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Unformatted text preview: Electric Circuits Lecture Notes 20 Goals To understand the concept of current. To study resistance and Ohms Law. To observe examples of electromotive force and circuits to learn Ohms Laws application. To calculate the energy and power in electric circuits. To study the similarity and differences in the combination of resistors in parallel and those connected in series. To apply Kirchhoffs Rules to combinations of resistors. To observe and understand devices which measure electricity in circuits. To combine resistors and capacitors then calculate examples of the results. 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). Electromotive Force In a fountain, the water emerges at the top, cascades down to the basin, and then is pumped back to the higher gravitational potential energy. Current defined Electric current is the amount of charge per unit time that passes through a surface that is perpendicular to the motion of the charges. 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. One Coulomb per second is referred to as an ampere (A). Direct current (dc) is the current produced from the motion of the charges around a circuit in the same direction. Alternating current (ac) occurs when the charges move first one way and then the opposite way, changing direction from moment to moment. 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) ( ) ( ) ( ) C 61 . s 3600 A 10 17 . 3 = = = t I q ( ) ( ) J 8 . 1 V . 3 C 61 . Charge Energy Charge Energy = = = 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. Ohms Law The potential difference V between the ends of a conductor is proportional to the current I through the conductor; the proportionality factor is the resistance R : =constant or V=IR R is the resistance of the piece of material. SI Unit of Resistance: volt/ampere (V/A) = ohm ( ) 1 k = 10 3 (kilohm) & 1 M = 10 6 (megohm) V R I = Resistance is defined by Ohms Law....
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This note was uploaded on 04/01/2011 for the course PHY 135 taught by Professor Wagihghobriel during the Spring '11 term at University of Toronto Toronto.
 Spring '11
 WagihGhobriel
 Current, Resistance, Energy, Force, Power

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