You are Here: Home >< Physics

Official OCR New Spec AS Level Physics: Depth in physics - 9th of May watch

1. (Original post by physicsamor)
Can someone explain how to answer this question step by step:

The photocell is connected to a 12V supply and a sensitive ammeter which can detect a current of 1x10^-9A. Only 5% of the photons of average energy 4×10^-19J incident on the plate A cause electrons to be released. Calculate the minimum light energy that plate A must absorb per second for the photocell circuit to detect a current.
Well first, you need to work out how many electrons are emitted because 4x10^-19 J of energy will release 1 electron.

Q = It = 1.0x10^-1 x 1 = 1.0x10^-9 C

Then we work out number of electrons by using no. of electrons = Q/elementary
charge

=1.0x10-9 / 1.6x10-19 = 6.25x 10^9 electrons released.

5% of the photons had average energy of the energy required to release a photon.That means number of photons x 0.05 = number of electrons emitted.

No. of photons = 6.25x10^9 / 0.05 = 1.25x10^11 photons.

Total energy = no. of photons x average energy= 1.25x10^11 x 4x10^-19 = 5x10-8 J of energy required to detect a current.
2. (Original post by HassanMirza)
Well first, you need to work out how many electrons are emitted because 4x10^-19 J of energy will release 1 electron.

Q = It = 1.0x10^-1 x 1 = 1.0x10^-9 C

Then we work out number of electrons by using no. of electrons = Q/elementary
charge

=1.0x10-9 / 1.6x10-19 = 6.25x 10^9 electrons released.

5% of the photons had average energy of the energy required to release a photon.That means number of photons x 0.05 = number of electrons emitted.

No. of photons = 6.25x10^9 / 0.05 = 1.25x10^11 photons.

Total energy = no. of photons x average energy= 1.25x10^11 x 4x10^-19 = 5x10-8 J of energy required to detect a current.
You are amazing! Thank you, I understand now
3. http://cowenphysics.com/wp-content/u....1.pdf#page=10

can anyone geniuses work out where the mark scheme got the answer that the variation in p.d. in the left circuit across the LED is 5.6V to 6V???????

(hi there HassanMirza)
4. 9th of June?
5. (Original post by Rubberbandit)
http://cowenphysics.com/wp-content/u....1.pdf#page=10

can anyone geniuses work out where the mark scheme got the answer that the variation in p.d. in the left circuit across the LED is 5.6V to 6V???????
This question threw me also, I tried doing (6/110)*100 but that gave me 5.5V?
6. When do you know if its a potential divider question and not just resistors? This is confusing me because im insure when exactly you can use Voltage out = Voltage in * (Resistance of x/Total resistance of other components)
7. (Original post by voltz)
When do you know if its a potential divider question and not just resistors? This is confusing me because im insure when exactly you can use Voltage out = Voltage in * (Resistance of x/Total resistance of other components)
Potential dividers are used when you need to get the minimum or maximum reading from a compent e.g a voltmeter. When doing the characteristics of a diode it is better to use a potential divider as the characteristics happen at a low voltage but for an ohmic resistor where the characteristics of it is a straight line through zero a potential divider is not needed as the trend is obvious at any reading on a voltmeter.

Posted from TSR Mobile
8. Guys I am unsure on how to figure out if a component is in parallel and series. You know the potential divider circuit.. If they asked you to add the resistors.. how would you know which ones are in parallel and which ones are in series?
9. Has anyone got links to super hard moments questions? Thanks!
10. (Original post by TheRealLBA)
Has anyone got links to super hard moments questions? Thanks!
https://isaacphysics.org/questions/c...lls_book_ch_b5

Give the last few questions a shot.
11. (Original post by Rubberbandit)
http://cowenphysics.com/wp-content/u....1.pdf#page=10

can anyone geniuses work out where the mark scheme got the answer that the variation in p.d. in the left circuit across the LED is 5.6V to 6V???????

(hi there HassanMirza)
(Original post by therealbatman365)
This question threw me also, I tried doing (6/110)*100 but that gave me 5.5V?
This is a mistake in the mark scheme!
12. (Original post by Rubberbandit)
http://cowenphysics.com/wp-content/u....1.pdf#page=10

can anyone geniuses work out where the mark scheme got the answer that the variation in p.d. in the left circuit across the LED is 5.6V to 6V???????

(hi there HassanMirza)
I get 5.5V, so I'm not sure really. Sorry!
13. For the moments question of that locked paper, b)ii) where you had to find the tension:
why couldn't you do a triangle of forces, since the object is in equilibrium?
Also,(for moments around A), instead of finding perpendicular distances, can't you just resolve to find the components of the forces which act in a perpendicular direction to the arm. This will give Tsin30=1800gcos30??
14. any links to all the information on the practicals and how to do them?
15. (Original post by BobAhmed)
For the moments question of that locked paper, b)ii) where you had to find the tension:
why couldn't you do a triangle of forces, since the object is in equilibrium?
Also,(for moments around A), instead of finding perpendicular distances, can't you just resolve to find the components of the forces which act in a perpendicular direction to the arm. This will give Tsin30=1800gcos30??
There were 2 unknown forces N and T. So the triangle wouldn't be solvable.

You can resolve forces for moments about A. Same amount of work as other way.
16. (Original post by tWSol)
There were 2 unknown forces N and T. So the triangle wouldn't be solvable.

You can resolve forces for moments about A. Same amount of work as other way.
yh thats true, but with trig all u need is an angle and 1 known side, we know the angle 30 and the weight...
17. Guys in the official practice paper for depth I noticed there were a number or experiment related question where you would have to present uncertainties and systematic errors etc.... Anyone got any revision stuff for that?

So like maybe how to find uncertainties and definitions of precision/reliability/accuracy and what exactly things like random or systematic errors are just because it seems you could get quite a few marks for knowing things like this.

Any help would b emus happreciated and Good luck everyone!
18. (Original post by dodgewc25)
Guys in the official practice paper for depth I noticed there were a number or experiment related question where you would have to present uncertainties and systematic errors etc.... Anyone got any revision stuff for that?

So like maybe how to find uncertainties and definitions of precision/reliability/accuracy and what exactly things like random or systematic errors are just because it seems you could get quite a few marks for knowing things like this.

Any help would b emus happreciated and Good luck everyone!
Check the Module 2 notes
19. (Original post by dodgewc25)
Guys in the official practice paper for depth I noticed there were a number or experiment related question where you would have to present uncertainties and systematic errors etc.... Anyone got any revision stuff for that?

So like maybe how to find uncertainties and definitions of precision/reliability/accuracy and what exactly things like random or systematic errors are just because it seems you could get quite a few marks for knowing things like this.

Any help would b emus happreciated and Good luck everyone!

Not to be rude mate, the exam is in 4 hours and you don't know any of that? Why would you leave revision to literally the last minute?
20. (Original post by dodgewc25)
Guys in the official practice paper for depth I noticed there were a number or experiment related question where you would have to present uncertainties and systematic errors etc.... Anyone got any revision stuff for that?

So like maybe how to find uncertainties and definitions of precision/reliability/accuracy and what exactly things like random or systematic errors are just because it seems you could get quite a few marks for knowing things like this.

Any help would b emus happreciated and Good luck everyone!
here is a video I just watched on Uncertainty: https://www.youtube.com/watch?v=zrGSDHSOM5E

Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 13, 2016
Today on TSR

Edexcel C3 Maths Unofficial Markscheme

Find out how you've done here

984

students online now

Exam discussions