June 03, 2020, 10:21:51 am

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#### K.Smithy

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##### Re: QCE Physics Questions Thread
« Reply #15 on: April 28, 2020, 10:50:56 pm »
+3
Question: A charged ball of mass 0.32 g hovers 2.0 cm above a charged plate that has a charge of −4.72 × 10-7 C. Calculate the magnitude of the charge on the ball.

Hey A.Rose!

I'm gonna start off by saying that: I am not super confident in physics right now! It is just one of those subjects I have put on the back burner, and have been meaning to work on. So, I would take my reply with a grain of salt (just incase I have no clue what I'm talking about).

Anywho, I don't remember mass ever coming into play for questions like these, so I believe that is just an added (and unnecessary) piece of information trying to throw you off. So, I would ignore the 0.32g. If you are a bit rusty, as you said, and have no clue whether or not the mass is necessary, your go to is the formula sheet If you look under the "electromagnetism" section there are no formulas involving mass. So you can just rule out that bit of information.
But lets just have a look at what we think is important:
- the ball hovers 2cm above charged plate
- plate has a charge of -4.72x10-7 C

So now we can head to the formula sheet and see what we can use.
You'll see that there are two formulas that look very good:
F = 1/4πɛ0 x Qq/r2 AND E = 1/4πɛ0 x q/r2
(keeping in mind that 1/4πɛ0 = k = 9x109 - also be careful with units, I'm fairly certain that the distance should be in meters so just make sure you do any necessary conversions)

Because we aren't looking for the force we can rule out the first formula - leaving us with the second one.
Therefore, I would start off by using: E = 1/4πɛ0 x q/r2

(Once again, I could be entirely wrong. But this is what I would do anyway... I hope I'm not wrong tho and I hope this helps - good luck!)
« Last Edit: April 29, 2020, 12:07:10 am by K.Smithy »
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#### Bri MT

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##### Re: QCE Physics Questions Thread
« Reply #16 on: April 29, 2020, 03:04:39 pm »
+2
Hello!
I am revising Electrostatics from Unit 3 Topic 2 at the moment and I am a bit rusty!  I would really appreciate a little bit of help with this question, just at least to start me off

Question: A charged ball of mass 0.32 g hovers 2.0 cm above a charged plate that has a charge of −4.72 × 10-7 C. Calculate the magnitude of the charge on the ball.

Thank you!!

K. Smithy you weren't quite there but you were on the right track.

The trick to this question is recognising that since the ball is hovering, it is stationary with 0 net force. Thus, force due to gravity and force to the electric field strength cancel out.

If we take the magnitude of those two forces as being equal:

$mg = \frac{1}{4 \pi \varepsilon_0} \frac{qQ}{r^2}$

all you need to do is sub in values appropriately (as K.Smithy said, be careful with units) and solve for q

Edit: for showing fractions neatly in latex it's \frac{}{} . I didn't learn it for a while on the forums but there's a great guide by Rui  here
« Last Edit: April 29, 2020, 03:10:16 pm by Bri MT »

#### K.Smithy

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##### Re: QCE Physics Questions Thread
« Reply #17 on: April 29, 2020, 07:31:08 pm »
+2
K. Smithy you weren't quite there but you were on the right track.

Aha worth a shot! Hopefully i'll get there before the externals
Thanks for clearing it up!
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#### Bri MT

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##### Re: QCE Physics Questions Thread
« Reply #18 on: April 29, 2020, 07:48:26 pm »
+1
Aha worth a shot! Hopefully i'll get there before the externals
Thanks for clearing it up!

No worries!

I'm sure you will

#### A.Rose

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##### Re: QCE Physics Questions Thread
« Reply #19 on: April 29, 2020, 08:01:18 pm »
+2
The trick to this question is recognising that since the ball is hovering, it is stationary with 0 net force. Thus, force due to gravity and force to the electric field strength cancel out.

Thank you both for your help! Yes, I did work out that I needed the mass to calculate the force eventually.
I definitely need to do some thorough electrostatics revision!
Thanks!!

#### Bri MT

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##### Re: QCE Physics Questions Thread
« Reply #20 on: April 29, 2020, 08:14:18 pm »
+1
Thank you both for your help! Yes, I did work out that I needed the mass to calculate the force eventually.
I definitely need to do some thorough electrostatics revision!
Thanks!!

No worries!

This stuff is pretty standard across states so if you want more revision you should be able to find a bunch from other states.

