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physics help!

Hi I need some help with this physics question




From the circuit in the figure calculate the following:

a) Currents through resistors R1, R2 and R3
b) Power dissipated in R3



Note: values of the resistances are in Ohms.



im stuck on a and b... please help!
be9d9c3033c5648ae5361c36f530e13d.png1.5KB
For A:
R1, R3: 10 = I(3 + 7)
I = 1A

R2, R3: 5 = I(3 + 14)
I = 0.29A
Just one more step to work out the current of each resistor.


For B:
Use v = IR with the current from A.
hey! what do you mean one more step for question a.. dont get that part
Original post by SpikeSpiegel
hey! what do you mean one more step for question a.. dont get that part

You have IR1 and IR2.
IR3 = IR2 + IR1.
Cool ... thank you so much!!!!
For part a) i got

a) R1, R3 : 10 = I(3+7)
Therefore I = 1A


R2, R3 : 5 = I(3+14)
I= 0.29 A


I have - IR1 and IR2
: IR3 = IR2 + IR1

So, R3 = R1+R2
R3= 1A + 0.29A = 1.29A



is this how I would approach this?


I don't know how to go about part b since there is 2 voltages for R3
Original post by SpikeSpiegel
For part a) i got

a) R1, R3 : 10 = I(3+7)
Therefore I = 1A


R2, R3 : 5 = I(3+14)
I= 0.29 A


I have - IR1 and IR2
: IR3 = IR2 + IR1

So, R3 = R1+R2
R3= 1A + 0.29A = 1.29A



is this how I would approach this?


I don't know how to go about part b since there is 2 voltages for R3

Yes.

For b, use v = IR, you know I and R.
but it asks for power dissipated ... not voltage?
Original post by SpikeSpiegel
but it asks for power dissipated ... not voltage?

Sorry I didn't read that part of the question properly, but the idea is the same. There are two equivalent methods:
1. Calculate voltage as I said and use p = Iv.
2. Use p = I2R.
I would go with 2.
(edited 9 years ago)
so.. this is how i did it

p= I^2 R

p= 1.29 x 3

so, p = 3.87?
Original post by SpikeSpiegel
so.. this is how i did it

p= I^2 R

p= 1.29 x 3

so, p = 3.87?

You forgot to square the I
It should be p = 1.292x3=4.9923W

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