The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

Physics questions

17 (a) A student uses a computer for an average of 5 hours every day. The battery supplies a current of 3.5 A to the computer. Calculate how many electrons flow through the computer’s battery in 5 hours.

b) The computer’s screen emits visible light photons with an average frequency of
5.5* 10^14 Hz. The power of the light emitted is 10 W.
Calculate the number of photons emitted per second from the computer screen.


Please can some explain their workign as well
Reply 1
Avatar for jaroc
jaroc
6
Original post by jsmith6131
17 (a) A student uses a computer for an average of 5 hours every day. The battery supplies a current of 3.5 A to the computer. Calculate how many electrons flow through the computer’s battery in 5 hours.


What is the definition of current?


b) The computer’s screen emits visible light photons with an average frequency of
5.5* 10^14 Hz. The power of the light emitted is 10 W.
Calculate the number of photons emitted per second from the computer screen.


Each photon has some energy - how does it depend on frequency? Power is energy per second. How much energy is emitted each second? How does it depend on the number of photons emitted each second?
a) Current = rate of flow of charge (NOT NUMBER OF ELECTRONS)

b)
energy of each photon = 3.6 * 10^-19 J
therefore power per photon (per second) = 3.6*10^-19

10 Watts per second

Total watt / watt per photon
10 / 3.6*10^-19 = 2.78 * 10^-19

??
Reply 3
Avatar for limetang
limetang
19
Original post by jsmith6131
a) Current = rate of flow of charge (NOT NUMBER OF ELECTRONS)

b)
energy of each photon = 3.6 * 10^-19 J
therefore power per photon (per second) = 3.6*10^-19

10 Watts per second

Total watt / watt per photon
10 / 3.6*10^-19 = 2.78 * 10^-19

??


If you have rate of flow of charge you can calculate the amount of charge passing in a certain amount of time, and if know the charge of an electron you can figure out how many times the charge of an electron goes into the total charge 'used'
Q = It
Q = 3.5 * 5 * 60 * 60
Q = 63000 C
Q/1.6*10^-19 = 3.9 * 10^23
Reply 5
Avatar for jaroc
jaroc
6
Original post by jsmith6131
Q = It
Q = 3.5 * 5 * 60 * 60
Q = 63000 C
Q/1.6*10^-19 = 3.9 * 10^23


Correct :smile:


Original post by jsmith6131
b)
energy of each photon = 3.6 * 10^-19 J
therefore power per photon (per second) = 3.6*10^-19

10 Watts per second

Total watt / watt per photon
10 / 3.6*10^-19 = 2.78 * 10^-19


There is a miscalculation, but generally correct. One reservation would be that it doesn't make sense to speak about 'watts per second' (in this context). 1 watt is 1 joule per second.
I dont get part b at all.

P = E/t <<< so how come we are dividing power by energy?

somebody explain please.
Reply 7
Avatar for SsEe
SsEe
13
Original post by ANTI-GADDAFI
I dont get part b at all.

P = E/t <<< so how come we are dividing power by energy?

somebody explain please.


The energy is that of a single photon. The power is total energy output per second. So the number of photons is power / energy.

Spoiler

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you