When is power absorbed or dissipated in a DC Voltage source and resistors?

Discussion in 'Homework Help' started by exidez, Aug 24, 2008.

Aug 22, 2008
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In one of my questions it asks if power is absorbed or dissipated in certain voltage sources in the circuit.
I have calculated the power using P = E*I
and the current is leaving the positive terminal and entering the negative terminal. This means that the voltage source is generating power or delivering power right?
and if the current is passing into the positive terminal and leaving from the negative, power is being absorbed or dissipated right?
I always get confused on the wording, especially when it comes to the resisters.

i have in my notes (i may have written in down incorrectly) that when current is passing through the a positive side of the resistor, power is generated or it is delivering power. When it is passing through the negative side and out through the positive it is absorbing / dissipating power...

Is this correct?

2. beenthere Retired Moderator

Apr 20, 2004
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I would be curious to learn your text's definition of "absorbed power". Possibly, that would refer to the power actually transfered to do work, as opposed to heat losses - "dissipated" power.

Resistors with current through them dissipate power. The voltage dropped across it multiplied by the current gives power in the resistor. That power shows up as heat, and is dissipated by warming the environment.

Her is a link to the power chapter in out Ebook - http://www.allaboutcircuits.com/vol_1/chpt_2/4.html.

Aug 22, 2008
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this is where i was getting confused with the resistors. Am i right to say that no matter what direction the current flows through the resistor that it will always dissipate power...?

i will have to check to see what they actually meant by power absorbed.

4. PatDarden New Member

Aug 23, 2008
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Power Absorbed means power taken from the source by a device. How the device uses the power depends on the device. For example, a resistor uses the power by converting it to heat. A motor ... something else. etc ...

E*I = Power (watts) ONLY if both E and I are RMS (root-mean-square) values or if the circuit is pure DC (Direct current).

There is more but I will stop for now and see if this helps any.

/ ... JCD

5. PatDarden New Member

Aug 23, 2008
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Now matter what direction is correct as long as you have a pure DC situation or if E and I are both RMS values.