CH 20 Notes - CH.20 Electric Circuits 20.1 Electromotive...

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CH.20 Electric Circuits 20.1 Electromotive Force and Current Under static (or equilibrium) conditions, electric field E inside a conductor is zero. What happens if charges +Q and –Q are suddenly deposited on opposite ends of conductor? No longer static conditions, as charges on ends generate electric field E inside the conductor. Charges +Q and –Q will then move towards each other and eventually charge distribution will even out such that electric field E is zero again. However, if charges are continuously supplied to ends of conductor through a battery, e.g., then conductor not in equilibrium. Electric field inside conductor and charges are in continuous motion giving rise to an electrical 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. SI unit: Ampere (A). If the charges move around a 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). Ex 1 : The current from 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? Energy Source: 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 = ε). Battery and Water Pump Analogy: Pump (battery) sends water up to higher potential (positive terminal) so water (current) can flow down to lower potential (negative terminal). Thus pump (battery) is source for energy for water (current) to flow. 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. 20.2 Ohm’s Law For many devices, the current I is proportion to the voltage V across the device. Such devices are said to be ohmic. The electrical resistance R of the device is ratio of the voltage across the device to the current through the device. V=IR or R=V/I SI Unit of Resistance: volt/ampere (V/A) = ohm (Ω) Ex 2: 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 0.05 A in the filament. Determine the resistance of the glowing filament. 3. Resistance and Resistivity Resistance is a measure of resistance to the flow of current. Resistance essentially results from the collisions of electrons with the ions in the lattice structure

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This note was uploaded on 06/21/2011 for the course PH 202 taught by Professor Nordlund during the Spring '08 term at University of Alabama at Birmingham.

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CH 20 Notes - CH.20 Electric Circuits 20.1 Electromotive...

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