PHY108 - Chapter 27

# PHY108 - Chapter 27 - C hapte 27 DCC nt C r urre ircuits...

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Chapter 27: DC Current Circuits Questions to Answer o What is an EMF? Is your wall outlet an EMF? o What are Kirchoff’s Rules? o How do you calculate the currents through a many branched circuit? o What is an RC circuit? o What is the RC time constant? When is this important?

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Ch. 27 Direct Current Circuits In this chapter we will learn how to analyze some simple direct current circuits. Direct current here means that the Emf (the source of electrical energy) has a constant voltage across its terminals. In order to analyze the circuits we will discuss Emf Kirchoff’s Rules RC Circuits Electrical Measurement devices
Electromotive Force is a source of electrical energy such as a battery. Think of it as a charge pump. Charge has gained potential energy q V If no internal resistance for the EMF source, then V =Emf=C If there is internal resistance, then there is some energy loss due to this resistance and the net potential difference at the terminals is reduced. b a potential gain of the battery V V V = potential difference between the terminal V s Ir ε ε = - = -

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Work, Energy, and Emf Work, Energy, and Emf 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 the device does during this process is dW . Then, the emf of the emf device is defines as dq dW = E 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. The SI unit of emf is 1 J/C = 1 V. An ideal emf device is one that lacks any internal resistance to the internal movement of charge from terminal to terminal. A real emf device has internal resistance to the internal 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.
Work, Energy, and Emf (cont’d) Work, Energy, and Emf (cont’d)

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PHY108 - Chapter 27 - C hapte 27 DCC nt C r urre ircuits...

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