Lecture10_Kim

# Lecture10_Kim - Lecture 10-1 Lecture 10-2 Chapter 25...

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2/13/2012 1 Lecture 10-1 Batteries We use batteries as devices that provide direct currents in circuits The voltage written on it is the potential difference that it can provide to a circuit You will also find its rating in units of mAh This rating provides information on the total charge that a single battery can deliver over its lifetime The quantity mAh is another unit of charge: 1 mAh = (10 - 3 A)(3600 s) = 3.6 As = 3.6 C Chapter 25 Electric Current and Direct-Current Circuits Lecture 10-2 Demo – Internal Resistance of a Battery a b a b a b V V Ir V V Ir IR V V Ir ε ε ε = + - - = - = - = - Lecture 10-3 In this circuit, electrons flow around the circuit counterclockwise. (The conventionally defined current is clockwise; remember, electrons are negative charges.) The electrons can’t disappear so the current requires a whole loop! Chemical action pumps electrons from the positive terminal (+) to the negative terminal ( - ) in the battery. The emf (electromotive force, or electric field) pushes electrons around the wire from (-) to (+). + Circuits lightbulb Direct electric current flows in one direction Lecture 10-4 Microscopic View of Electric Current in Conductor All charges move with some velocity v e random motion with high speeds (O(10 6 )m/s) but with a drift in a certain direction on average if E is present Drift velocity v d [O(10 -4 m/s)] is orders of magnitudes less than the actual velocity of charges [O(10 6 m/s)]. A Why random motion? thermal energy scattering off each other, defects, ions,

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2/13/2012 2 Lecture 10-5 Drift Velocity (1) Consider a conductor with cross sectional area A and electric field E Suppose that there are n electrons per unit volume The negatively charged electrons will drift in a direction opposite to the electric field We assume that all the electrons have the same drift velocity v d and that the current density J is uniform .
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