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# Higher physics help watch

1. Could someone explain to me how QV=hf can be derived from W=Ephoton?

Thanks
2. QV=hf gives you the maximum possible energy the photon can give to an electron, assuming this topic involves excitation?
QV=hf gives you the maximum possible energy the photon can give to an electron, assuming this topic involves excitation?
I think so? It's about LEDs. I have a graph of switch on voltage against frequency and I have to derive Planck's constant from it. I get that the I can derive it from 'gradient x charge of an electron' but I don't get the first step. How does it give me the maximum energy?
4. Oh never mind. I think I got it now

5. Can any of you explain this question to me because I have no idea what to do here thanks.
6. (Original post by Hey234)

Can any of you explain this question to me because I have no idea what to do here thanks.

Straight out of course notes.
The gradient of the graph = -r, where r is internal resistance of the battery.
Extend the line with a ruler til it touches the axes. The y-axis intercept i the emf, the x-axis intercept is the sshort-circuit current, which is what the question asks for.

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