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

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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?!

Reply 221

JoshC.

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 :smile:

(edited 12 years ago)

Reply 222

sweetascandy

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...

Reply 223

Dale12

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

Reply 224

Dale12

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 :smile:

and Josh. C is right

Reply 225

A for Andromeda

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? :s-smilie:

(edited 12 years ago)

Reply 226

donzy

safe to say i'm going to fail any photon questions like a boss

Reply 227

Dale12

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 :smile:

Reply 228

sweetascandy

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.

Unparseable latex formula:

V_{out} = \frac{R_1}{R_{total}} \x V_{in}

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.

Reply 229

Dale12

whats up sweetascandy???

Reply 230

sweetascandy

Original post by Dale12

^This is the experiment for the 2 source interference of microwaves...are they the same?

Thanks for your help though. :smile:

Reply 231

Dale12

Original post by sweetascandy

^This is the experiment for the 2 source interference of microwaves...are they the same?

Thanks for your help though. :smile:

with 2 source interference, you move the probe along the 'screen' (where the screen would be in youngs double slit) and you'll get the interference pattern...

however to detect a stationary wave you move it between the transmitters and the metal sheet, sort of back and forth and you will be able to detect low signal (node) and high signal (antinode)

also the 2 source interference uses two transmitters connected to one source, for stationary waves only 1 transmitter is used to reflect off a metal sheet... that help?

Reply 232

sweetascandy

Original post by Dale12

Okay, but the progressive waves which form a stationary wave are coherent, right?!

Reply 233

Pangol

Original post by sweetascandy

Okay, but the progressive waves which form a stationary wave are coherent, right?!

Not really. They are identical, but travel in opposite direcctions. If they were coherent, they would have a constant phase difference, but as they travel in opposite directions, their phase difference is constantly changing. Look at the animation in Stonebridge's sig for more details!

Reply 234

jake9382

can someone plz help me understand this. If sound waves are longitudinal, then how come when you pluck a violin string a stationary transverse wave is produced? So the sound is transverse? i dont understand how sound is longitudinal then? Thanksss

Reply 235

sweetascandy

Original post by ebmaj7

Go on.

Haha.

Firstly, in that equation, if the resistance increases, then as you said, R1 also increases. But doesn't R.total also increase? (I'm assuming R.total=sum of resistances?). Correct me if I'm wrong.
And secondly, only for a metal does resistance increase with a higher temp (which is due to more collisions impeding the electron flow).
Thirdly, how does resistivity come into it? What does resistivity have to do with this?

P.S. Go easy on me please; Physics is my weakest subject, ha. :colondollar:

Reply 236

sweetascandy

Original post by Pangol

Oh okay, cool, that does kinda make sense. Also, who's stonebridge?! :p:

Reply 237

sweetascandy

Is a microwave probe just simply a microwave detector?
And is microwave generator the same thing as a microwave transmitter?

Reply 238

Joseppea

Original post by sweetascandy

Is a microwave probe just simply a microwave detector?
And is microwave generator the same thing as a microwave transmitter?

I think so :smile:

Don't think they'll use "probe" in the exam though?

Our college gave us loads of practice papers with the old syllabus stuff on! I was wondering why I wasn't understanding half of it :frown:

Do we need to know how to work out conductivity? (I know resistivity)

Reply 239

SamXi

Original post by Joseppea

I think so :smile:

Don't think they'll use "probe" in the exam though?

Our college gave us loads of practice papers with the old syllabus stuff on! I was wondering why I wasn't understanding half of it :frown:

Do we need to know how to work out conductivity? (I know resistivity)