(40) Power and Circuits

(40) Power and Circuits - Lecture 40 Power. Circuits. A....

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Unformatted text preview: Lecture 40 Power. Circuits. A. Equal to the time needed by the electrons in the socket to get to the lamp. B. Zero, its 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 Power dissipation in a resistor Battery Supplied energy (tries to accelerate electrons) Resistor Work by the electric field when a charge dq moves across a potential difference V: supplied W Vdq = - Power supplied: supplied supplied W dq P V VI dt dt = == - = - Power dissipated (in resistor): dissipat ed supplied P P = - P I V = = - Supplied energy Dissipated energy (slows electrons down. Energy is transformed into thermal energy, light, etc) 2 2 V P I R R = = For Ohmic resistors Example: Resistance of a bulb Example: Resistance of a bulb Power dissipation can be bad (loss in transmission wires) or good (bulb, hair drier, etc) Example: 75-W bulb ( 29 2 2 US 120 V 192 75 W V R P = = = DEMO: Current through a pickle Or for fun: A. I t will work just as in the US. B. I t will be weaker. C. Youll have to buy a new hairdryer because this one will burn down as soon as you connect it....
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This note was uploaded on 03/27/2008 for the course PHYS 221 taught by Professor Herrera-siklody during the Fall '08 term at Iowa State.

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(40) Power and Circuits - Lecture 40 Power. Circuits. A....

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