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Advanced level physics voltage help?

It is necessary to maintain a p.d. of at least 3.4 volts across a device of resistance 0.5 ohms. What is the least number of cells, each of emf 2.0 V and internal resistance 0.20 ohms, which must be used in series?
(answer is 6 cells calculated at 5.3)

First I calculated the current with the pd 3.4 volts and the resistance 0.5 ohms, giving a current of 6.8 amps. Because they are asking in series that means the current will stay the same.
Then I worked out 1/Rtotal 1/0.2 which is 5
but im not sure what to do next :/

Help is much-appreciated thanks
Original post by Montenzuma
It is necessary to maintain a p.d. of at least 3.4 volts across a device of resistance 0.5 ohms. What is the least number of cells, each of emf 2.0 V and internal resistance 0.20 ohms, which must be used in series?
(answer is 6 cells calculated at 5.3)

First I calculated the current with the pd 3.4 volts and the resistance 0.5 ohms, giving a current of 6.8 amps. Because they are asking in series that means the current will stay the same.
Then I worked out 1/Rtotal 1/0.2 which is 5
but im not sure what to do next :/

Help is much-appreciated thanks

At the required current, what is the terminal voltage of each (series) cell? How many are required to reach 3.4V?
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Montenzuma
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Original post by RogerOxon
At the required current, what is the terminal voltage of each (series) cell? How many are required to reach 3.4V?

I'm not sure because the number of cells is what we have to figure out :frown:
Original post by Montenzuma
I'm not sure because the number of cells is what we have to figure out :frown:

Each cell has an internal resistance of 0.2 Ohm, so how much of it's 2V EMF is lost at the required current?
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Montenzuma
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Original post by RogerOxon
Each cell has an internal resistance of 0.2 Ohm, so how much of it's 2V EMF is lost at the required current?

that would be v/r so 2/0.2 = 10 amps?
Original post by Montenzuma
that would be v/r so 2/0.2 = 10 amps?

No. You calculated the minimum current required to maintain 3.4V across the load. Now calculate how much of each cell's EMF will be lost at that minimum current.
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Montenzuma
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Original post by RogerOxon
No. You calculated the minimum current required to maintain 3.4V across the load. Now calculate how much of each cell's EMF will be lost at that minimum current.

ahh so V=IR = 2 x 10 = 20 volts?
Original post by Montenzuma
It is necessary to maintain a p.d. of at least 3.4 volts across a device of resistance 0.5 ohms. What is the least number of cells, each of emf 2.0 V and internal resistance 0.20 ohms, which must be used in series?
(answer is 6 cells calculated at 5.3)

First I calculated the current with the pd 3.4 volts and the resistance 0.5 ohms, giving a current of 6.8 amps. Because they are asking in series that means the current will stay the same.


Original post by Montenzuma
ahh so V=IR = 2 x 10 = 20 volts?

No.

To maintain a voltage of 3.4V across the 0.5 Ohm load, you calculated that a minimum current of 6.8A is required. That current flows through all the cells in series, so each cell loses the same amount of its EMF.

Each cell has an internal resistance of 0.2 Ohm, so how much of its EMF is lost when 6.8A is flowing through it?
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Montenzuma
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Original post by RogerOxon
No.

To maintain a voltage of 3.4V across the 0.5 Ohm load, you calculated that a minimum current of 6.8A is required. That current flows through all the cells in series, so each cell loses the same amount of its EMF.

Each cell has an internal resistance of 0.2 Ohm, so how much of its EMF is lost when 6.8A is flowing through it?

oh:colondollar:
0.2 ohms, 6.8A so voltage is IR = 0.2 x 6.8 = 1.36V ? is that the emf lost or the total emf:confused:
Original post by Montenzuma
oh:colondollar:
0.2 ohms, 6.8A so voltage is IR = 0.2 x 6.8 = 1.36V ? is that the emf lost or the total emf:confused:

That's what is lost of each cell's 2V EMF. Now calculate what's left, per cell. Finally, find how many cells you need in series to get 3.4V.
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Montenzuma
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Original post by rogeroxon
that's what is lost of each cell's 2v emf. Now calculate what's left, per cell. Finally, find how many cells you need in series to get 3.4v.

aaaa thank u sm i got it now

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