Bazza's 3/4 Physics question thread

[Lasercookie]

So it looks like you're having trouble with the concepts.

Yeah, the textbooks are a bit dense, once you begin to have a bit of an idea about what it's saying, then the textbooks are a bit easier to understand. The stuff you're talking about is all mentioned in the textbook.

At a glance, the question seems to be badly place to a small extent (there seem to be easily missed references about the opposite business, paragraph 3 of page 359 for example). Perhaps have a read of 10.2 (Faraday's Law) and 10.3 (Lenz's Law) first, the answer to the question might be clearer after that.

For that question, you have a current, that induces a magnetic flux (chapter 9 content), which induces a current in the other end of the solenoid.

You'll do demonstrations like these in class, but have a look at the ones here about electromagnetic induction and magnetic fields. http://www.youtube.com/ibphysicshelp (youtubing electromagnetic induction will come up with quite a few videos).

Right hand column, the videos on magnetism on khanacademy could be of use to you at the moment: http://www.khanacademy.org/#physics

The iTute notes might also be of help too.

edit: I didn't notice you had posted two questions, mistook 1) and 2) to refer to 7 and 8:

With the second Q, how do you determine the direction of the current? Book say's initially clockwise then later anticlockwise, but how can you determine this?

Right hand grip rule.

You know the direction of the field, so you curl your fingers in that direction. The direction of the current is given by your thumb. The clockwise/anticlockwise here would refer to the direction of the current in the loop. Also remember that the EMF generated is given by the negative rate of change of flux.

[WhoTookMyUsername]

For that question, you have a current, that induces a magnetic flux (chapter 9 content), which induces a current in the other end of the solenoid.

Thanks laserred
i'm still fuzzy on this one after going back , could you please explain the theoretical basis and steps to solve the question?

(also i just added a question before you responded)
thanks!

[Lasercookie]

For that question, you have a current, that induces a magnetic flux (chapter 9 content), which induces a current in the other end of the solenoid.

Thanks laserred
i'm still fuzzy on this one after going back , could you please explain the theoretical basis and steps to solve the question?

(also i just added a question before you responded)
thanks!

You might want to double check this (particularly the directions):

10.1 Question 6
You turn on the switch, you keep it turned on and then you turn it off.

You turn on the switch. So current flows into the circuit X. Assume conventional current, so it comes out of long end of the battery. A magnetic flux will be induced in the steel rod for a moment. Trace along the current until you get to the coil bit and see what direction it is there (If you were to take the left end of the solenoid and peer down it, the current would be travelling clockwise). Right hand grip rule to find the direction of the field (I get pointing towards the right, I'm not overly refreshed on this so don't take my word as gospel).

So this change in magnetic flux induces a current in Coil Y. Right hand grip rule to find the direction of the current. Remember that the direction of current will be such that the magnetic flux it (the induced current creates) will be opposite to the change in flux that produced it. So the flux that it'll create will point to the left, and hence we find the current will be anticlockwise, if we peered down the left end of the solenoid.

The switch is left on, you're not changing the current so there's no magnetic flux being induced, hence no current induced in coil Y.

You turn off the switch - the current decreases (instead of increasing as it did before) in Coil X, hence a change in the magnetic flux. This induces a current in Coil Y again. This current in Coil Y this time will be in the opposite direction to the one we had before when we turned on the switch. So it'd be travelling clockwise if we peered down the left end of the solenoid.

[paulsterio]

It's pretty much a transformer, if you understand how a transformer works, you will be able to understand how that works. Just thought I'd add that to Laseredd's explanation.

Anyways, succinct summary is that, a change in current direction will cause a change in flux and by Faraday's law, a change in flux will cause an induced voltage.

[FlorianK]

Didn't wanted to make a new thread

1. How could naturally-occuring magnets have been formed?

2. What is the purpose of each of the following in a DC motor?
a) The magnet
b)the brushes
c)the commutator (three aspects)
d)The large number of turns of wires

3. Stronger magnetic fields can be obtained with an electromagnet. The same DC power source can supply current to the electromagnet as well as to the circuit, the electromagnet and the rotating coil, can be connected to the power source in two different ways.
a) what are these ways?
b) How do you think the starting and operating characteristics of these two types will differ?

