1. Phys102 Lecture 9 Electric Currents and Resistance
Key Points
Ohm’s Law
Resistivity
Electric Power
Alternating Current
References
18-1,2,3,4,5,6,7

2. Unit of electric current: the ampere, A:
Electric current is the rate of flow of charge through a conductor:
The instantaneous current is given by:
Unit of electric current: the ampere, A:
1 A = 1 C/s.

3. Ohm’s Law The ratio of voltage to current is called the resistance:
1.8kW
1.5V RL VL IL

4. Resistivity
The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area:
The constant ρ, the resistivity, is characteristic of the material. A l

5. Resistivity
This table gives the resistivity and temperature coefficients of typical conductors, semiconductors, and insulators.

6. Electric Power
Power, as in kinematics, is the energy transformed by a device per unit time: or

7. Electric Power The unit of power is the watt, W.
For ohmic devices, we can make the substitutions:

8. Electric Power Example: Headlights.
Calculate the resistance of a 40-W automobile headlight designed for 12 V.
Solution: R = V2/P = 3.6 Ω.

9. Alternating Current
Current from a battery flows steadily in one direction (direct current, DC). Current from a power plant varies sinusoidally (alternating current, AC).
Figure (a) Direct current. (b) Alternating current.

10. Alternating Current The voltage varies sinusoidally with time:, ,
as does the current:

11. Alternating Current
Multiplying the current and the voltage gives the power:
Figure Power transformed in a resistor in an ac circuit.

12. Alternating Current
Usually we are interested in the average power: .

13. Alternating Current
The current and voltage both have average values of zero, so we square them, take the average, then take the square root, yielding the root-mean-square (rms) value:

14. Alternating Current Example: Hair dryer.
(a) Calculate the resistance and the peak current in a 1000-W hair dryer connected to a 120-V line. (b) What happens if it is connected to a 240-V line in Britain?
Figure A hair dryer. Most of the current goes through the heating coils, a pure resistance; a small part goes to the motor to turn the fan. Example 25–13.
Solution: a. The rms current is the power divided by the rms voltage, or 8.33 A, so the peak current is 11.8 A. Then R = V/I = 14.4 Ω.
b. Assuming the resistance would stay the same (probably wrong – the heat would increase it), doubling the voltage would mean doubling the current, and the power (P = IV) would increase by a factor of 4. This would not be good for the hair dryer.