Power wasted/delivered to a resistor

AI Thread Summary
The discussion revolves around calculating the power delivered to two 8-ohm resistors connected in series with a 12V battery that has an internal resistance of 8 ohms. Initially, the user calculated the power incorrectly as 18 watts, but through clarification, it was established that the total resistance in the circuit is 24 ohms, leading to a current of 0.5A. The correct total power dissipated is 6 watts, which, when divided among the three resistors (including the internal resistance), results in 2 watts per resistor. The conversation emphasizes the importance of using the correct formulas for power calculations and understanding the impact of internal resistance. Ultimately, the user gains clarity on the calculations and the distribution of power across the resistors.
chris858
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Homework Statement


two 8 ohm resistors and a battery of emf 12.0V and internal resistance of 8ohms are connected in series. Calculate the power delivered to each external resistor and then calculate the power wasted due to internal resistance.
I keep coming up with the answer 18 but apparently that is not right, and I am strugglying, so help would be much appreciated.

Homework Equations


W=IV
I=V/R

The Attempt at a Solution


VxV/R
12 X 12/8 =18...
current = 1.5A
 
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chris858 said:

Homework Statement


two 8 ohm resistors and a battery of emf 12.0V and internal resistance of 8ohms are connected in series. Calculate the power delivered to each external resistor and then calculate the power wasted due to internal resistance.
I keep coming up with the answer 18 but apparently that is not right, and I am strugglying, so help would be much appreciated.

Homework Equations


W=IV
I=V/R

The Attempt at a Solution


VxV/R
12 X 12/8 =18...
current = 1.5A
Sketch the circuit. How many resistors does the current flow through? What's the total resistance that the current flows through? So what then the current?
 
flows through two resistors, total resistance = 16 ohms. Current = 12/16 =0.75A
 
chris858 said:
flows through two resistors, total resistance = 16 ohms. Current = 12/16 =0.75A
What about the battery's internal resistance?
 
16+8=24 so 12/24=0.5A
 
chris858 said:
16+8=24 so 12/24=0.5A
Right. So now what will you do now that you know the current through each resistor?
 
pwer= voltage times voltage over current. p=VxV/R so 12x 12/0.5=2
 
chris858 said:
pwer= voltage times voltage over current. p=VxV/R so 12x 12/0.5=2
R is not current in the formula P = V2/R, it represents resistance.

There are other formulas for power. What are they?

Oh, and be sure to include units with all results.
 
power = work/time
P=IV
P=V²/R
P=RI²
 
  • #10
power is measured in watts
current in amps
resistance in ohms
voltage in volts
 
  • #11
Okay, so what is the power dissipated by one of the resistors?
 
  • #12
I get it now, so basically the equation i was using gave the total power watts, which is actually 6 not 2, and so there are 3 resistors, including the internal resistance, so therefore you have to divide 6 by 3 which gives 2.0Watts per resistor.
Thankyou very much!
 

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