4. What is the difference between magnetic field and magnetic flux?

5. When a metal rod falls through a magnetic field, charge is seperated and a voltage is established between the two ends of the rod. This requires energy. Where did the energy come from?

[max payne]

Since this thread gets a lot of traffic, would anyone mind doing this question.

Question 3
20 kW of power is available from a generator, at 250 V, for
transmission to consumers in a town some distance from the
generator. The transmission lines over which the power is
transmitted have a resistance of 1.2 .
a How much power is lost if the power is transmitted at 250 V?
b What would be the voltage at the end of the transmission lines?
c Would the townspeople be able to use normal 240 V electrical
appliances? Why?
d How much power would be lost if, instead, the voltage was
stepped up by a transformer at the generator to 6.0 kV?
e What would be the voltage at the town if the power was
transmitted at 6.0 kV? >answer: 240V

[WhoTookMyUsername]

A record collector plays an Elvis LP, made of platinum, on a turntable that rotates at 33 revolutions per minute. Assume that the direction of the Earth's magnetic field is always perpendicular to the plane of the record, and has a uniform strength of 5.0 × 10–5 T. The radius of the record is 26 cm. What EMF would be induced between the centre of the record and the rim? Would an EMF be induced between opposite edges of the record


1) Anyone know how to do this question?
2) From a physics conceptual point of view why does cutting "lines of flux" count as a change in flux?
Also
3) Not sure why/why not ( conceputually no voltage is induced between opposite sides of the record.
4) Why does a local magnetic field act to counteract change in flux? (current carrying)
If it doesn't, and the magnetic fields add together, will this combined magnetic field induce a stronger current and so on and so forth? If yes, doesn't this mean a local magnetic field induces a current in the wire that created the magnetic field? If not why can't the magnetic fields add together?



Thanks !

[WhoTookMyUsername]

Is a baffle the front board of the enclosure or the whole enclosure of the speaker itself?
Also do baffles and ports reduce resonance? If yes, how? If no, how is resonance reduced? ( textbook doesn't explain this part in much detail)

Thanks

[WhoTookMyUsername]

Why is voltage not considred a "force" ? E.g. For stopping voltage, the opposing voltage acts as a "force"... Doesn't it? (also please see baffles q directly above)
Thanks!

[paulsterio]

Why is voltage not considred a "force" ? E.g. For stopping voltage, the opposing voltage acts as a "force"... Doesn't it? (also please see baffles q directly above)
Thanks!

Because it is not a force?

It is simply the electrical potential difference between two points on a circuit.

[WhoTookMyUsername]

M... Fair enough
But how would you describe stoppng voltage (what is it / what does it represent)
In a trial exam yesterday i wrote something like
"the stopping voltage is an measure of the force required to stop the photoelectron with the highest kinetic energy, and is therefore a measure of the kinetic energy of the fastest electrophoton"
Which i can see is wrong on numerous levels but i was just unsure how to phrase it


Also WHY does reducing slit separation increase fringe spacing D: would we ever have to discuss why? Or just know the effects
 Thanks

[paulsterio]

You're not far off, I'd say something like:

Stopping voltage is the measure of the reverse voltage required to stop the most energetic photoelectron. Thus, numerically, it is equal to the kinetic energy of the fastest photoelectron.

Just don't call the voltage a force, it is a voltage, or a potential difference.

Slit separation will increase fringe spacing due to a concept known as Fraunhofer Diffraction - http://en.wikipedia.org/wiki/Fraunhofer_diffraction

You won't need to know that.

[WhoTookMyUsername]

Thanks paul

No idea what to do for the following question
( attached)
Could someone please explain the reasoning?
Thanks!

[Aurelian]

Thanks paul

No idea what to do for the following question
( attached)
Could someone please explain the reasoning?
Thanks!

Zzz I'll tentatively say this is a very poor question, and one which you wouldn't get in VCE for a whole host of reasons. I had to recourse to the line integral formulation of Faraday's law to think about this...

To put it simply, though, from what I can tell, this question is unanswerable, because it depends on where you measure the emf...

Is the answer 0.67V by any chance?

[WhoTookMyUsername]

i've never seen a question like it before D:

yeah answer is 0.68; answers do something dodgy
(which i do not understand)
E=faraydaylenzlaw = -3x (30x10^-3 x 0.15 x 0.30)/(6x10^-3) = 0.68 V

no idea whats going on

thanks for the help