## Electric power & EnergyBy: Maximo De Hoyos

Summary: Power is used by many people. Knowing that power is the rate of energy use or energy conversion, what is the the expression for electric power? Power lines are an example and also people think of light bulbs in terms of their power ratings in watts.

Formulas :Electric energy depends on both the voltage involved and the charge moved. This is expressed as PE = qV , q is the charge moved and V is the voltage. Power is the rate at which energy is moved, and so electric power is, P=PE/t=qV/t. Recognizing that current I = q/t ( Delta t = t ) the expression for power becomes P = IV. Three expressions for electric power are listed together here. P = IV , P = V^2 / R , P = I^2R

Vocabulary: Electric power - Electric power is defined as the rate at which electrical energy is consumed in an electrical circuit.

Conceptional questions: Why do incandescent lightbulbs grow dim late in their lives, particularly just before their filaments break? If you’ve ever seen the glass of the bulb ending up with a darker coating is because the filament is going away. When the filament is evaporating it gets thinner and its resistance increases, this reduces the current through the bulb and makes it dimmer. The power dissipated in a resistor is given by P = V^2 / R , which means power decreases if resistance increases. Yet this power is also given by P = I^2 R , which means power increases if resistance increases. Explain why there is no contradiction here. I is proportional to R , as R changes so does I.

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