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Lecture%2021A

# Lecture%2021A - Physics 1B Lecture 21A Current Up until now...

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Physics 1B Lecture 21A

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Current Up until now, we have dealt with electrostatics (i.e. charges that do not move). We now define electric current, I, to be the rate at which electric charge passes through a surface or volume. The SI unit for current is the Ampere.
Current What causes electric current to flow? An electric potential difference, usually created by some sort of battery or a generator. The battery uses chemical energy to create ΔV. Current is defined as positive charges moving in a certain direction. If negative charges are actually moving, then current is defined as flowing in the direction opposite to the motion of the negative charges. + + + I I

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Current What physically flows through wires: positive charges or negative charges? It is the tiny electrons that move in the wire. The massive positively charge protons stay where they are. But due to historical reasons, we use positive current when performing calculations. How fast do electrons move along a wire under current flow? Not fast at all, electrons in a well-conducting wire move with a drift speed of about 0.1 – 1.0 mm/s.
Current

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Current If electrons move so slowly, then why do lights turn on exactly when I hit the switch? The electron at the switch isn’t the one that will give energy to the lights. When you turn on a light switch you are establishing an electric field that moves all the free electrons in the wire circuit. You need a closed loop with the wire. An open loop means no electric field in the wire and no current flow. This is known as an open circuit.
Current When a battery is placed in a closed loop, an electric field is established inside the conducting wire. This electric field starts on the positive terminal and ends on the negative terminal.

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Lecture%2021A - Physics 1B Lecture 21A Current Up until now...

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