Page 924 - College Physics For AP Courses

912 Chapter 20 | Electric Current, Resistance, and Ohm's Law
rms current: the root mean square of the current, , where I0 is the peak current, in an AC system rms voltage: the root mean square of the voltage, , where V0 is the peak voltage, in an AC system
semipermeable: property of a membrane that allows only certain types of ions to cross it
shock hazard: when electric current passes through a person
short circuit: also known as a “short,” a low-resistance path between terminals of a voltage source
simple circuit: a circuit with a single voltage source and a single resistor
temperature coefficient of resistivity: an empirical quantity, denoted by α, which describes the change in resistance or resistivity of a material with temperature
thermal hazard: a hazard in which electric current causes undesired thermal effects Section Summary
20.1 Current
• Electric current is the rate at which charge flows, given by

where is the amount of charge passing through an area in time .
• The direction of conventional current is taken as the direction in which positive charge moves.
• The SI unit for current is the ampere (A), where
• Current is the flow of free charges, such as electrons and ions.
• Drift velocity is the average speed at which these charges move.
• Current is proportional to drift velocity , as expressed in the relationship . Here, is the current through a wire of cross-sectional area . The wire's material has a free-charge density , and each carrier has charge and a drift velocity .
• Electrical signals travel at speeds about times greater than the drift velocity of free electrons.
20.2 Ohm’s Law: Resistance and Simple Circuits
• A simple circuit is one in which there is a single voltage source and a single resistance.
• One statement of Ohm's law gives the relationship between current , voltage , and resistance in a simple circuit to
be
• Resistance has units of ohms ( ), related to volts and amperes by .
• There is a voltage or drop across a resistor, caused by the current flowing through it, given by . 20.3 Resistance and Resistivity
• The resistance of a cylinder of length and cross-sectional area is , where is the resistivity of the
material.
• Values of in Table 20.1 show that materials fall into three groups—conductors, semiconductors, and insulators.
• Temperature affects resistivity; for relatively small temperature changes , resistivity is , where
is the original resistivity and is the temperature coefficient of resistivity.
• Table 20.2 gives values for , the temperature coefficient of resistivity.
• The resistance of an object also varies with temperature: , where is the original resistance,
and is the resistance after the temperature change.
20.4 Electric Power and Energy
• Electric power is the rate (in watts) that energy is supplied by a source or dissipated by a device.
• Three expressions for electrical power are
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