OCR Physics A G482, Electrons, Waves and Photons, 25th May 2012

Does anyone else feel as though this exam is mainly based upon regurgitating information?

huh resistivity never changes as its only a ratio and is the same for the same material.

Okay I have now forgotten the basic principle of Physics:
WHAT HAPPENS TO V, I OR R WHEN EITHER OF THEM INCREASE? (Don't V=IR cos thats really misleading; it doesn't always work with this).

So when R increases, I decreases, but what happens to V?
- Does it also increases since V is proportional to R (V=IR, sorry )
- Or does it increase since there's a smaller I so the voltage needs to push the current around the circuit?

Wait, I'm contradicting myself here; they're both the same thing but (I THINK) what I'm trying to ask is why the voltage increases... (reason 1 or reason 2 or both or neither?!)

Also, when V increases, WHAT happens to I or R? They can't both increase (...can't they? )

I think I'm going crazzyy.

Is stopping potential on our syllabus, someone mentioned it earlier and worried me! I saw it in the textbook but we never covered it in school?

JUST to make me happy eh ?
a driving oscillator just waggles the string up and down... basically the same as a vibrations generator

If there is only one component in the circuit and the resistance increased then the current would fall and V would remain constant

And if V is increased then current will increase but R will remain constant

These two assume that temperature remains constant ^^^^

If there are two or more components in series then it becomes a potential divider circuit where increasing the resistance of one component increases the P.d. (voltage) across it and lowers the P.d. across the other components. It will also lower the current

When V is the EMF, V = IR works.

V is proportional to I. R is a constant. V, where V is the EMF, stays the same.

When V is a PD, things can alter slightly depending upon conditions (eg. temperature comes up a lot.)

Then there's the other formula, involving V out and V in. This shows, where V out is the EMF, and V in is the PD across a component, that as the R of a component increases, as does the PD across a component.

Thanks alot guys, will +rep you when i get a refill

Btw ebmaj, in what situations would you put V equal to the EMF? Is that when there is just one component in the circuit, or is there some other rule?

when the battery has negligible internal resistance

Yeh it is ... but it doesnt tend to be worth as many marks as other experiments it tends to be an "add on question" to the photoelectric effect

here is how its done..

Photoelectric effect
-em radiation shone on metal
-one photon acts with one electron
-if the photon is below the threshold frequency no electrons are displaced even if the intensity is altered
-if the photon is above the threshold frequency a surface electron is freed
-when the frequency is above the threshold the amount of electrons displaced is effected by intensity
-if the kinetic energy transfered from the photon to the electron is great enough it will travel through the vacuum to complete the circuit

Stopping potential
- The stopping potential can be found by passing a current the opposite direction to the freed electron
- To find the max KE of an electron we need to know find voltage is needed to cancel out the photoelectron
- We then multiply the voltage by e (electron charge / 1.6x10^-19) to find energy (Max KE)

I dont have the book mate.. but i think thats stopping potential which ive explained above il try and go into more detail

- The stopping potential can be found by passing a current the opposite direction to the freed electron
- To find the max KE of an electron we need to know find voltage is needed to cancel out the photoelectron
- We then multiply the voltage by e (electron charge / 1.6x10^-19) to find energy

A good hint when they're gonna want you to mention the internal resistance is when they put 'Battery A, which has a very small internal resistance.'

If they don't say the word negligible, it's because they're gonna want you to mention it in one of the long essay-like questions.

Same if they say something like 'A voltmeter, with a very high resistance is connected in parallel.'

what experiments are you meant to know?? it's so confusing cause they all seem the same

Also yes, stopping potential - yer' what?

Never heard this be mentioned.



Just throw it in if you can, they've given you that information for a reason.

Say there's one component in a circuit and the PD doesn't = the EMF, the reason could be due to internal resistance etc.

Also some energy is lost etc. Dunno what they could ask tbh.

I think instead of the signal generator and vibration thingey, it's called an oscillator. Makes things oscillate.

But otherwise yeah that's great. There's a good question to do with that on the June 2010 paper I think.