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Unformatted text preview: 1 Chapter 28 Direct Current Circuits 2 Chapter 28 DC circuits EMF and Terminal Voltage Resistors in Series and Parallel Kirchhoffs Rules RC Circuits 3 What do we need to have current in an electric circuit? A device that provides a potential difference, such as battery or generator They normally convert some types of energy into electric energy These devices are called source of electromotive force (emf) This does NOT refer to a real force. Potential difference between terminals of emf source, when no current flows to an external circuit, is called the emf ( ) of the source. Battery itself has some internal resistance ( ) due to the flow of charges in the electrolyte Why does the headlight dim when you start the car? The starter needs a large amount of current but the battery cannot provide charge fast enough to supply current to both the starter and the headlight EMF and Terminal Voltage 4 Since the internal resistance is inside the battery, we can never separate them out. EMF and Terminal Voltage So the terminal voltage difference is V ab =V aV b . When no current is drawn from the battery, the terminal voltage equals the emf which is determined by the chemical reaction; V ab = . However when the current flows naturally from the battery, there is an internal drop in voltage which is equal to . Thus the actual delivered terminal voltage is ab V Ir =  5 Resistors are in series when two or more resistors are connected end to end These resistors represent simple resistors in circuit or electrical devices, such as light bulbs, heaters, dryers, etc Resistors in Series What is common in a circuit connected in series? Current is the same through all the elements in series Potential difference across every element in the circuit is V 1 =IR 1 , V 2 =IR 2 and V 3 =IR 3 Since the total potential difference is V, we obtain V=IR eq =V 1 +V 2 +V 3 =I(R 1 +R 2 +R 3 ) Thus, R eq =R 1 +R 2 +R 3 eq i i R R = Resistors in series When resistors are connected in series, the total resistance increases and the current decreases. 6 Why is it true that V=V 1 +V 2 +V 3 ? Energy Losses in Resistors What is the potential energy loss when charge q passes through the resistor R 1 , R 2 and R 3 U 1 =qV 1 , U 2 =qV 2 , U 3 =qV 3 Since the total energy loss should be the same as the energy provided to the system, we obtain U=qV= U 1 + U 2 + U 3 =q(V 1 +V 2 +V 3 ) Thus, V=V 1 +V 2 +V 3 7 Example 28 1 Battery with internal resistance. A 65.0 resistor is connected to the terminals of a battery whose emf is 12.0V and whose internal resistance is 0.5 . Calculate (a) the current in the circuit, (b) the terminal voltage of the battery, V ab , and (c) the power dissipated in the resistor R and in the batterys internal resistor....
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 Fall '09
 WilfredNgwa
 Physics, Current, RC Circuits

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