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# Electricity EMF and Internal Resistance question watch

1. A Battery of EMF 9.0V and internal resistance r, 6.0ohms is connected across a 30 ohm resistor.

what will V subscript t be if the cell is on open circuit?
2. (Original post by thefatone)
A Battery of EMF 9.0V and internal resistance r, 6.0ohms is connected across a 30 ohm resistor.

what will V subscript t be if the cell is on open circuit?
I think you have to find out the total current and then use that to find V.
3. (Original post by Science_Girl)
I think you have to find out the total current and then use that to find V.
pls talk me through what to do i have no idea what i'm doing...

i found the current going through the resistor which is 0.25A
4. (Original post by thefatone)
pls talk me through what to do i have no idea what i'm doing...

i found the current going through the resistor which is 0.25A
That's right!
You then need to use V=IR again to find out the total external V. To do this, you use the external resistor value of 30 ohms
5. (Original post by Science_Girl)
I think you have to find out the total current and then use that to find V.
If it's an open circuit would Vt not equal EMF?
6. (Original post by Science_Girl)
That's right!
You then need to use V=IR again to find out the total external V. To do this, you use the external resistor value of 30 ohms
so i got 0.25x30=7.5V so what do i do now?
7. thanks for all your helps guys i figured it out
8. (Original post by Xsk)
If it's an open circuit would Vt not equal EMF?
That's only the case if the internal resistance is zero or negligible, is it not?
9. (Original post by Asuna Yuuki)
That's only the case if the internal resistance is zero or negligible, is it not?
I have the exact question and answer in front of me here. The answer is apparently 9V, which matches the EMF
10. (Original post by Xsk)
I have the exact question and answer in front of me here. The answer is apparently 9V, which matches the EMF
Hmm. What precisely does Vt refer to? I'm unfamiliar with the terminology
11. (Original post by Asuna Yuuki)
Hmm. What precisely does Vt refer to? I'm unfamiliar with the terminology
Vt is the voltage as measured across the two terminals of the battery. It is normally calculated by Vt =emf - ir, where r is the internal resistance. In this case, since it it is an open circuit (i.e. battery isn't connected to anything), the current is 0 so the equation will simplify to Vt=emf
12. (Original post by samb1234)
Vt is the voltage as measured across the two terminals of the battery. It is normally calculated by Vt =emf - ir, where r is the internal resistance. In this case, since it it is an open circuit (i.e. battery isn't connected to anything), the current is 0 so the equation will simplify to Vt=emf
A battery of emf 12V and internal resistance 1.5 ohms was connected to a 4.5ohm resistor

i worked out total resistance of circuit 6 ohms and current through battery 2A

how do i work out lost p.d and pd across the cell terminals?
13. the 'lost' p.d. will be the voltage drop due to the internal resistance of the battery, so would be equal to the current x internal resistance. The voltage across the terminals of the battery is therefore the emf-ir (i.e. the emf of the cell - the drop due to the internal resistance of the battery itself
14. (Original post by samb1234)
the 'lost' p.d. will be the voltage drop due to the internal resistance of the battery, so would be equal to the current x internal resistance. The voltage across the terminals of the battery is therefore the emf-ir (i.e. the emf of the cell - the drop due to the internal resistance of the battery itself
thanks
15. (Original post by thefatone)
thanks
no worries, anything else i can help with?
16. (Original post by samb1234)
no worries, anything else i can help with?
not at the moment i might comes up with something else in a few minutes though
17. (Original post by samb1234)
no worries, anything else i can help with?
okay so emf 1.5v internal resistance 0.5ohm are connected to 2.5 ohm resistor

i worked out current as 0.5A terminal p.d as 1.25v
how do i find the power delivered to 2.5 ohm resistor and power wasted in the cell?
18. (Original post by thefatone)
okay so emf 1.5v internal resistance 0.5ohm are connected to 2.5 ohm resistor

i worked out current as 0.5A terminal p.d as 1.25v
how do i find the power delivered to 2.5 ohm resistor and power wasted in the cell?
well what formula relating power, current and resistance could you apply (you can use other ones but it's probably the easiest one to apply here)
19. (Original post by samb1234)
well what formula relating power, current and resistance could you apply (you can use other ones but it's probably the easiest one to apply here)
i got p=vi which i used and got 0.63w but i'm not sue about the 2nd part
20. (Original post by thefatone)
i got p=vi which i used and got 0.63w but i'm not sue about the 2nd part
It works exactly the same for the internal resistance, you can just use a standard power formula. If you don't already know them, it would be worth getting comfortable with using p=I^2 R and p=v^2 /R as they make the written questions much easier

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