Help Physics Power and velocity PLEASE
This is form 2825 June 2005 paper - Its the very last part confusing me.
So I was told to work out the the power of the electron beam which I worked out to be correctly 12,000W.
Then it says, "Calculate the velocity of the electrons if the rate of arrival of electrons is 7.5*10^17 s^-1. Relativistic effects may be ignored."
So what I've is done is, I know P=e/t so to find E which will be equal to KE I did 18,000 (p) multiplied by the time to get E.
And then rearranged 1/2mv^2 = E to find velocity. However, answer says it is wrong and what they wrote in the markscheme is 1/2mv^2 * 7.5*10^17 = 12,000
This makes no sense to me - why would they multiply the KE by time to get power?
It doesn't let me post a screenshot of the question of markscheme but I will be able to do so if someone replies. Thanks.
So I was told to work out the the power of the electron beam which I worked out to be correctly 12,000W.
Then it says, "Calculate the velocity of the electrons if the rate of arrival of electrons is 7.5*10^17 s^-1. Relativistic effects may be ignored."
So what I've is done is, I know P=e/t so to find E which will be equal to KE I did 18,000 (p) multiplied by the time to get E.
And then rearranged 1/2mv^2 = E to find velocity. However, answer says it is wrong and what they wrote in the markscheme is 1/2mv^2 * 7.5*10^17 = 12,000
This makes no sense to me - why would they multiply the KE by time to get power?
It doesn't let me post a screenshot of the question of markscheme but I will be able to do so if someone replies. Thanks.
They haven't multiplied energy by time. What they have done is to say that if the power is 12,000 W, then the amount of energy being developed every second is 12,000 J, using the fact that (as you say) P = E/t. So, each second (i.e. setting t = 1), P = E. But the energy of the beam is made up of the KE of all of the electrons that arrive, and since there are 7.5 x 10^17 arriving each second, this needs to be taken into account.
Original post by Pangol
They haven't multiplied energy by time. What they have done is to say that if the power is 12,000 W, then the amount of energy being developed every second is 12,000 J, using the fact that (as you say) P = E/t. So, each second (i.e. setting t = 1), P = E. But the energy of the beam is made up of the KE of all of the electrons that arrive, and since there are 7.5 x 10^17 arriving each second, this needs to be taken into account.
Ohhhh so 7.5*10^17 is actually the NUMBER OF ELECTRONS not time, - so it's the number of electrons and each electron will have KE 1/2mv^2 which will be equal to the total power here? Thank you so much
Original post by MrToodles4
Ohhhh so 7.5*10^17 is actually the NUMBER OF ELECTRONS not time, - so it's the number of electrons and each electron will have KE 1/2mv^2 which will be equal to the total power here? Thank you so much
Yes, this is it. There are a couple of ways that you can notice this in the future. One is the fact that they describe this number as the rate of arrival of electrons, that is, now many are turning up in a particular time. The other is to look at the unit - it is s^-1, so it is to do with something that happens every second. If it were a time, it would be s (seconds).
Original post by Pangol
Yes, this is it. There are a couple of ways that you can notice this in the future. One is the fact that they describe this number as the rate of arrival of electrons, that is, now many are turning up in a particular time. The other is to look at the unit - it is s^-1, so it is to do with something that happens every second. If it were a time, it would be s (seconds).
That makes perfect sense
really appreciate it.Quick Reply
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