Power wasted/delivered to a resistor

[chris858]

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

 

[gneill]

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?

 

[chris858]

flows through two resistors, total resistance = 16 ohms. Current = 12/16 =0.75A

 

[gneill]

flows through two resistors, total resistance = 16 ohms. Current = 12/16 =0.75A

What about the battery's internal resistance?

 

[chris858]

16+8=24 so 12/24=0.5A

 

[gneill]

16+8=24 so 12/24=0.5A

Right. So now what will you do now that you know the current through each resistor?

 

[chris858]

pwer= voltage times voltage over current. p=VxV/R so 12x 12/0.5=2

 

[gneill]

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.

 

[chris858]

power = work/time
P=IV
P=V²/R
P=RI²

 

[chris858]

power is measured in watts
current in amps
resistance in ohms
voltage in volts

 

[gneill]

Okay, so what is the power dissipated by one of the resistors?

 

[chris858]

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!