Lecture 15 - Resistors. Circuits. Power

# Lecture 15 - Resistors. Circuits. Power - ACT Switch on the...

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1 Lecture 15 Power. Circuits. A. Equal to the time needed by the electrons in the socket to get to the lamp. ACT: Switch on the lamp When a lamp is switched on, the time it takes to light up is: Typical drift B. Zero, it’s instantaneous. C. Very short but not zero, the current is established in the wires at the speed of light. Electric fields propagate at the speed of light… speed is ~ 1 mm/s!! Power dissipation in a resistor Battery Supplied energy (tries to accelerate electrons) Resistor = - Supplied energy Dissipated energy (slows electrons down. Energy is transformed into thermal energy, light, etc) 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 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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2 ACT: Traveling to Europe 22 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? 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.
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Lecture 15 - Resistors. Circuits. Power - ACT Switch on the...

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