# Phys102 Lecture 9 Electric Currents and Resistance Key

Phys102 Lecture 9 Electric Currents and Resistance Key Points Ohms Law

Resistivity Electric Power Alternating Current

References 18-1,2,3,4,5,6,7 Electric Current 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. Ohms Law The ratio of voltage to current is called the resistance:

1.8kW IL 1.5V VL RL

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

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

Power, as in kinematics, is the energy transformed by a device per unit time: or Electric Power The unit of power is the watt, W.

For ohmic devices, we can make the substitutions: Electric Power Example: Headlights. Calculate the resistance of a 40-W automobile headlight designed for 12 V.

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

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

Multiplying the current and the voltage gives the power: Alternating Current Usually we are interested in the average power: .

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: 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?

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