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# chapter28notes - 1 Chapter 28 •Direct Current Circuits 2...

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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 • Kirchhoff’s 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 a-V 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 ε = - 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 – Δ U 1 =qV 1 , Δ U 2 =qV 2 , Δ U 3 =qV 3 • Since the total energy loss should be the same as the energy provided to the system, we obtain – Δ U=qV= Δ U 1 + Δ U 2 + Δ 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- Ω resistor is connected to the terminals of a battery whose emf is 12.0V and whose internal resistance is 0.5- Ω . 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 battery’s internal resistor....
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## This note was uploaded on 11/01/2009 for the course PHY phy2049 taught by Professor Wilfredngwa during the Fall '09 term at University of Central Florida.

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chapter28notes - 1 Chapter 28 •Direct Current Circuits 2...

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