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Ohm’s Law

Ohm’s Law. PH 203 Professor Lee Carkner Lecture 11. Ohm’s Law. How much current do you get if you put a potential difference V across a wire with resistance R? i = V/R This relationship is called Ohm’s Law V = iR Ohm’s law is very important, memorize it!

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Ohm’s Law

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  1. Ohm’s Law PH 203 Professor Lee Carkner Lecture 11

  2. Ohm’s Law • How much current do you get if you put a potential difference V across a wire with resistance R? i = V/R • This relationship is called Ohm’s Law V = iR • Ohm’s law is very important, memorize it! • Every individual piece of a circuit obeys Ohm’s law

  3. Energy in Electric Circuits • The battery produces potential difference (DV) • This causes current to flow (i) • This energy can be extracted by resistors (R) • We should be able to relate the potential difference, current and resistance to the energy produced

  4. Energy Transfer Rate • The energy per electron is DPE = qDV so energy per coulomb is DV • (Energy/Coulomb)(Coulomb/Second) = (Energy/Second) iDV = P

  5. Dissipative Power P = i2R and P = (DV)2/R • In general we will know the values of R (since it depends on the properties of the resistor) and DV (since we should know the voltage of our source or battery)

  6. Lightbulbs • A common circuit element is the lightbulb • Household lightbulbs are rated in watts • In the US, most power outlets produce 120 volts of potential difference • household electrical devices either operate at 120 V or use a transformer

  7. Ohmic • Ohm’s law does not apply to all materials • V = iR is true for all devices • Ohmic devices have R that is constant with V • Wires are pretty close to being ohmic

  8. Non-Ohmic • A good example of non-ohmic materials is a diode • so the resistance is infinite for negative voltage and finite for positive • Much of what happens in integrated circuits is non-ohmic

  9. Next Time • Read 27.1-27.6 • Problems: Ch 26, P: 41, 43, 57, Ch 27, P: 2, 6

  10. Suppose a copper wire is cut in half while keeping the cross sectional area the same. If this change doubles the resistance of the wire, how does the resistivity change? • It is quartered • It is halved • It is doubled • It is quadrupled • It does not change, this situation is impossible

  11. 1,3,6,8 4,5 4 2,4,5,7 1,2,3,4,5,6,7,8 Which correctly lists all of the junctions in this circuit? 3 2 1 4 5 6 7 8

  12. Consider a wire that has a constant current maintained in it. Which of the following would double the drift speed of the electrons? • Halve the area of the wire • Halve the density of the charge carriers • Halve the charge on the charge carriers • All of the above • None of the above

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