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Chapter 19 DC Circuits current, emf, resistance AQ current is the rate of flow of charge / = oe" Unit: 1 C/s= 1A (ampere) In a wire AQ is the magnitude of charge that flows past some point in the wire in a time interval Az. L_—= 49> By convention , / is in the direction of flow of positive charge. In a metal wire (conductor) it is the electrons that move. —g moving to the left is equivalent to +g moving to the right so the electrons flow opposite to the current direction. 3 —> VO emf € An emf source (a chemical battery is an example) is a device that maintains a constant potential difference between its terminals. Circuit symbol for a battery: + € oS _——— al b Long line is positive terminal (high potential) and short line is negative terminal (low potential). The potential difference between terminals is V,, =V, -V, =E Charge can flow into and at the same time out of the emf source. The emf source remains neutral. resistance R Electrons don’t flow freely through a conductor. There is resistance to the flow. Sort of like friction. Conductor has some electrons that are free to move and an underlying lattice of positive ions. The electrons that make up the current have to move through the lattice of positive ions. 
