# Power in circuit

The designer of a heating element for the rear window of a car decides to connect six separate
heating elements together as shown in Figure 2. Each element has a resistance of 6.0Ω and the
unit is connected to a 12V dc supply having zero internal resistance. See: http://img219.imageshack.us/img219/7709/cirrg6.th.jpg [Broken]

In order to calculate the power output of the cicuit, I found the current in one of the resistors (1A), and using P=(I^2)R, found the power output of each resistor to be 6W. 6*6 = 36W for total power output.

I was then asked to compare what would happen if the resistors were in series, which I found Total R = 36 ohms, therefore current = 12/36 = 1/3A. Again, using P=(I^2)R, I found the total power output to be 4 W.

My question is, are the calculations above correct, and if so, how come the output of a supply is varied by the contents of the circuit?

Thanks

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Doc Al
Mentor
Your calculations are correct. Realize that while the battery provides a fixed voltage, the amount of power delivered depends on the resistive load. (The maximum current that the battery can put out is limited.) This should make some sense: What about when you disconnect the circuit? Does the power keep getting drawn? (Disconnecting the circuit is the same as inserting an infinite resistor across the battery.)

Ah yes, thank you. So when a battery is short-circuited, is the current flowing very high (ie. towards infinity), resulting in very high power disspation?

Thanks

Doc Al
Mentor
Right!