Essential Pre-Uni Physics C6a.8 please help

https://isaacphysics.org/questions/phys19_c6a_q8?stage=a_level

Still confused on how to get to the answer. The only way you could have a P.D of 1 across the combination is if 1 resistor has a pd of 1.1 and the other has .1. I’ve used that to try and find the ratio of currents and then used that to find the resistance of the other resistor but none of the answers I’ve given worked. Please help.

Original post by Jsvi6v6

https://isaacphysics.org/questions/phys19_c6a_q8?stage=a_level

Still confused on how to get to the answer. The only way you could have a P.D of 1 across the combination is if 1 resistor has a pd of 1.1 and the other has .1. I’ve used that to try and find the ratio of currents and then used that to find the resistance of the other resistor but none of the answers I’ve given worked. Please help.

When the p.d. across the resistor R1 is 1.2 V with a current of 0.40 A, we can find the resistance of the resistor R1 using V/I.
Next, we are told that another resistor say R2 is connected in parallel to R1, the p.d. across them is 1.0 V while the total current is 0.40, the effective resistance of the combination of R1 and R2 can be found using V/I, again.
Recall that the effective resistance Reff of R1 and R2 in parallel is
1/Reff = 1/R1 + 1/R2
which allows you to find the resistance of R2.

Reply 2

Joinedup

20

Original post by Jsvi6v6

https://isaacphysics.org/questions/phys19_c6a_q8?stage=a_level

Still confused on how to get to the answer. The only way you could have a P.D of 1 across the combination is if 1 resistor has a pd of 1.1 and the other has .1. I’ve used that to try and find the ratio of currents and then used that to find the resistance of the other resistor but none of the answers I’ve given worked. Please help.

Your reasoning is going wrong - the resistors are in parallel so they must both have the same PD as each other.
and you're told the current remains 0.4A but the PD changes to 1.0V - you don't have to worry about what else might or might not be in the circuit dropping the 0.2V

you can use R=V/I to get the resistance of the original single resistor (R1)

R1=1.2/0.4

then you can calculate the effective resistance of the 2 resistors in parallel
Reff = 1.0/0.4

then you can use the formula for resistors in parallel to calculate the second resistor R2

1/Reff=1/R1 + 1/R2

Reply 3

Jsvi6v6

OP

2

thank you,I got it correct.