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March 29, 2024, 01:20:38 am

Author Topic: VCE Physics Question Thread!  (Read 603244 times)  Share 

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rlenora

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Re: VCE Physics Question Thread!
« Reply #525 on: July 15, 2014, 09:52:30 pm »
+3
can someone please help me?
An electron moving north enters a magnetic field
that is directed vertically upwards.
If the electron’s motion was inclined upwards
at an angle, as well as travelling north, what
would be the path of the electron?

Use right hand palm rule (slap rule :P)

Direction of magnetic field: up (use fingers of right hand to point up)

Direction of current, i.e. direction of positive charge: south (opposite to direction of electron) (point thumb to point south on right hand)

Direction of force on electron: West (given by the direction of the palm of right hand)

Hence electron will initially move West, and since force applied on the electron will always be perpendicular to its motion (like it is in circular motion), the electron will travel in a circular path.

We do not have to worry about the component of the electron's motion upwards (i.e. parallel to the magnetic field), since components of current  (velocity of the electron in this case) parallel to the magnetic field doesn't result in a force being applied on the electron.

Hope this helps :)

Let me know if you'd like clarification on part!
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rlenora

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Re: VCE Physics Question Thread!
« Reply #526 on: July 15, 2014, 10:27:41 pm »
+2
Hello knightrider,

When asking questions and for help, it is better if you let us know what you are struggling with specifically, rather than just saying you need help. For example, you could tell us how you attempted the problem and that way we would know where a mistake may have been made and point you in the right direction.

I remember that question with the bus and Anna from last year, I had to ask my teacher.

You can usually find the worked solutions by googling "heinemann physics 11 worked solutions chapter X" where X is obviously the chaper you're up to.

I hope this helps.

Cheers,
Stewart

Exactly!

This way, we can help you out better than just giving you the answers knightrider :)
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hyunah

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Re: VCE Physics Question Thread!
« Reply #527 on: July 15, 2014, 11:18:02 pm »
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hey Lenora,
the part which I didn't get is the inclines angle bit and yet as well as travelling north, does that when my hand is like slanted north - west or north east?

rlenora

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Re: VCE Physics Question Thread!
« Reply #528 on: July 15, 2014, 11:51:44 pm »
+2
You don't have to worry about the inclined bit of the motion of the electron, since it is parallel to the direction of the magnetic field (which is also upwards), it doesn't result in a force being applied on the electron.

So all you have to worry about is that the electron is moving north.
Which is the same as if a positive charge was moving south.

and since the thumb points in the direction of the conventional current (i.e. direction of movement of positive charge), thumb will need to point south.

So, fingers up. thumb south. palm faces west. Force applied on electron is to the west :)
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knightrider

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Re: VCE Physics Question Thread!
« Reply #529 on: July 18, 2014, 11:27:47 pm »
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hi guys,
For these 2 equations how do you know when to use the one with the plus 1/2 and the one with -1/2

What does the plus and minus mean and how would you know to use which formula

Thorium

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Re: VCE Physics Question Thread!
« Reply #530 on: July 19, 2014, 02:37:32 am »
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hi guys,
For these 2 equations how do you know when to use the one with the plus 1/2 and the one with -1/2

What does the plus and minus mean and how would you know to use which formula

Apparently the second one is not officially recognised (maybe only by my school), but it is quite useful in some situations.

They are both used under one main condition, that is, when acceleration is constant. You can notice that one involves u (initial velocity) and the other involves v (final velocity). So if you have one quantity missing out of x, u, t, a, then you use the equation x=ut+1/2at^2. If you have one quantity missing out of x, v, t, a, then you use the equation x=vt-1/2at^2.

Hope that helps :)
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knightrider

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Re: VCE Physics Question Thread!
« Reply #531 on: July 19, 2014, 09:07:57 am »
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Apparently the second one is not officially recognised (maybe only by my school), but it is quite useful in some situations.

