# Lect_40 - ACT: Switch on the lamp Lecture 40 Power....

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Lecture 40 Power. Circuits. A. Equal to the time needed by the electrons in the socket to get to the lamp. B. Zero, it’s instantaneous. C. Very short but not zero, the current is established in the wires at the speed of light. ACT: Switch on the lamp ACT: Switch on the lamp When a lamp is switched on, the time it takes to light up is: Electric fields propagate at the speed of light… (more about this in Phys222!) Typical drift speed is ~ 1 mm/s!! Power dissipation in a resistor Battery Supplied energy (tries to accelerate electrons) Resistor Work by the electric field when a charge d q moves across a potential difference V : supplied W Vdq =− Power supplied: supplied supplied dq PV I dt = ==− Power dissipated (in resistor): dissipated supplied PP PIV = = - Supplied energy Dissipated energy (slows electrons down. Energy is transformed into thermal energy, light, etc) 2 2 PIR R == For Ohmic resistors Example: Resistance of a bulb Power dissipation can be bad (loss in transmission wires) or good (bulb, hair drier, etc) Example: 75-W bulb ( ) 2 2 US 120 V 192 75 W P = DEMO: Current through a pickle Or for fun:

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A. It will work just as in the US. B. It will be weaker. C. You’ll have to buy a new hairdryer because this one will burn down as soon as you connect it. ACT: Traveling to Europe ACT: Traveling to Europe 22 VV PR RP =⇔ = Your hairdryer is labeled 1500 W. If you take it with you on your summer tour through Europe, where the outlets have 220 V, (and buy the adapters for the different outlet shapes), what will happen to the hairdryer?
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## This note was uploaded on 04/16/2008 for the course PHYSICS 221 taught by Professor Johnson during the Fall '06 term at Iowa State.

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Lect_40 - ACT: Switch on the lamp Lecture 40 Power....

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