Chapter_7 - General Physics II Electromotive Force and...

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General Physics II Electromotive Force and Circuits Lecture Outline 1. Electromotive Force 2. Kirchoff's Rules 2. Resistors in Series 3. Resistors in Parallel 4. Multi loop circuits 5. Electrical Meters 6. RC Circuits Electromotive Force (EMF) A direct current circuit is defined as a type of circuit in which charge flows smoothly, connected to a potential source, with simple circuit elements connected in series or parallel. EMF: A device which increase the potential energy of charge circulating within an electric circuit is termed a source of emf , symbolized as ε and sometimes referred to as “electromotive force”. EMF has the same unit as the potential, which in SI unit is the volt. As an example, a battery is a source of emf, converting chemical potential energy into electrical potential energy. The potential across the terminals of a battery is not in general equal to the battery emf, due to the non-zero internal resistance within a battery. Terminal voltage for a battery is given as: r I V × = Batteries and Electromotive force (a) A water circuit consists of a pump and a continuous stream of water that flows downhill through a pile of rocks. (b) In an electric circuit, the battery raises the electric potential of charge in the same way that pump raises the gravitational potential of water. Here the arrow shows the direction of electric current I. The electrons move in the opposite direction. We can make analogy between the flow of electrons in a wire and fluid flow. Pump ÅÆ battery Pile of rocks ÅÆ light bulb Within a battery, a chemical reaction occurs that transfers electrons from one terminal (leaving it positively charged) to another terminal (leaving it negatively charged). Gaziantep University Faculty of Engineering Department of Engineering Physics 1
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General Physics II The electromotive force of a battery or other electric power source is the value of the potential difference it maintains between its terminals in the absence of current. In a typical car battery, the chemical reaction maintains the potential difference at a maximum of 12 volts between the positive and negative terminals, so the emf is 12 V. In a typical flashlight battery the emf is 1.5 V. The batteries consist of an internal small resistance r. Circuit symbols Batteries r Resistors Kirchoff’s Rules In a circuit, charges move from one place to another carrying energy. These charges can be thought of as buckets that carry energy around a circuit. The battery fills the buckets. The buckets are emptied at various places around the circuit, but the buckets themselves never disappear. They return to the battery to be refilled. These basic ideas are summarized in Kirchoff's Rules and are applicable to even the most complicated circuits.
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This note was uploaded on 02/29/2012 for the course PHYS 227 taught by Professor Rabe during the Fall '08 term at Rutgers.

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Chapter_7 - General Physics II Electromotive Force and...

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