VCE Physics Question Thread!

[zsteve]

Flux IS a scalar, but it's a scalar that takes positive and negative values. If defined properly, it has to be defined with a consistent choice of surface direction. What I mean is, you need to measure flux in a way such that if the surface flips 180 degrees, the flux changes sign, obviously. So in this way, you do have to specify a direction for the flux, but this direction is only a convention for how your flux was defined.

Mathematically, flux is a scalar because it arises from a dot product of two vectors: the magnetic field vector and the surface normal.

I thought I would add - 'flux' through a surface is pretty much the 'flow' of the B-field through your given surface. In this sense, you can have flux in two directions - out of surface, and into surface (you define what directions 'out' and 'in' represent).

[HopefulLawStudent]

How do I approach this question?

A coin will rest on a long-playing record rotating at 45 rpm provided that it is not more than 10cm from the centre of the record. How far away from the centre may it be placed if it is to remain on the record when rotated at 33.3 rpm?

The answer is apparently

Spoiler

18cm.

How do they get to this answer???

[zsteve]

How do I approach this question?

The answer is apparently

Spoiler

18cm.

How do they get to this answer???

Let the coin be of mass m, the record rotating at 45rpm, so one revolution is made in 1/45 min = 60/45 = 1.33s.
Hence:

is the centripetal force (supplied by maximal friction).

Now, if it's rotated at 33.3 rpm (i.e. one rotation each 1.802s), we've still got the same friction, so


Hence r = 0.18m

[wyzard]

How do I approach this question?

The answer is apparently

Spoiler

18cm.

How do they get to this answer???

Zsteve answered this really well, but I'll present an alternative method which doesn't involve the calculation of the force, which is awkward because you don't know the coin's mass.

The centripetal force acting against the friction on the coin to prevent it from spinning away from the record is:


Since mass remains unchanged, centripetal force only depends on the distances squared over the rotational period square. So for the centripetal force to stay the same, we arrive at the following equation:


Which can be used to to find the second radius, which is the distance from the records since we know its RPM.

[YellowTongue]

Two perpendicular wires are arranged as shown in the attached diagram. The upwards current has a magnitude of one ampere whilst the horizontal current has a magnitude of two amperes. Point P is two units downwards from the horizontal wire and one unit right from the vertical wire. What is the direction of the magnetic force at P?

I just can't figure out how I would go about answering this question. Thank you for your assistance 

[HopefulLawStudent]

Hey guys:

Please help.

Just the one question (even though there are three attachments + the fourth attachment which is the supplied answer).

I know that work per unit mass is the area under the graph but I don't quite understand where they're getting their numbers from.

PS: Realised I never thanked y'all for the help you gave me for the last question so thank you (better late than never, right?)

[wyzard]

Hey guys:

Please help.

Just the one question (even though there are three attachments + the fourth attachment which is the supplied answer).

I know that work per unit mass is the area under the graph but I don't quite understand where they're getting their numbers from.

PS: Realised I never thanked y'all for the help you gave me for the last question so thank you (better late than never, right?)

The reason why the area underneath the gravitational field strength graph with respect to distance is the work done per unit mass is the mass times gravitational field strength is the gravitational force, and recall that area underneath the force-distance graph is the work done to counteract the gravitational force to get further away from Earth

Think the gravitational field strength as force per unit mass, hence the you're finding work done per unit mass when you're evaluating the area.

As to where the graph comes from, it's nothing more than the formula for gravitational field strength:

[HopefulLawStudent]

Haaaaai

Two quick questions:

1) Why is interference not seen when the beams of a car’s headlights overlap?

2) To demonstrate the interference of light waves, Young allowed light from an illuminated single slit to fall on a pair of narrow closely-spaced slits. Why does light from two separate lamp bulbs never give an interference pattern?

I asked my teacher and his answer was something along the lines of "it's because x, y, z, oh wait... maybe it's because of l, m, n. Oh I don't know. I'll get back to you on those." And when I brought it back up in the following class, he blinked at me and was basically just like "I'm busy haven't thought about it yet, I'll get back to you."

[jamonwindeyer]

Haaaaai

Two quick questions:

1) Why is interference not seen when the beams of a car’s headlights overlap?

2) To demonstrate the interference of light waves, Young allowed light from an illuminated single slit to fall on a pair of narrow closely-spaced slits. Why does light from two separate lamp bulbs never give an interference pattern?

I asked my teacher and his answer was something along the lines of "it's because x, y, z, oh wait... maybe it's because of l, m, n. Oh I don't know. I'll get back to you on those." And when I brought it back up in the following class, he blinked at me and was basically just like "I'm busy haven't thought about it yet, I'll get back to you."

I know the first one would be because headlights are a wide spectrum light source! They aren't coherent and they don't have a single frequency, instead spread over a wide spectrum shifted towards the infra-red end of the spectrum. Light bulbs are like mini, not quite proper, black bodies, that emit a spectrum of radiation rather than a specific frequency. No interference occurs for separate sources like those

Quick Definition: A coherent light source is a source of light that produces waves that have a close to constant phase difference between them. Another way to think about it is that the light waves more in sync with one another, even if they aren't in phase.

This also answers the second question. Essentially, two separate lamps will not be able to produce light waves that have a consistent phase difference, there is too much variation in frequency and too much fluctuation from the source. That means no noticeable interference. I mean, it could occur, but it's never consistent enough to actually be noticeable

[MB_]

Does anyone have a practice data analysis SAC for light?

[HopefulLawStudent]

Hi.

Could someone please tell me how realistic this question is as a potential VCAA exam question?

Would a proton microscope be able to achieve a higher degree of resolution than an electron microscope with the same accelerating potential difference of 10kV?

[zsteve]

Hi.

Could someone please tell me how realistic this question is as a potential VCAA exam question?

Yeah, that sounds like a legit VCAA question, sounds somewhat familiar to me actually (might have encountered that in some ancient pre-1990s VCAA questions). Just compare the wavelengths.

[HopefulLawStudent]

Okey doke. Thank you.

[MB_]

What is the purpose of the armature in a DC motor?

[wyzard]

What is the purpose of the armature in a DC motor?

The simple design of the DC motor learned in VCE physics has a major flaw of having a very uneven rotation (or in more technical term uneven torque), due to the fact that torque is maximum when the coil is parallel to the field, and zero when it is perpendicular; so the rotation is very rough and shakes around a lot.

An armature is basically an improvement in the design, where the torque applied to the coil not only more smoothed out, the motor is a lot more compact as well