They are both used under one main condition, that is, when acceleration is constant. You can notice that one involves u (initial velocity) and the other involves v (final velocity). So if you have one quantity missing out of x, u, t, a, then you use the equation x=ut+1/2at^2. If you have one quantity missing out of x, v, t, a, then you use the equation x=vt-1/2at^2.

Hope that helps :)

Didn't realize the equations were different i thought they bith had u's that were negative and positive thankyou  for your help  :)

knightrider

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Re: VCE Physics Question Thread!
« Reply #532 on: July 19, 2014, 05:29:13 pm »
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Hi how would you do the following question

I have worked out the initial speed to be 20m/s and the maximum height to be 20m

how would you find the speed of the cork as it returned to its
starting point?

Butterscotch

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Re: VCE Physics Question Thread!
« Reply #533 on: July 20, 2014, 04:09:17 pm »
0
Hi how would you do the following question

I have worked out the initial speed to be 20m/s and the maximum height to be 20m

how would you find the speed of the cork as it returned to its
starting point?

An easy way to go about solving such types of questions is to ignore the rise of the cap. Let's just look at the fall of the cork from the maximum point.
You already have:
u = 0 ms^-1
a = 9.8 ms^-2
t = 2 s (remember we're only looking at the return journey of the cork)
v = ?

You'd plug these into the formula: v = u + at
Your answer rounded off would be 20 ms^-1.

EDIT: Orrr an alternative (also one which saves time) is one that PB has said below :)
« Last Edit: July 21, 2014, 08:13:37 pm by Butterscotch »

PB

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Re: VCE Physics Question Thread!
« Reply #534 on: July 21, 2014, 11:58:40 am »
+2
Well, technically you don't even have to do any working out... i mean, the cork decelerates on the way up and accelerates by the same amount on the way down.  It is bound to have the same speed at the end of the flight as it did at the beginning.

edit: speed, not velocity
« Last Edit: July 28, 2014, 02:26:22 am by PB »
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Re: VCE Physics Question Thread!
« Reply #535 on: July 21, 2014, 02:46:12 pm »
+2
Well, technically you don't even have to do any working out... i mean, the cork decelerates on the way up and accelerates by the same amount on the way down.  It is bound to have the same velocity at the end of the flight as it did at the beginning.

You've invoked the conservative nature of the gravitational force, and a rigorous proof of that isn't going to be given before second year university :P

Technically, it should reach the starting height slower than its initial speed because of the non-conservative nature of air resistance. In addition, PB I think you've made a typo there that I'll highlight. I trust you can see what is wrong with it :P
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yang_dong

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Re: VCE Physics Question Thread!
« Reply #536 on: July 21, 2014, 09:11:35 pm »
+1
why no force is produced when the wire is parallel to the magnetic field?

thank you

PB

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Re: VCE Physics Question Thread!
« Reply #537 on: July 22, 2014, 01:04:09 am »
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You've invoked the conservative nature of the gravitational force, and a rigorous proof of that isn't going to be given before second year university :P

Technically, it should reach the starting height slower than its initial speed because of the non-conservative nature of air resistance. In addition, PB I think you've made a typo there that I'll highlight. I trust you can see what is wrong with it :P

Well yeah of course, but I didn't want to confuse anyone else given that this is VCE Physics we are talking about and not real life physics.
But your second point is valid. My apologies people, it should really say speed instead of velocity because 'same velocity' would imply that the cork has the same direction of travel too..which is of course false - it is the opposite on the way down.
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Re: VCE Physics Question Thread!
« Reply #538 on: July 22, 2014, 01:19:14 am »
+2
why no force is produced when the wire is parallel to the magnetic field?

thank you

Interesting phenomenon and sadly way outside the course content of VCE Physics :P
If it were to be asked though, just prove it using the formula for the force acting on  current carrying wire "F=nILBsin(theta)".  When M-field and current is parallel, theta is 0 which makes the whole left side of the equation equal 0. Thus, force is zilch when wire is parallel to M-field.
Always good to link it back to a formula to prove a point.
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Re: VCE Physics Question Thread!
« Reply #539 on: July 22, 2014, 11:28:00 am »
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thank you PB,

can someone please help me with question 26a)

thank you