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OCR Physics A AS Forces+Motion/Electrons & Photons revision thread watch

1. (Original post by goku999)
This is a great revision thread ...
Though im dreading all my exams
me too
2. ''Sketch And explain the I/V characteristics of a metallic conductor at constant temperature, a semiconductor diode and a filament lamp''

I've attached a graph for filament but I cant do the others, help

Also

''Sketch the variation with temperature of the resistance of a pure metallic conductor and of a negative temperature coefficient (NTC) Thermistor''

Attached Images

3. ''Sketch And explain the I/V characteristics of a metallic conductor at constant temperature, a semiconductor diode and a filament lamp''
Metallic conductor - straight line through the origin
semiconductor diode - curved exponential graph that has a negative current with the voltage in the opposite direction.
filament lamp - s shaped graph with a flat gradient with a higher voltage.

''Sketch the variation with temperature of the resistance of a pure metallic conductor and of a negative temperature coefficient (NTC) Thermistor''
Resisitance of a pure metallic conductor increases uniformly with temperature. Graph is a straight line with the graph crossing the y-axis at + c.
Resistance of a NTC thermistor decreases as temperature increases. Graph shaped like y = 1/x
4. that is great, thanks

What could I say about the I/V characteristics of a metallic conductor and semiconductor diode?

for example for the filament lamp
-Line passes through origin
-For small currents and p.d's, graph is roughly straight line
-At higher p.d's line starts to curve, current flows less, resistance has increased
-As filament gets hotter, resistance increases.

I'll check tommorrow, goodnight
5. Metallic conductor
- Line passes through origin.
- A metallic conductor is an ohmic conductor since it obeys Ohm's law at constant temperature.
- Line is a straight line since voltage across the metallic conductor is directly proportional to current through the metallic conductor.
- Therefore using V = IR, R = V/I which is a constant for an ohmic conductor at constant temperature
- The gradient of the line = I/V which is the reciprocal of V/I so is also a constant.
- Gradient of curve also equals 1/R.

Semiconductor diode
- The graph is an exponential curve that passes through the origin.
- When the voltage is negative, the current is very low since the semiconductor only allows the current to flow in one direction.
- As the voltage increases, the current increases rapidly, since the diode allows the current to flow in the forward direction.
- At high voltages, the graph starts to curve and the current is very high.
- So using R = V/I we can see that as voltage increases slightly, current increases rapidly since the graph is exponential, so resistance decreases with increasing voltage/current.
6. that is perfect, thanks
7. Thanks!!! I have to say thats most usefull infomation i've found on forces and motion so far. I am good with calculations thanks to m1 but now this just completed it for me!
8. 3 days away, still need help

Electrons + Photons

''Describe and explain the use of thermistors and LDR's in potential dividers to provide a p.d which is dependant on temp and on light intensity respectively''

What do I actually need to know here?

''Show an appreciation of the particulate nature of electromagnetic radiation, ie a photon model''

Do I just need to appreciate that em radiation has a particulate nature? what do they mean a photon model?
9. PLEASE someone help, i'll give rep
10. check PM sis
11. ''Describe and explain the use of thermistors and LDR's in potential dividers to provide a p.d which is dependant on temp and on light intensity respectively''

I havent got a good description of this.

Just need to say when you have either a LDR or thermistor the p.d depends on the light intensity and temp respectively.

Anyone got a good description??
12. Here are the notes I promised people, All credit due to Widowmaker, so thanks to him.

''Describe and explain the use of thermistors and LDR's in potential dividers to provide a p.d which is dependant on temp and on light intensity respectively''

In a series circuit;
Vout/Vin = IR2/IRT
Vout = (R2 x Vin)/RT (1)

RT = R1 + R2 (2)

(2) into (1)
Vout = R2/(R1+R2) x Vin

- In a potential divider circuit, you have a normal resistor in series and a NTC thermistor also in series and you measure the p.d. across the NTC thermistor.
- Resistance through normal resistor is represented by R1
- Resistance through NTC thermistor is represented by R2
- Warming the NTC thermistor will decrease its resistance.
- Looking at the formula, if R2 decreases, then Vout decreases.
- If you want Vout to increase, then switch the resistor and the NTC thermistor around.

- In a potential divider circuit, you have a normal resistor in series and a LDR also in series and you measure the p.d. across the LDR.
- Resistance through normal resistor is represented by R1
- Resistance through LDR is represented by R2
- Warming the LDR will decrease its resistance.
- Looking at the formula, if R2 decreases, then Vout decreases.- If you want Vout to increase, then switch the resistor and the LDR around.
13. Good luck to everyone doing either Forces, Electrons and/or Waves tommorrow btw.
14. does anyone know if the right hand grab rule is for conventional current or the flow of electrons?
15. (Original post by stuey)
does anyone know if the right hand grab rule is for conventional current or the flow of electrons?
Its for the direction of conventional current.

Thanks to whoever neg repped me for this thread btw, idiots

Exam in under 6 hours

m0.
17. Can someone give me a good definition please.

''Describe qualitatively motion in a curved path due to a perpendicular force, and understand that centripetal accelartion in the case of uniform motion in a circle''
18. (Original post by chats)
Can someone give me a good definition please.

''Describe qualitatively motion in a curved path due to a perpendicular force, and understand that centripetal accelartion in the case of uniform motion in a circle''
you want a definition of circular motion?

an object moves with uniform circular motion when the force providing the motion originates from a fixed point and is always perpendicular to the velocity of the object.
19. (Original post by El Chueco)
you want a definition of circular motion?

an object moves with uniform circular motion when the force providing the motion originates from a fixed point and is always perpendicular to the velocity of the object.
is that all they want us to know?
20. (Original post by chats)
is that all they want us to know?
what else is there? you might want to put that the force being perpendicular means the speed is constant but velocity is continually changing...

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