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 (  V)   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  PE = q  V so energy per coulomb is  V   (Energy/Coulomb)(Coulomb/Second) = (Energy/Second) i  V = P

5. Dissipative Power  P = i 2 R and P = (  V) 2 /R  In general we will know the values of R (since it depends on the properties of the resistor) and  V (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? A)It is quartered B)It is halved C)It is doubled D)It is quadrupled E)It does not change, this situation is impossible

11. A)1,3,6,8 B)4,5 C)4 D)2,4,5,7 E)1,2,3,4,5,6,7,8 Which correctly lists all of the junctions in this circuit? 1 45 8 76 3 2

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