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

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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.

(edited 7 years ago)

Reply 81

physicsamor

Original post by HassanMirza

You are amazing! Thank you, I understand now :h:

Reply 82

Rubberbandit

http://cowenphysics.com/wp-content/uploads/2016/05/H156-02-paper-Dec-15-v1.1.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)

(edited 7 years ago)

9th of June?

Original post by Rubberbandit

This question threw me also, I tried doing (6/110)*100 but that gave me 5.5V?

Reply 85

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)

Reply 86

DSG7799

Original post by voltz

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.

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Reply 87

Starmock99

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?

Reply 88

TheRealLBA

Has anyone got links to super hard moments questions? Thanks!

Reply 89

M0nkey Thunder

Original post by TheRealLBA

Has anyone got links to super hard moments questions? Thanks!

https://isaacphysics.org/questions/ch_b_p5?board=physicsskills_book_ch_b5

Give the last few questions a shot.

Reply 90

dylan1016

Original post by Rubberbandit

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!

Reply 91

HassanMirza

Original post by Rubberbandit

I get 5.5V, so I'm not sure really. Sorry!

Reply 92

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??

Reply 93

dididid

any links to all the information on the practicals and how to do them?

Reply 94

tWSol

Original post by BobAhmed

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.

Reply 95

BobAhmed

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...

Reply 96

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!