The Student Room Group

AS physics - electricity

There are 4 identical cells each with e.m.f 3.0V and internal resistance of 1 ohm.

With all 4 cells in series, would it be 4 x 3 = 12V ?
And would the internal resistance be 4 x 1 = 4ohms ?

With all 4 cells in parallel, would it be 1/3 + 1/3 1/3 + 1/3 = 4/3 = 1.33V ?
How would I work out the internal resistance?

If they are short circuited and the terminals are connected by a thick wire (neglible resistance), what would be the current? And which battery would go flat quicker? Parallel or series?
Reply 1
The internal resistance for the parallel is also 1+1+1+1 because for resistors in parallel, Total Resistance= 1/(1/R1 +1/R2 +1/R3....)

For the total EMF of the 4 cells in parallel, it would just be 3v as the potential difference is the same for all the cells.
Reply 2
malolis
The internal resistance for the parallel is also 1+1+1+1 because for resistors in parallel, Total Resistance= 1/(1/R1 +1/R2 +1/R3....)

For the total EMF of the 4 cells in parallel, it would just be 3v as the potential difference is the same for all the cells.


I thought it was 4 but the book claims the total interntal resistance is 0.25
Reply 3
Poshtotty
I thought it was 4 but the book claims the total interntal resistance is 0.25


Yeah sorry it is. I just failed really hard :P

The total resistance is 1/ (1+1+1+1) = 1/4 =0.25

I wasn't concentrating too hard when I did it in my head :frown:
Reply 4
malolis
Yeah sorry it is. I just failed really hard :P

The total resistance is 1/ (1+1+1+1) = 1/4 =0.25

I wasn't concentrating too hard when I did it in my head :frown:


Ah yeah makes sense now.

Also, if the terminals of a battery are connected with a wide of resistance 2 ohms, the parallel cells will go flat more slowly, why is this? Any idea?
Reply 5
Poshtotty
Ah yeah makes sense now.

Also, if the terminals of a battery are connected with a wide of resistance 2 ohms, the parallel cells will go flat more slowly, why is this? Any idea?


the power from the batteries will have to overcome the resistance in the wire. less power flows due to this resistance therefore in a given amount of time less power is able to travel from the battery so the batteries last longer :smile:

hope that helps.
Reply 6
Mitch92uK
the power from the batteries will have to overcome the resistance in the wire. less power flows due to this resistance therefore in a given amount of time less power is able to travel from the battery so the batteries last longer :smile:

hope that helps.


Sorry i missed out a section. I'm actually wanting to know why it goes flat more slowly than it would if it was in series.
Reply 7
Poshtotty
Sorry i missed out a section. I'm actually wanting to know why it goes flat more slowly than it would if it was in series.


cus in series you add the resistances but in parallel you calculate resistance by : 1 / Rtotal = 1/R1 + 1/R2 and so there's less resistance in a parallel arrangement. Less internal resistance means less volts are lost so batteries last longer :smile: sorry for the delay btw :smile:

Latest