HSC Physics Question Thread

[Brenda0708]

Hi, for Quanta to quarks do we leave our answer for mass defect in kg or amu? And for the energy that that mass defect is equivalent to, do we calculate in in MeV or in J? Alsoooo how many significant figures should I leave my answer in?

[jakesilove]

Hi, for Quanta to quarks do we leave our answer for mass defect in kg or amu? And for the energy that that mass defect is equivalent to, do we calculate in in MeV or in J? Alsoooo how many significant figures should I leave my answer in?

Unless the question specifies, it doesn't matter what units you use! Sig figs do matter though; look at the FEWEST number of sig figs you've got in the question (ie. if mass defect is 1.0g, then you have two sig figs) and use that in your answer.

[Goodwil]

How would we do both parts to this question?

[noonedoesnt]

How would we do both parts to this question?

for part a)
- By using the Np/Ns = Vp/Vs = Is/Ip we see that the largest voltage is induced when the ratio of primary to secondary coils is low. ie, 5 primary turns to 100 secondary. Thus arrangement B would produce a spark, as a large voltage is required to jump the gap

for part b)

using the same formula from above, by moving different lengths, the induced emf is altered. ie, at arrangement a, ratio of primary to secondary coils is much higher than at arrangement b, thus a lower voltage is induced than that of in arrangement b the more of the primary coil is inside the secondary coil.

[FallonXay]

Just a quick question: Does back EMF occur in both DC AND AC motors or only in DC motors?
Thanks.

[Phillorsm]

Hey guys, for the medical physics dot point about bone scans vs x rays,  are bone scans CAT scans or PET scans?
I'm getting some conflicting information.
Thanks.

[jakesilove]

Just a quick question: Does back EMF occur in both DC AND AC motors or only in DC motors?
Thanks.

We say that it only occurs in DC motors, because of what back EMF really is. An opposing current implies a consistent direction, which wouldn't be the case for any induced back EMF in AC motors. Like, there needs to be a consistent current to oppose, if that makes sense. So, we only talk about Back EMF in DC motors.

[jakesilove]

Hey guys, for the medical physics dot point about bone scans vs x rays,  are bone scans CAT scans or PET scans?
I'm getting some conflicting information.
Thanks.

Ahahaha you have no idea how close to home this question hits. In my trials, we got a question about a 'Bone scan', which I had never heard of before. I just assumed it was a PET scan, and I got the marks, but others used CAT scan and also got the marks. As far as I understand, you really can use either/or. However, someone is welcome to correct me!

[Phillorsm]

It makes more sense to be a PET scan haha

[jakesilove]

It makes more sense to be a PET scan haha

Yeah I totally agree

[Mei2016]

How would we do both parts to this question?

This is the 2008 Q26 question, and due to Faraday's law of EM induction ϵ=−ndϕ/dt, as Arrangement A has more number of coils inside it, it has a greater induced EMF, therefore producing a spark in Arrangement A.

For part b) again using Faraday's law, different amounts of coil produces different amounts of EMF

(However, I don't get how this works with  Np/Ns = Vp/Vs = Is/Ip as the answers to the question is Arrangement A)

I think it's something to do with : in Arrangement B, they're showing you that there's no primary coil within the secondary coil so you get Np=0.

[Brenda0708]

Unless the question specifies, it doesn't matter what units you use! Sig figs do matter though; look at the FEWEST number of sig figs you've got in the question (ie. if mass defect is 1.0g, then you have two sig figs) and use that in your answer.

Oh so even if the fewest number of sig figs is 6 we should leave it to 6? The answer in the book did it to 3

[noonedoesnt]

This is the 2008 Q26 question, and due to Faraday's law of EM induction ϵ=−ndϕ/dt, as Arrangement A has more number of coils inside it, it has a greater induced EMF, therefore producing a spark in Arrangement A.

For part b) again using Faraday's law, different amounts of coil produces different amounts of EMF

(However, I don't get how this works with  Np/Ns = Vp/Vs = Is/Ip as the answers to the question is Arrangement A)

well shit, that doesn't make any sense. How is it arrangement A?

Jake can you explain?

[jakesilove]

Oh so even if the fewest number of sig figs is 6 we should leave it to 6? The answer in the book did it to 3

Yep, the book is likely wrong. It's always the fewest number of sig figs in the question!

[jamonwindeyer]

well shit, that doesn't make any sense. How is it arrangement A?

Jake can you explain?

It makes perfect sense let me explain! In Arrangement B, the two coils are completely separated from each other (remember there is no connection between the two besides flux linkage. Thus, when we pull them apart, the EMF in the secondary coil is tiny (the absence of the core being near the secondary coil in Arrangement B makes this worse).

It's like having a transformer with the coils way too far apart, essentially