Prozorov_07 - PHYSICS 222 Introduction to Classical Physics...

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Unformatted text preview: PHYSICS 222 Introduction to Classical Physics II Prof. Ruslan Prozorov Iowa State University Fall 2011 LECTURE 7 Resistors. Circuits. EMF. Electric power. switching on the lamp When a lamp is switched on, the time it takes to light up is: A. Equal to the time needed by the electrons in the socket to get to the lamp. (drift velocity – submm/s) B. Fermi velocity (3% of speed of light) C. speed of light Electric fields propagate at the speed of light… PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 2 resistors are color-coded PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 3 current–voltage relationships PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 4 symbols for circuit diagrams PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 5 “open” and “complete” circuits PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 6 internal resistance o We generalize at the outset, but the truth of a battery is that you only get 12 V when a 12 V battery isn’t connected. o Making a connection allows electrons to flow, but internal resistance within the battery actually delivers incrementally less than 12 V. PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 7 source in an open circuit voltmeter has infinite (very large) resistance ammeter has zero (very small) PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 8 source in a close circuit PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 9 a source with a short circuit PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 10 potential changes around a circuit PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 11 The water circuit analogy Current (water flow) Battery fixed potential increase (water pump) Resistors potential drops (ramps with obstacles) Ideal wires constant potential (flat sections) PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 12 Power dissipation in a resistor Supplied energy Battery (tries to accelerate electrons) Dissipated energy Resistor = - Supplied energy (slows electrons down. Energy is transformed into thermal energy, light, etc) Work by the electric field when a charge dq moves across a potential difference V: Power supplied: Psupplied Power dissipated (in resistor): Wsupplied Pdissipated V dt Psupplied Wsupplied Vdq dq VI dt P IV V2 For Ohmic resistors P I 2R R PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 13 energy conversion PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 14 Example: Resistance of a bulb Power dissipation can be bad (loss in transmission wires) or good (bulb, hair drier, etc) Example: 75-W bulb 120 V 192 , I V 120 0.63 A V R P 75 W R 192 2 US R 2 EUROPE V P 2 220 V 75 W 2 V 220 645 , I 0.34 A R 645 PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 15 (bad) resistance of a wires etc How much power is lost to heating? For any device to operate we need to supply certain power Pd. Since P VI , we will need only half current if the voltage is doubled. Bad (parasitic) losses in circuitry, wires etc (with total resistance R is Ploss RI 2 Therefore, for the same device 120 loss 230 loss P P 2 2 I 220 V 230 3.4 I 120 V 120 So, lower voltage systems are much less efficient… PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 16 Traveling to Europe Your hairdryer is labeled 1500 W. If you take it with you on your trip to Europe, where the outlets have 230 V, (and buy the adapters for the different outlet shapes), what will happen to the hairdryer? 2 VUS (120 V) 2 R 9.6 PUS 1500 W V2 V2 P R R P PEurope 2 VEurope R (220 V) 2 5042 W 9.6 Probably a little too much for your hairdryer… PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 17 High voltage transmission lines A Transmission line resistance R B Electrical plant I C Pdel = power to be delivered to the Pdel IVBC costumer (fixed). The power loss in the line is Ploss I Pdel VBC P I R R V 2 2 del 2 BC decreases for larger voltages PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University 7 September 2011 18 Short circuit Another way of burning things… If you connect the ends of a battery, the generated current is huge (the resistance of the wires is usually low). A lot of energy is released in a short time. Current takes the easiest path PHYS222 - Lecture 7 - Prof. Ruslan Prozorov - Iowa State University Can even produce an explosion! 7 September 2011 19 ...
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This note was uploaded on 11/14/2011 for the course PHYS 5863005 taught by Professor Meyer during the Fall '09 term at Iowa State.

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