#### A.Rose

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##### Re: QCE Physics Questions Thread
« Reply #21 on: April 30, 2020, 12:56:38 pm »
0
Hi! Me again with another question!

I am very confused about this question. (The diagram is attached). The question is to 'explain using physics concepts how the system will operate to close and open the tubing'.
A student designs a simple solenoid valve system to control the flow of a liquid through flexible tubing by closing and opening the tubing. It is comprised of an iron rod with a spring coiled around it at one end. One end of the spring is attached at one end of the rod and its other end is attached to a rectangular housing. The tubing is fixed to a solid support.

If you could at least direct me to where I can find a good explanation of how these kinds of experiments work because I don't remember learning about solenoids in this kind of context last year. I'm assuming its something to do with the establishment of a magnetic field that opens or closes the tube - but I am not confident. Just some clue or somewhere I can research this (I didn't have much luck in my textbook or searching online) that would be amazing!!
Thank you!

#### Bri MT

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##### Re: QCE Physics Questions Thread
« Reply #22 on: April 30, 2020, 02:03:12 pm »
+1
Hi! Me again with another question!

I am very confused about this question. (The diagram is attached). The question is to 'explain using physics concepts how the system will operate to close and open the tubing'.
A student designs a simple solenoid valve system to control the flow of a liquid through flexible tubing by closing and opening the tubing. It is comprised of an iron rod with a spring coiled around it at one end. One end of the spring is attached at one end of the rod and its other end is attached to a rectangular housing. The tubing is fixed to a solid support.

If you could at least direct me to where I can find a good explanation of how these kinds of experiments work because I don't remember learning about solenoids in this kind of context last year. I'm assuming its something to do with the establishment of a magnetic field that opens or closes the tube - but I am not confident. Just some clue or somewhere I can research this (I didn't have much luck in my textbook or searching online) that would be amazing!!
Thank you!

Hey!

You're on the right track! Current carrying solenoids create magnetic fields like the field around bar magnets.

This exact question type isn't common but there are often questions involving magnetic fields around solenoids in VCE physics so it might be worth looking at some of those.

I'm aware I've been pretty vague here (you seemed to be on the right track) - please feel free to follow up with anything you're unsure of

#### elmobluey

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##### Re: QCE Physics Questions Thread
« Reply #23 on: May 02, 2020, 10:33:41 pm »
+1
Hi!
I'm confused on the working that my teacher has provided on a recent practise data test we did.
I have attached the dataset and the question. I really don't have a clue on what my teacher has done. I thought I would have seen "9.81m/s/s" somewhere in the working. But... Yeah I really have no idea what's going on here. Can anyone explain to me why the acceleration due to gravity is 12.5m/s/s?
Thank you!
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#### Bri MT

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##### Re: QCE Physics Questions Thread
« Reply #24 on: May 03, 2020, 09:57:24 am »
+1
Hi!
I'm confused on the working that my teacher has provided on a recent practise data test we did.
I have attached the dataset and the question. I really don't have a clue on what my teacher has done. I thought I would have seen "9.81m/s/s" somewhere in the working. But... Yeah I really have no idea what's going on here. Can anyone explain to me why the acceleration due to gravity is 12.5m/s/s?
Thank you!

Hey!

This is a question saying if the data we had was x, then what would that mean about y? Sometimes this will end in a reasonable value and sometimes it won't.  The reason you don't see the actual value for g is because the question wants you to pretend you don't know what this is & figure out what it would be for the provided fake situation.

Do you understand where g = sin(theta) comes from?

#### A.Rose

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##### Re: QCE Physics Questions Thread
« Reply #25 on: May 05, 2020, 03:22:41 pm »
+1
Hi, I have another Electromagnetism question! Please see attached.
I just need a clue to where to start. Since its asking for speed I'm assuming I'm going to need to use F=qvB at some stage...? But it also gives me a potential difference which makes me think I need to use ∆U=∆Vq? Am I at least on the right track or is there something I'm missing?
Thank you!

#### Bri MT

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##### Re: QCE Physics Questions Thread
« Reply #26 on: May 19, 2020, 01:51:29 pm »
+1
Hi, I have another Electromagnetism question! Please see attached.
I just need a clue to where to start. Since its asking for speed I'm assuming I'm going to need to use F=qvB at some stage...? But it also gives me a potential difference which makes me think I need to use ∆U=∆Vq? Am I at least on the right track or is there something I'm missing?
Thank you!

Sorry I somehow missed this earlier!

You may have resolved this by now, but my approach would be to use the change in energy to get the kinetic energy and thus speed