HSC Physics Question Thread

[jamonwindeyer]

Hi for back emf, so when a motor is running at a constant speed with no load attached, the back emf will equal the supply emf and so the net emf is zero. Thus current in the coil should be zero right? ( http://www.physics.usyd.edu.au/teach_res/hsp/u6/t6_lenz.pdf )
But if this is the case, how come current is measured when an ammeter is attached to the motor? Is this because the galvanometer is considered a load?

TIA

Because in real life, we have friction.

If we want a motor to keep spinning, we actually need a supply EMF slightly larger than the Back EMF, to provide a torque large enough to overcome friction

If anyone has any unanswered questions from above, please let me know, because I can't see if anything got missed!

[Cindy2k16]

Because in real life, we have friction.

If we want a motor to keep spinning, we actually need a supply EMF slightly larger than the Back EMF, to provide a torque large enough to overcome friction

If anyone has any unanswered questions from above, please let me know, because I can't see if anything got missed!

I see thank you!

[Albertenouttaten]

Wait so in HSC should we say that:
 1) Back emf equals supply emf, resulting in zero net current in coil ,or
2) Supply emf is slightly greater than the back emf?

[JemexR]

Quick last minute quetsion; can someone differentiate Faraday from Lenz for me please?

[JemexR]

Wait so in HSC should we say that:
 1) Back emf equals supply emf, resulting in zero net current in coil ,or
2) Supply emf is slightly greater than the back emf?

Back EMF is produced due to lenz's law, and is essentially the force stopping the motor from just going on forever (conservation of energy). Hence supply EMF is greater than the back EMF; otherwise the motor won't run

[Albertenouttaten]

Quick last minute quetsion; can someone differentiate Faraday from Lenz for me please?

Faradays law: When a conductor experiences a change in flux, an Emf is induced

Lenz law (sort of like a consequence of lenz law, not sure how to word it): If the circuit is closed, the conductor gives rise to a current which creates a magnetic field to oppose the original change in flux.

So Faraday's just talks about an Emf being induced however Lenz law talks about a current and why it produces a mag field.

[nimasha.w]

For your second one to demonstrate the production of AC, this is going to be very simplified and basic, but I reckon something along the lines of moving a magnet in and out of a solenoid connected to a galvanometer should do. Unless the moderators, the actual smart people, have a genius way of producing an alternating current .

could you please expand on this if possible :-)

[jamonwindeyer]

Wait so in HSC should we say that:
 1) Back emf equals supply emf, resulting in zero net current in coil ,or
2) Supply emf is slightly greater than the back emf?

If they specify a motor with friction, (2), if they specify an ideal motor, (1) you can justify your choice either way, if it is an MC they will alert you to which

[jamonwindeyer]

could you please expand on this if possible :-)

If you move a magnet near a solenoid, you expose the solenoid to a changing magnetic flux. This generates an induced EMF (and current) according to Faraday's Law. Moving the magnet in, then out, generates current in opposite directions each time. You've generated AC

[proficles]

Hi, for the mass defect, do you include the mass of electrons?
thanks in advance

[$Billi$]

Hey man can you quickly explain which one is T₀ and which one is t_v in time dilation formula? Appreciate it bro

[jamonwindeyer]

Hey man can you quickly explain which one is T₀ and which one is t_v in time dilation formula? Appreciate it bro

Think of T0 as the observer, (o=observer), the thing that ISN'T moving at relativistic speed. Then tv is moving at speed.

Remember though, we could say "An astronaut on a ship moving at 0.9c measures a time of 16 days on his ship. How long is that on earth?"

In that scenario, we have to let the astronaut be the observer (t0), because he's measured it in his own reference frame. Thus, the earth is actually the thing moving past HIM, that's the idea of relativity

Try to get a sense of whether your time should get bigger or smaller; that should help prevent errors!

[$Billi$]

Think of T0 as the observer, (o=observer), the thing that ISN'T moving at relativistic speed. Then tv is moving at speed.

Remember though, we could say "An astronaut on a ship moving at 0.9c measures a time of 16 days on his ship. How long is that on earth?"

In that scenario, we have to let the astronaut be the observer (t0), because he's measured it in his own reference frame. Thus, the earth is actually the thing moving past HIM, that's the idea of relativity

Try to get a sense of whether your time should get bigger or smaller; that should help prevent errors!

You're a life saver 🔑!! Thanks alot

[nancy_cc]

For Einstein's contribution to quantum theory, am I missing anything?
- extended quantum theory to all forms of EMR not just black body radiation
- proposed light interacted with matter as photons (and hence proposed the photoelectric effect)
(This is for the ideas to implementation topic )

Hey i think you might have been missed! If you want to look at Jamon's guide to the photoelectric effect I've attached a link to the forum below, you can check what you have against what he said

Physics: A Complete Guide to the Course!

[jamonwindeyer]

Hey i think you might have been missed! If you want to look at Jamon's guide to the photoelectric effect I've attached a link to the forum below, you can check what you have against what he said

Physics: A Complete Guide to the Course!

Legend Nancy! Thanks for the help!