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# Lec18 - Today Finish Chapter 22(Electrostatics Start...

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Today Finish Chapter 22 (Electrostatics) Start Chapter 23 (Electrical Circuits) Looking ahead: Tuesday Nov 10: Review Session for Midterm 2 Friday Nov 13: Midterm 2 (Chs: 11,13,14, 15, 19, 20, 22, 23) Final Exam: Tuesday Dec 15, 11.30 -1.30pm

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Chapter 23: Electric Current
Electric Current Is the flow of electric charge (i.e. charged particles, usually electrons) between regions of different electric potential. When there is no potential difference, there is no charge flow, no current. Consider high potential object, eg van de Graaff dome. Connect to earth with wire – charge flows down to the lower potential of the ground. But this current is brief, because it stops once dome reaches the same potential as the ground. In order to get a sustained current flow, need some kind of pump (i.e. battery) Before getting into this, consider fluid analogy: Water flows from high pressure to low pressure, but stops flowing once pressures (heights) are equal. Can get sustained water flow if pressure difference maintained via a pump

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Electric current cont . What actually is flowing? Depends on the circuit: In circuits of metal wires, its electrons (called conduction electrons ), free to move (whereas positive ions are fixed in a lattice). In car batteries, it’s positive ions in a fluid (sulfuric acid and distilled water mix) Rate of flow measured in amperes – amount of charge passing a given point per second: 1 ampere = 1 Coulomb/sec Note that we often call “potential difference” simply “voltage” : need a voltage across two points in order to get a current between them. Recall from Ch. 22, that the unit is volt. Usually, symbol for current is I (but not used in your book)
Question In a circuit carrying a 1-ampere current, how many electrons pass a given point in a second? 1 Ampere means 1 Coulomb per second, and recall

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Clicker Question Is there a net charge on a current-carrying wire? A)Yes B)No Answer: B No (not usually). The negative conduction electrons flow through the atomic lattice made of positively charged nuclei. The atoms were originally neutral, so the # electrons = # protons.
Voltage sources Recall, need an “electrical pump” for sustained current flow, to maintain a potential difference. Called a “voltage source”: eg chemical battery, or generator Batteries do work to pull negative charges away from positive ones. Eg. In chemical battery, a chemical reaction between zinc or lead in acid releases chemical energy in broken bonds – this energy is converted to electric potential energy. Eg. Generators use electromagnetic induction (see later Ch 25) to separate charges. This work is stored in the battery, and is available at the battery terminals. We characterize batteries by their “voltage” = “electrical pressure” (eg car battery is 12-V) Note terminology: Voltage across a circuit vs current through circuit c.f. fluids Pressure difference across a pipe vs water flow through pipe

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Electrical Resistance c.f. fluids – eg a short and wide pipe has less resistance than a long and skinny one. Similarly with wires:
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Lec18 - Today Finish Chapter 22(Electrostatics Start...

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