HSC Physics Question Thread

[bsdfjnlkasn]

Hiii,
Can someone please give me a quick summary of how back emf works? No matter how many times I go through it I just cant seem to get it
Thank you ! 

Hey there, back-EMF isn't all that tricky

Hopefully this explanation helps:

I just thought to go through the basic to make sure you know why back EMF is induced

When a current carrying conductor experiences a change in magnetic flux, an EMF is induced causing a current to flow (if a closed circuit is formed). The interaction between the magnetic fields from the moving charges and external field interact cause the ends of the coil to experience a force. This force (Torque) causes the magnetic flux to continue to change around the coil.

We need a Back EMF to be induced otherwise the coil will continue to turn faster and faster. Think of it this way:

1. The coil experiences a change in flux
2. Current induced in coil
3. Torque experienced
4. Rotation causes increases in magnetic flux
5. Current in coil increases
-------> Cycle continues, with the coil getting faster and faster due to the rate of change in flux continuously increasing. This implies that without adding energy, the coil is gaining speed and so energy. This contravenes the LCE!! :O

This leads us to Lenz's law: According to Lenz's law, which is really just an extension of the LCE, a back EMF will be induced in the coil to oppose the change in flux and ensure that the coil isn't gaining energy without it being supplied. This induced current is otherwise known as induced emf.

For example, if there is an increase in magnetic flux due to the introduction of an external magnetic field, then the back EMF in the coil will be such that it produces a magnetic field in the opposite direction to the incoming field lines.

Or, if there is a decrease in the number of magnetic field lines (flux), then the induced EMF (current, because there's a closed circuit) will be such that a magnetic field will be directed in the way of the decreased magnetic field (so that the magnetic flux can be restored to it's amount before the change).

The idea is that back EMF will always be induced to counteract the change in flux which caused it to be 'created' in the first place. It wants to restore the magnetic flux to the state it was before the change occurred.

If you have any examples that you'd want us to look at, that might be a more proactive way for us to really help refine your understanding.

P.S. Not sure if you do chem, but it's really similar to Le Chatelier's principle

[bsdfjnlkasn]

Hey there!

Does anyone know how I might answer this? I thought of using that change in gravitational potential = -E_k + heat, friction, etc... But still didn't get any of the answers

[edvinat]

Hey there, back-EMF isn't all that tricky

Hopefully this explanation helps:

I just thought to go through the basic to make sure you know why back EMF is induced

When a current carrying conductor experiences a change in magnetic flux, an EMF is induced causing a current to flow (if a closed circuit is formed). The interaction between the magnetic fields from the moving charges and external field interact cause the ends of the coil to experience a force. This force (Torque) causes the magnetic flux to continue to change around the coil.

We need a Back EMF to be induced otherwise the coil will continue to turn faster and faster. Think of it this way:

1. The coil experiences a change in flux
2. Current induced in coil
3. Torque experienced
4. Rotation causes increases in magnetic flux
5. Current in coil increases
-------> Cycle continues, with the coil getting faster and faster due to the rate of change in flux continuously increasing. This implies that without adding energy, the coil is gaining speed and so energy. This contravenes the LCE!! :O

This leads us to Lenz's law: According to Lenz's law, which is really just an extension of the LCE, a back EMF will be induced in the coil to oppose the change in flux and ensure that the coil isn't gaining energy without it being supplied. This induced current is otherwise known as induced emf.

For example, if there is an increase in magnetic flux due to the introduction of an external magnetic field, then the back EMF in the coil will be such that it produces a magnetic field in the opposite direction to the incoming field lines.

Or, if there is a decrease in the number of magnetic field lines (flux), then the induced EMF (current, because there's a closed circuit) will be such that a magnetic field will be directed in the way of the decreased magnetic field (so that the magnetic flux can be restored to it's amount before the change).

The idea is that back EMF will always be induced to counteract the change in flux which caused it to be 'created' in the first place. It wants to restore the magnetic flux to the state it was before the change occurred.

If you have any examples that you'd want us to look at, that might be a more proactive way for us to really help refine your understanding.

P.S. Not sure if you do chem, but it's really similar to Le Chatelier's principle

Hey! Thank you so so much! I think i've just never understood it before because no ones ever properly explained it to me, but it makes a lot more sense now. Thanks again, you're a legend  

[jamonwindeyer]

Hey there!

