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# HR27_post - Chapter 27 Circuits Cover the following topics...

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Cover the following topics: • Electromotive force ( emf ) • Ideal and real emf devices • Kirchoff’s loop rule • Kirchoff’s junction rule Chapter 27 Circuits 1 • Multiloop circuits • Resistors in series • Resistors in parallel • RC circuits, charging and discharging of a capacitor

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“Pumping” Charges In order to create a steady current through a resistor, potential difference must be applied and maintained across its terminals . One way to do so is to connect the resistor to a battery A device that can maintain a potential difference between two terminals is called an “emf device” or “seat of emf” 2 “Emf” stands for “ electromotive force ”. We say that “emf device “ provides emf E Examples of emf devices are a battery, an electric generator, a solar cell, a fuel cell, etc. Emf devices all act like “charge pumps” in the sense that they move positive charges from low potential (negative) terminal to the high potential (positive) terminal
Figure shows an emf device in a circuit with a resistor. The emf device keeps one of its terminals at higher electric potential. The polarity of emf device is represented by an arrow with a small circle on its tail. The arrow points from the negative to positive terminal When an emf device is not connected to a circuit, there is no net flow Work, Energy and Emf 3 of charge carriers within it. However, when it is connected to a circuit, its internal chemistry (e.g. in case of battery) causes a net flow of positive charge carriers from the negative to positive terminal, in the direction of emf arrow . This flow is part of the current that is set up around the circuit in that same direction Within the emf device, positive charge moves from a low to high potential region, i.e. from low potential energy to high potential energy. This motion is opposite to what the electric field between the terminal would cause the charge carriers to do

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In a time interval dt , a charge dq passes through any cross section of the circuit (e.g. aa ). This charge dq enters at low-potential end and leaves at its high-potential end. The amount of work done by the device during this process is dW . Then, the emf of the emf device is defined as dq dW = E Work, Energy and Emf (cont’d) The SI unit of emf is 1 J/C = 1 V 4 The emf of an emf device is the work per unit charge that device does in moving charge from its low-potential terminal to its high-potential terminal An ideal emf device is one that lacks any internal resistance to the internal movement of charge from terminal to terminal. Potential difference between the terminals of ideal emf device is equal to emf of the device A real emf device has internal resistance to the movement of charge. When a real emf device is not connected to a circuit, the potential difference between its terminals is equal to its emf. However, when the device is connected in a circuit, the potential difference does not equal to the emf

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