HSC Physics Question Thread

[bsdfjnlkasn]

Step into my office

Legit, the moment in my first year ELEC lecture when I finally figured out why this is, best moment ever. Not difficult! Just a little intricate.

Right, so \(P=VI\) gives the power dissipated/lost in a circuit element (EG - the wires of a transmission network), as a product of the current through the element and the voltage across the element. The formula is identical to \(P=I^2R\) and \(P=\frac{V^2}{R}\). So why does only one work?

Notice what I emphasised in the text above - Across the element. Meaning, the voltage at one end minus the voltage at the other, the potential difference. This goes right back to Electrical Energy in the Home, circuit analysis in Year 11 involved analysing voltage drops across resistors.

The problem is that we are almost always given the voltage going into the transmission wires, never the voltage ACROSS those wires. We might put 100,000V into the wires, but that's not the voltage across them. The voltage across them is the difference between 100,000V and the voltage at the other end, which is calculable as \(V=IR\), Ohm's Law!

If we use the voltage across the wires, those other formulas work - \(P=VI\) and \(P=\frac{V^2}{R}\). If we just use the input voltage, they break. THIS is why we usually use \(P=I^2R\), because we don't need to look at the other end of the wires. We know how much current goes in, we know how much resistance there is - And that is all we need

This is really hard until you have a click moment, then it is really easy. If it is still a little confusing let me know, in which case I'll do a numerical example using Year 11 techniques to show you the difference

Hey Jamon,

This definitely is a gap in my knowledge from Year 11 so I would really appreciate some additional examples (even though some really bright people have already moved on ). I just never understood the reason or method for calculating potential drops across resistors, so any help would be greatly appreciated 

[Sukakadonkadonk]

Hey guys, 

Could someone please explain to me what an electromotive force actually is? I keep seeing it used in strange ways and would appreciate an accurate definition.

Thanks!!

[jamonwindeyer]

Hey Jamon,

This definitely is a gap in my knowledge from Year 11 so I would really appreciate some additional examples (even though some really bright people have already moved on ). I just never understood the reason or method for calculating potential drops across resistors, so any help would be greatly appreciated 

Sure!! I think a worked example in a video will be best - I'll put something together tonight and upload it

[jamonwindeyer]

Hey guys, 

Could someone please explain to me what an electromotive force actually is? I keep seeing it used in strange ways and would appreciate an accurate definition.

Thanks!!

Hey! So basically, it's a voltage. It's an abstract concept; force does not mean Newtons, but instead energy per unit charge. So, an electromotive force is something that gives things with charge, energy. Just like gravity gives things with mass, energy!

This could be a voltage generated by a battery, or induction, or an electric field. But it's a voltage, an electric field, and it causes things with charge to gain energy!

It is genuinely difficult to explain this in a semi-informal way, without the proper formal definitions of it (which wikipedia has and does a decent job with if you like), but does that help at all?

[jakesilove]

Hey! So basically, it's a voltage. It's an abstract concept; force does not mean Newtons, but instead energy per unit charge. So, an electromotive force is something that gives things with charge, energy. Just like gravity gives things with mass, energy!

This could be a voltage generated by a battery, or induction, or an electric field. But it's a voltage, an electric field, and it causes things with charge to gain energy!

It is genuinely difficult to explain this in a semi-informal way, without the proper formal definitions of it (which wikipedia has and does a decent job with if you like), but does that help at all?

Just to add to that; without a formal understanding of the underlying mathematics, it's really hard to explain the difference between EMF and voltage. In fact, I'm supposed to understand the underlying mathematics and I'm still bloody confused. So, for the sake of the HSC, think of it as the push on an electron

[strawberriesarekewl]

Hey jake/jamon

How is the physics poster going? Also how do I study effectively by looking at a poster? (serious question because I am genuinely curious to know)

I like how the chemistry one was super summarised and what not but I think those posters just have the bare bones

[jamonwindeyer]

Hey jake/jamon

How is the physics poster going? Also how do I study effectively by looking at a poster? (serious question because I am genuinely curious to know)

I like how the chemistry one was super summarised and what not but I think those posters just have the bare bones

A poster is always going to be bare bones content - To put all the minor details of the whole course on a single piece of paper would cover a wall, aha The poster isn't meant to be the same as the set of notes - The notes cover the intricacies, the poster is a (super effective) tool for memorising the basic concepts in a visual and succinct way. You can't teach yourself the course from the poster, like you could the Notes

Having the posters in places you view often (above your study space, for example), gives you the content in an easy to digest format. I used to have posters above my bed for main concepts and formulas - I'd just study them while I was moving around my room, just to jog my memory for free while I wouldn't normally be studying

[Sukakadonkadonk]

Hey! So basically, it's a voltage. It's an abstract concept; force does not mean Newtons, but instead energy per unit charge. So, an electromotive force is something that gives things with charge, energy. Just like gravity gives things with mass, energy!

This could be a voltage generated by a battery, or induction, or an electric field. But it's a voltage, an electric field, and it causes things with charge to gain energy!

