related to the idea of having a current induced to oppose the change in flux.
So it would be incorrect to cite lenz law by itself because that would only tell you the direction of the emf, whereas Faraday actually explains why there would be no emf without a change in flux

[Calculus]

Lenz law only deals with the direction of the induced current, while faradays law is related to the idea of having a current induced to oppose the change in flux.
So it would be incorrect to cite lenz law by itself because that would only tell you the direction of the emf, whereas Faraday actually explains why there would be no emf without a change in flux

Thanks Odeaa. 

[zsteve]

@Odeaa: Lenz's law specifies emf and hence currently (indirectly). I think explanations have to be done carefully to specify that it is the sign of the voltage, that produces current of a specific direction.

Also, what is the difference between an AC generator and an alternator? From what I've read before, alternator = AC generator, 'generator' = DC generator. However, the TSSM exam in front of me says differently ...