# OCR A - Level Physics 2018 Breadth - Section A: Question 2

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#1
https://imgur.com/a/dzc5Fv6

I figured out the question with a relative a while back, and i know the answer but i'd like ( if possible ) for someone to give an explanation as to why the answer is D and the steps you took to reach your conclusion

EDIT: I figured it out with a relative BUT i've forgotten it now.
Last edited by Inmate_I; 1 year ago
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1 year ago
#2
The question states quantities rather than units, thus immediately eliminating A and B. Both "second" and "ampere" are units. If you're not sure about this, I'd recommend going back to chapter 1 of your physics textbook and rereading the part about units, particularly SI units - theyre extremely useful later on! Continuing, the definition of intensity is "Power per unit area" (I dont remember if this is how it is worded in OCR Physics, long time since I sat those papers!), NOT "energy per unit area", hence C is incorrect. This leaves D remaining.

If you wish to take a more direct approach, finding which one is correct rather than which ones are incorrect, you must remember the difference between electromotive force and potential difference. iirc, for me, i always remembered what potential difference was, and then from that, recalled that electromotive force was the opposite. Potential difference is in someways easy to remember - think of a simple resistor. Electrical current passes through the resistor, and as a result it heats up. Therefore, there is a transfer from electrical energy to heat energy. This is quantified using giving the resistor a volt reading, for example, a 5V resistor. By definition, E = QV, hence V = E/Q, or electrical energy per unit charge. Putting this together, potential difference is defined as the energy per unit charge from electrical energy to other forms. Now, recall that electromotive force is the opposite to potential difference, it can be said that electromotive force is the energy per unit charge from other forms to electrical energy.

I hope that helps clear things up.
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