Lecture10_Kim - 1 Lecture 10-1 Batteries • We use...

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Unformatted text preview: 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 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, … 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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This note was uploaded on 02/21/2012 for the course PHY 241 taught by Professor Virgil.e.barnes during the Spring '11 term at Purdue.

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Lecture10_Kim - 1 Lecture 10-1 Batteries • We use...

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