# Electricity physics question help? :(Watch

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Thread starter 3 weeks ago
#1
What is the maximum current that 12 cells, each of emf 2.0 V and internal resistance 0.6 ohms, can produce in a resistor of 0.80 ohms?

The answer is 5 A.

I’m not sure how to do this question because I thought it’d be a simple V=IR equation and then multiplying the final answer by 12 but I do not seem to get 5 amps.

Help is much appreciated
Last edited by Montenzuma; 3 weeks ago
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3 weeks ago
#2
What resistance are you plugging in for R in your V=IR equation?

(also it helps if you say what level this is for, e.g. GCSE/A-level/degree level etc ).
Last edited by artful_lounger; 3 weeks ago
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Thread starter 3 weeks ago
#3
(Original post by artful_lounger)
What resistance are you plugging in for R in your V=IR equation?

(also it helps if you say what level this is for, e.g. GCSE/A-level/degree level etc ).
Hi, this is for Advanced level physics
I would plug in the 0.8 resistor value
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3 weeks ago
#4
(Original post by Montenzuma)
Hi, this is for Advanced level physics
I would plug in the 0.8 resistor value
What about the battery's internal resistance?
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3 weeks ago
#5
(Original post by Montenzuma)
Hi, this is for Advanced level physics
I would plug in the 0.8 resistor value
Do you see where multiplying that by 12 would then cause an issue? Remember there is only one 0.8 ohm resistor in the circuit

You also don't seem to have accounted for the internal resistance there. I assume all cells and the resistor are connected in series? While I'm pretty rusty I don't actually get 5A either if the cells and resistor are in series though, although it's been quite a few years since I had to do any of this
Last edited by artful_lounger; 3 weeks ago
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3 weeks ago
#6
(Original post by Montenzuma)
What is the maximum current that 12 cells, each of emf 2.0 V and internal resistance 0.6 ohms, can produce in a resistor of 0.80 ohms?

The answer is 5 A.

I’m not sure how to do this question because I thought it’d be a simple V=IR equation and then multiplying the final answer by 12 but I do not seem to get 5 amps.

Help is much appreciated
If you place the cells in series with the load, what is the total resistance? EMF? Current?

How else can you arrange the cells?
Last edited by RogerOxon; 3 weeks ago
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3 weeks ago
#7
(Original post by Montenzuma)
What is the maximum current that 12 cells, each of emf 2.0 V and internal resistance 0.6 ohms, can produce in a resistor of 0.80 ohms?

The answer is 5 A.

I’m not sure how to do this question because I thought it’d be a simple V=IR equation and then multiplying the final answer by 12 but I do not seem to get 5 amps.
Does the question specify a circuit arrangement, e.g. cells in series or parallel ? For a question like this its always useful to start with a sketch of the circuit(s) as that can give you a very clear hint how to start throwing maths at the problem. The answer will definitly require Ohms law, but which circuit to apply it to should be the starting point .
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3 weeks ago
#8
(Original post by Montenzuma)
What is the maximum current that 12 cells, each of emf 2.0 V and internal resistance 0.6 ohms, can produce in a resistor of 0.80 ohms?

The answer is 5 A.

I’m not sure how to do this question because I thought it’d be a simple V=IR equation and then multiplying the final answer by 12 but I do not seem to get 5 amps.

Help is much appreciated
This is going to entail arranging the cells in a mixture of series and parallel. Now it is a well known fact that we get maximum power (and so maximum current because of I2R) out of a supply when the external resistance is equal to the internal resistance. Put the cells into 4 groups of 3 parallel cells and then put these groups in series. This gives you?
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