Does anyone know how I might answer this? I thought of using that change in gravitational potential = -E_k + heat, friction, etc... But still didn't get any of the answers

Hey! So at this height, probably okay to use simplified GPE formula to calculate the energy lost:



Kinetic energy gained is:



Difference between these is about \(5.34\text{MJ}\), so the answer would be C. Slightly off, again, because of using the simplified version of GPE (we can see it didn't really make a huge difference)

[Baylsskool]

I have a question that involves the explanation of why in the suggested answers they(markers) think that nuclear fusion produces energy or expels energy, because it doesn't right?, it absorbs and the question specifically refers to releasing energy so are they wrong to put that in there, or am I being too specific??

[Baylsskool]

Are control rods a type of moderator?? Cause like deuterium is heavy water, a moderator, but absorb neutrons to slow them down?? Is that not what control rods do?

[samorchard]

Hi, I can't figure out why radius equals the distance in t = fd for this question (29b 2013)

A 0.05 kg mass is lifted at a constant speed by a DC motor. The motor has a coil of 100 turns in a 0.1 T magnetic field. The area of the coil is 0.0012 m2. The motor shaft has a radius of 0.004 m

a)  F= 0.49N

b) Calculate the minimum current required in the coil to lift the mass.
t=Fd

[samorchard]

Are control rods a type of moderator?? Cause like deuterium is heavy water, a moderator, but absorb neutrons to slow them down?? Is that not what control rods do?

Moderators slow down neutrons so that they can interact with the fissile material and cause fission. They don't abosrb, only slow down the neutron through inelastic collisions. Control rods absorb the neutrons and hence can control the reaction, slowing down or stopping the chain reaction whereas the moderators can't do this.

Hope that helps

[Baylsskool]

Moderators slow down neutrons so that they can interact with the fissile material and cause fission. They don't abosrb, only slow down the neutron through inelastic collisions. Control rods absorb the neutrons and hence can control the reaction, slowing down or stopping the chain reaction whereas the moderators can't do this.

Hope that helps

Yeah that helps, so is it fair to say that through inelastic collisions, deuterium momentarily absorbs a neutron becoming tritium but the collision is too great so it has to let go of the neutron but does so at a slower speed?

[ajd938]

Aim/purpose of the michelson morley experiment??? Was it to prove the existence of the aether or measure the relative velocity of earth through the aether?? All websites I have checked give a different answer. pls help

[Baylsskool]

Aim/purpose of the michelson morley experiment??? Was it to prove the existence of the aether or measure the relative velocity of earth through the aether?? All websites I have checked give a different answer. pls help

Should've been just to prove the existence of the aether wind by doing the light and mirror experiment where the light came back to the eye in equal increments after being separated, because they didn't come back at different times, there was the conclusion of no aether, since the conclusion answers the aim, the conclusion was there was no aether and therefore the aim was proving the existence of the aether.

[jamonwindeyer]

Hi, I can't figure out why radius equals the distance in t = fd for this question (29b 2013)

A 0.05 kg mass is lifted at a constant speed by a DC motor. The motor has a coil of 100 turns in a 0.1 T magnetic field. The area of the coil is 0.0012 m2. The motor shaft has a radius of 0.004 m

a)  F= 0.49N

b) Calculate the minimum current required in the coil to lift the mass.
t=Fd

Hey! The force due to the mass pulling down on the pulley is what generates the torque - \(F\) generating \(\tau\). The formula linking those is the distance from where the force is applied, and the centre of the rotation the torque is causing. That, in this case, is the radius!

[samorchard]

Yeah that helps, so is it fair to say that through inelastic collisions, deuterium momentarily absorbs a neutron becoming tritium but the collision is too great so it has to let go of the neutron but does so at a slower speed?

I'm not too sure about that reaction, i think it would be safer to say that the neutron transfers some of its kinetic energy to the deuteriumin the collision.
The deuterium shouldn't absorb the neutron at all, as moderators are made from materials with light nuclei. If it did absorb the neutron then the most likely reaction would be a fission reaction and that is not the case.

[samorchard]

Hey! The force due to the mass pulling down on the pulley is what generates the torque - \(F\) generating \(\tau\). The formula linking those is the distance from where the force is applied, and the centre of the rotation the torque is causing. That, in this case, is the radius!

MAkes sense, thankyou!

[Dante1091]

Hello guys,

From the dotpoint: Identify data sources, gather, process, analyse information and use available evidence to assess the impact
of the invention of transistors on society with particular reference to their use in microchips and
microprocessors

How much of the uses in microchips and microprocessors would need to be known, since I only know of the impacts of transistors onto society