TPR - Power Dissipated - Are they Wrong?

[LaughingMan]

Hey All,

Thought this was an easy point for a discrete. I think TPR is wrong though.

A 12-volt battery causes current to flow through a circuit containing just two 8-ohm resistors connected in parallel. How much power is dissipated in one of these resistors?
A. 9 watts
B. 18 watts
C. 36 watts
D. 72 watts

The correct answer is B but I put C.

I did,
In Parallel, so two 8R resistors have an effective of 4 R

V=IR,
12 = I x 4
I = 3 ohms

P = I^2 x R
P = 3^2 x 4 = 36.

The answer they give is,

B. The power dissipated in each resistor would be V^2 / R = 12^2 / 8 = 18 watts.

I'm thinking it should have been 3^2 x 8 now as well, but that is wrong too.

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[Postictal Raiden]

Hey All,

Thought this was an easy point for a discrete. I think TPR is wrong though.

A 12-volt battery causes current to flow through a circuit containing just two 8-ohm resistors connected in parallel. How much power is dissipated in one of these resistors?
A. 9 watts
B. 18 watts
C. 36 watts
D. 72 watts

The correct answer is B but I put C.

I did,
In Parallel, so two 8R resistors have an effective of 4 R

V=IR,
12 = I x 4
I = 3 ohms

P = I^2 x R
P = 3^2 x 4 = 36.

The answer they give is,

B. The power dissipated in each resistor would be V^2 / R = 12^2 / 8 = 18 watts.

I'm thinking it should have been 3^2 x 8 now as well, but that is wrong too.

I remember making the same mistake.

Because you used the equivalent resistance, your calculation will only yield the value for the power dissipation by the entire circuit (both resistors) which is double the value for each resistor. To calculate the power dissipated by each resistor, you need to figure out how much current passes through that resistor, because the current splits due to the parallel connection.

 

[LaughingMan]

I remember making the same mistake.

Because you used the equivalent resistance, your calculation will only yield the value for the power dissipation by the entire circuit (both resistors) which is double the value for each resistor. To calculate the power dissipated by each resistor, you need to figure out how much current passes through that resistor, because the current splits due to the parallel connection.

Bah, such an elementary mistake. Tunnel Vision--hate it.

(1.5)^2 x 8 = 18

Thanks a bunch.

 

[adrakdavra]

Can we find the total power = 36s watt then divided by 2 = 18 watts

 

[Temperature101]

Can we find the total power = 36s watt then divided by 2 = 18 watts

Yes you can since both resistors are equal...But if they were not, you would have to find the intensity that pass thru each resistor to calculate the power dissipated by each.

 

[LaughingMan]

Can we find the total power = 36s watt then divided by 2 = 18 watts

Only if the 2 are equal, in which this case they were. If one was 10 and one was 6, then I don't think you could.