It is genuinely difficult to explain this in a semi-informal way, without the proper formal definitions of it (which wikipedia has and does a decent job with if you like), but does that help at all?

Yep, thanks Jake, Jamon,
Definitely a better description than some other resources I've used.

So if during an exam, they ask a question on EMF's would the first few of your sentences be a good way of answering it?

[Bubbly_bluey]

hello! i'm having a great deal of trouble trying to figure out how "induced currents in rings and loops" work. I did a worksheet in class (photo) with answers but i obviously don't know whats going on. How do you determine the direction of the induced current of rings when they enter into a denser or less dense magnetic flux?

Another question: I know 3 hand grip rules
1) right hand palm slap rule
2) right hand grip rule
3) Left hand palm rule
I keep getting confused when to use which rule to use.

[jamonwindeyer]

Yep, thanks Jake, Jamon,
Definitely a better description than some other resources I've used.

So if during an exam, they ask a question on EMF's would the first few of your sentences be a good way of answering it?

As a backwards way of answering your question, I highly doubt they'd ask it

But if I got:

Question: What is an electromotive force?
Answer: A potential difference/electric field which increases the energy per unit charge of objects within said field.

Or, something similar to that but yep, highly doubt they'd ask it, just too finicky imo!

[jamonwindeyer]

hello! i'm having a great deal of trouble trying to figure out how "induced currents in rings and loops" work. I did a worksheet in class (photo) with answers but i obviously don't know whats going on. How do you determine the direction of the induced current of rings when they enter into a denser or less dense magnetic flux?

Another question: I know 3 hand grip rules
1) right hand palm slap rule
2) right hand grip rule
3) Left hand palm rule
I keep getting confused when to use which rule to use.

Hey! This is a really tricky thing, I remember doing a really similar task and having heaps of trouble. Thankfully, it doesn't get asked much in the HSC!

So in the scenarios shown on your sheet, you need to ask yourself one question: What extra magnetic field lines are being added to your loop? Is it additional field lines INTO the page, or OUT OF the page. The current will flow to create magnetic field lines in the loop in the opposite direction to what is being introduced - That is Lenz's Law.

Let's do Loop B as an example. It is moving into a less dense magnetic field, so, removing lines into the page. We can analogise this to mean adding lines out of the page - It means the same thing! So, the current will act to introduce lines INTO the page, to do the opposite to what the movement is doing.

Here, we turn to the right hand grip rule. We need to introduce lines INTO the page, meaning the North Pole should face INTO the page as well (remember, magnetic field lines flow towards north inside a loop/coil). So, our thumb faces into the page, which means our fingers wrap clockwise - There's your current direction

So there is two steps: Figure out the change that is being introduced; the current will do the opposite. Then, use the right hand grip rule (as shown above) to yield the current direction.

Oh, and on those rules:

Right Hand Grip: Direction of current in a loop/coil/solenoid
Right Hand Slap: Direction of current in any other scenario
Left Hand Slap: Don't use it unless you are confident, but it is the direction of electron flow in any other scenario. Think eLEFTron

Hope this helps!

[kiwiberry]

Hey! This is a really tricky thing, I remember doing a really similar task and having heaps of trouble. Thankfully, it doesn't get asked much in the HSC!

So in the scenarios shown on your sheet, you need to ask yourself one question: What extra magnetic field lines are being added to your loop? Is it additional field lines INTO the page, or OUT OF the page. The current will flow to create magnetic field lines in the loop in the opposite direction to what is being introduced - That is Lenz's Law.

Let's do Loop B as an example. It is moving into a less dense magnetic field, so, removing lines into the page. We can analogise this to mean adding lines out of the page - It means the same thing! So, the current will act to introduce lines INTO the page, to do the opposite to what the movement is doing.

Here, we turn to the right hand grip rule. We need to introduce lines INTO the page, meaning the North Pole should face INTO the page as well (remember, magnetic field lines flow towards north inside a loop/coil). So, our thumb faces into the page, which means our fingers wrap clockwise - There's your current direction

So there is two steps: Figure out the change that is being introduced; the current will do the opposite. Then, use the right hand grip rule (as shown above) to yield the current direction.

Oh, and on those rules:

Right Hand Grip: Direction of current in a loop/coil/solenoid
Right Hand Slap: Direction of current in any other scenario
Left Hand Slap: Don't use it unless you are confident, but it is the direction of electron flow in any other scenario. Think eLEFTron

Hope this helps!

Just to add, the right hand palm/slap rule is also used to find the direction of the force experienced by a current carrying conductor in a magnetic field - ie the motor effect

[jamonwindeyer]

Just to add, the right hand palm/slap rule is also used to find the direction of the force experienced by a current carrying conductor in a magnetic field - ie the motor effect

Ahh yep yep, good catch kiwiberry, tah!

[strawberriesarekewl]

I know this goes back to prelim but how do we determine the direction of certain things in the space and motors and generators module

[jamonwindeyer]

I know this goes back to prelim but how do we determine the direction of certain things in the space and motors and generators module

You might have to be a bit more specific than "certain things," aha