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

1. Quick question; something which the (useless) textbook hasn't helped me with.

With Waves, what is the difference between Amplitude and Displacement? Are they basically the same numerical value, except Amplitude is always positive whilst Displacement can be positive or negative, depending on whether it is above or below the line?

Cheers
2. (Original post by JoshC.)
Quick question; something which the (useless) textbook hasn't helped me with.

With Waves, what is the difference between Amplitude and Displacement? Are they basically the same numerical value, except Amplitude is always positive whilst Displacement can be positive or negative, depending on whether it is above or below the line?

Cheers
Amplitude = the maximum displacement from the equilibrium position.
Displacement = the displace of any point on a wave from its equilibrium position.

So amplitude is the peak, whereas displacement is just simply the distance of any point of the wave (e.g. it could be halfway between equilibrium position and amplitude) from its equilibrium position.

Hope that helps
3. (Original post by JoshC.)
Quick question; something which the (useless) textbook hasn't helped me with.

With Waves, what is the difference between Amplitude and Displacement? Are they basically the same numerical value, except Amplitude is always positive whilst Displacement can be positive or negative, depending on whether it is above or below the line?

Cheers
my textbook says AMPLITUDE = the maximum displacement of a particle from its equilibrium position... so for instance, if you think of a sin wave.. then its amplitude is at 90 degrees whilst at say 45 degrees its displacement would be 0.707... if that makes sense?
4. (Original post by Dale12)
my textbook says AMPLITUDE = the maximum displacement of a particle from its equilibrium position... so for instance, if you think of a sin wave.. then its amplitude is at 90 degrees whilst at say 45 degrees its displacement would be 0.707... if that makes sense?
Thanks to you both, but that Maths-y way of describing it really has helped.

Cheers!
5. Can someone explain to me the relationships between current, voltage, and resistance.

eg.

- If you change the voltage supplied, how does the resistance and current change?
- If you increase the resistance, does the current decrease but the voltage stay the same?

Struggling to get my head round all this!
6. (Original post by Wilko94)
Can someone explain to me the relationships between current, voltage, and resistance.

eg.

- If you change the voltage supplied, how does the resistance and current change?
- If you increase the resistance, does the current decrease but the voltage stay the same?

Struggling to get my head round all this!
I just learnt this today.
Think of voltage as something which helps to push the current round a circuit.
When there's a high resistance and low current, then the voltage needs to increase so that it can push the current around faster.
On the other hand, when there's a low resistance but high current, the voltage doesn't need to push the current round since it's already high; hence the voltage decreases.

Btw, I don't think this is the actual scientific way of explaining it; it's just to help you understand it.
7. Can someone please explain the experiments for stationary waves?
- Like how stationary waves are formed,
- And how to demonstrate them using microwaves, stretched strings and air columns. (+rep) Thankyou.
8. (Original post by Wilko94)
Can someone explain to me the relationships between current, voltage, and resistance.

eg.

- If you change the voltage supplied, how does the resistance and current change?
- If you increase the resistance, does the current decrease but the voltage stay the same?

Struggling to get my head round all this!
for a component, resistance is constant so increasing the voltage means the current increases

however if you increase the resistance the current decreases but the p.d stays the same

V=IR
9. So say there is a parallel circuit with a resistor on each, 1 thats 1k Ohms, another 5k Ohms.

The 5k Ohm resistor will have a smaller current.

Where as the 1k Ohm resistor will have a greater current.

And if the resistors are in series, then then;

The 5k Ohm will have a larger share of the potential difference?
10. (Original post by Dale12)
for a component, resistance is constant so increasing the voltage means the current increases

however if you increase the resistance the current decreases but the p.d stays the same

V=IR
So am i wrong then? ...Are you sure?!
11. (Original post by Wilko94)
Can someone explain to me the relationships between current, voltage, and resistance.

eg.

- If you change the voltage supplied, how does the resistance and current change?
- If you increase the resistance, does the current decrease but the voltage stay the same?

Struggling to get my head round all this!
I always think of it in terms of V=IR

So, if the Resistance increases and the Voltage of the circuit stays the same (the resistance does that affect the Voltage from the battery, only the PD of a component), then I will get smaller.

If you change the Voltage of a circuit, the current will also change, however the resistance of the components will be the same. If Voltage increases, so too will current.

I hope that makes sense
12. (Original post by Wilko94)
So say there is a parallel circuit with a resistor on each, 1 thats 1k Ohms, another 5k Ohms.

The 5k Ohm resistor will have a smaller current.

Where as the 1k Ohm resistor will have a greater current.

And if the resistors are in series, then then;

The 5k Ohm will have a larger share of the potential difference?
Yeah, I think that's right...
13. (Original post by sweetascandy)
Can someone please explain the experiments for stationary waves?
- Like how stationary waves are formed,
- And how to demonstrate them using microwaves, stretched strings and air columns. (+rep) Thankyou.
Stationary waves are formed when a progressive wave reflects of a boundary and the two waves travelling in opposite directions superpose and form a standing wave with nodes and anitnodes..

to demonstrate using a microwave transmitter, it reflects of a metal sheet... if u move a probe between the metal and transmitter u get signals of minimum and maxima, these are the nodes and anitnodes...

for air columns the sound wave reflects off the closed end and u can hear a maxima at the tope of the tube, the length between the open end of the tube and closed end is wavelength/4
14. (Original post by Wilko94)
Can someone explain to me the relationships between current, voltage, and resistance.

eg.

- If you change the voltage supplied, how does the resistance and current change?
- If you increase the resistance, does the current decrease but the voltage stay the same?

Struggling to get my head round all this!
First, V = IR. When V is EMF (voltage of a battery or power source), if resistance increases, current decreases. And the other way around.

When you're talking about Potential Difference (so, some sort of measurement by a voltmeter), things change.

V in is the EMF, V out is the reading on the voltmeter.

You'll see that if you plug numbers into that, as R1 increases, so does V out. So the potential difference of a component will increase as it's resistance increases.

Those are the two things I've sorted made myself remember.

Oh and, current heats things up. When things get hot, resistance increases. Meaning that when temperature is taken into consideration (and this exam loves throwing temperature into things), as current increases, temp increases, resistivity increases, thus resistance increases.
15. (Original post by sweetascandy)
So am i wrong then? ...Are you sure?!
you are not wrong, you explained how to undertsand the concepts of v, I and R you didnt answer the questions thats all

and Josh. C is right
16. Just a quick question: Is it voltage or current (or both), which causes metallic conductors to heat up?

I always thought it was (just) the current, but one of the questions in a past paper, seems to imply that it's voltage?
17. safe to say i'm going to fail any photon questions like a boss
18. (Original post by A for Andromeda)
Just a quick question: Is it voltage or current (or both), which causes metallic conductor to heat up?
current mate
19. (Original post by ebmaj7)
First, V = IR. When V is EMF (voltage of a battery or power source), if resistance increases, current decreases. And the other way around.

When you're talking about Potential Difference (so, some sort of measurement by a voltmeter), things change.

V in is the EMF, V out is the reading on the voltmeter.

You'll see that if you plug numbers into that, as R1 increases, so does V out. So the potential difference of a component will increase as it's resistance increases.

Those are the two things I've sorted made myself remember.

Oh and, current heats things up. When things get hot, resistance increases. Meaning that when temperature is taken into consideration (and this exam loves throwing temperature into things), as current increases, temp increases, resistivity increases, thus resistance increases.
I'm not convinced.
20. whats up sweetascandy???

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