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# Physics help????? watch

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1. Hey, When you have say a voltage-current graph with a sloping line and a question say asks you to work out maximum power. I know that P = IV but what do you use for 'I' and 'V'. Do you have to work out an average and how do you do this??
2. (Original post by need_help)
Hey, When you have say a voltage-current graph with a sloping line and a question say asks you to work out maximum power. I know that P = IV but what do you use for 'I' and 'V'. Do you have to work out an average and how do you do this??
i'm pretty sure you would work out the area under the graph.
you can always look at it this way: if the units on each axes multiply together to give the required result, then you need to find the area under the graph.
if the unit on the y-axis divided by the unit on the x-axis gives the required result, then you need to find the gradient of the line, since grad. = (change in y) / (change in x)
3. (Original post by need_help)
Hey, When you have say a voltage-current graph with a sloping line and a question say asks you to work out maximum power. I know that P = IV but what do you use for 'I' and 'V'. Do you have to work out an average and how do you do this??
Well, as you said, Power can be calculated as IV.

To calculate the average power i.e.) Average rate of energy transfer, you can calculate the area under the graph, providing that the graph is linear in this section.

Hence, you would have a triangle and the area of this can be used as the average power maintained over this section.

Hope that helps
4. Thanks for the quick responses - the product multiply/divide thing for area and gradient is very useful.

This particular question asks for the maximum power. The graph only remains linear/constant for a small section. How would i work this out.
5. (Original post by need_help)
Thanks for the quick responses - the product multiply/divide thing for area and gradient is very useful.

This particular question asks for the maximum power. The graph only remains linear/constant for a small section. How would i work this out.
Well, if they are asking for maximum power then this is the point on the linear graph where voltage and current are at maximum values.

Just find the value of V and I at this maximum point on the graph, and multiply them to give the Maximum power.
6. Hmm....I tried that at first. The question asks you to prove the maximum power is 1.4W and using the maximum values you get 1.8W - not very close really when its such small numbers involved. I've tried to attach the question. See what you think.
Attached Files
7. right.doc (43.5 KB, 88 views)
8. (Original post by need_help)
Hmm....I tried that at first. The question asks you to prove the maximum power is 1.4W and using the maximum values you get 1.8W - not very close really when its such small numbers involved. I've tried to attach the question. See what you think.
You have to do it where the product is the highest - ie both are as high as possible, so it looks like about .45ish Volts * 3.2ish Amps which comes out at 1.44 Watts
9. ahh...i see. Thanks for that.

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