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As level physics electricity help

I've already posted tho so somewhere else, no replies so I don't know it this is the right place. I need to find internal resistance is the equation r=E-IR or is that wrong? I know the answer for this question is 0.71 but can someone show me the method. Thank you.
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Reply 1
Avatar for KeyFingot
KeyFingot
Look at the dimensions of the equation you posted. r=E-IR is not correct

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Reply 2
Avatar for xsalllyy
xsalllyy
So if you consider just the 10 ohm resistor and the voltage, you've got
V=IR => 2.8=I*10
=> I= 2.8/10 = 0.28

now you know the current in the circuit.

so now you can use the current and the voltage for the internal resistance circuit.
V=IR => 3.0 = 0.28 * r
=> r = 3.0/0.28 = 10.714 ohms = 10.7 ohms to the correct number of decimal places.


but the proper equation for internal resistance is
E = I(R+r)
Reply 3
Avatar for xsalllyy
xsalllyy
or in the format you're looking for:

r = (E - IR)/I
or r = (E/I) - R

I struggled with electricity when I did it, you need to just look at what you know and see what equations you can put it into to find out ANYTHING then you might be able to get to the answer you need.
you can rarely get the answer with the proper equation for the variable you want.
if you'd have used the E = I(r+R) equation straight away there would be too many unknowns. :smile:
Reply 4
Avatar for xsalllyy
xsalllyy
sorry, the current is correct.
but the 2nd half, I did wrong.
for the internal resistance it should be
lost volts = Ir,
so it's 3-2.8 = 0.28*r
=> 0.2/ 0.28 = r
=> r = 0.71 ohms.
Reply 5
Avatar for be patient
be patient
OP
Thank you, you are very helpful, what exam board are you on image on ocr

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