# ch20 - Chapter 20 Electric Circuits 20.1 Electromotive...

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Chapter 20 Electric Circuits

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20.1 Electromotive Force and Current In an electric circuit, an energy source and an energy consuming device are connected by conducting wires through which electric charges move.
20.1 Electromotive Force and Current Within a battery, a chemical reaction occurs that transfers electrons from one terminal to another terminal. The maximum potential difference across the terminals is called the electromotive force (emf) .

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20.1 Electromotive Force and Current The electric current is the amount of charge per unit time that passes through a surface that is perpendicular to the motion of the charges. t q I = One coulomb per second equals one ampere (A).
20.1 Electromotive Force and Current If the charges move around the circuit in the same direction at all times, the current is said to be direct current ( dc ). If the charges move first one way and then the opposite way, the current is said to be alternating current ( ac ).

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20.1 Electromotive Force and Current Example 1 A Pocket Calculator The current in a 3.0 V battery of a pocket calculator is 0.17 mA. In one hour of operation, (a) how much charge flows in the circuit and (b) how much energy does the battery deliver to the calculator circuit? (a) (b) ( 29 ( 29 ( 29 C 61 . 0 s 3600 A 10 17 . 0 3 = × = = - t I q ( 29 ( 29 J 8 . 1 V 0 . 3 C 61 . 0 Charge Energy Charge Energy = = × =
20.1 Electromotive Force and Current Conventional current is the hypothetical flow of positive charges that would have the same effect in the circuit as the movement of negative charges that actually does occur.

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20.2 Ohm’s Law The resistance (R) is defined as the ratio of the voltage V applied across a piece of material to the current I through the material.
20.2 Ohm’s Law OHM’S LAW The ratio V/I is a constant, where V is the voltage applied across a piece of mateiral and I is the current through the material: SI Unit of Resistance: volt/ampere (V/A) = ohm (Ω) IR V R I V = = = or constant

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20.2 Ohm’s Law To the extent that a wire or an electrical device offers resistance to electrical flow, it is called a resistor.
20.2 Ohm’s Law Example 2 A Flashlight The filament in a light bulb is a resistor in the form of a thin piece of wire. The wire becomes hot enough to emit light because of the current in it. The flashlight uses two 1.5-V batteries to provide a current of 0.40 A in the filament. Determine the resistance of the glowing filament. = = = 5 . 7 A 0.40 V 0 . 3 I V R

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20.3 Resistance and Resistivity For a wide range of materials, the resistance of a piece of material of length L and cross- sectional area A is A L R ρ = resistivity in units of ohm·meter
20.3 Resistance and Resistivity A L R ρ =

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20.3 Resistance and Resistivity Example 3 Longer Extension Cords The instructions for an electric lawn mower suggest that a 20-gauge extension cord can be used for distances up to 35 m, but a thicker 16-gauge cord should be used for longer distances. The cross sectional area of a 20-gauge wire is
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ch20 - Chapter 20 Electric Circuits 20.1 Electromotive...

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