Current, Ohm’s Law, Etc.

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The Continuity Equation for Steady State Currents

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Currents and current densities are constant in time – steady state. The flux of out of any closed surface must be zero.

j

Drift velocity

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jv

For steady state situation

1.Kirchhoff’s junction rule: The algebraic sum of the currents into any junction is zero.

2.Kirchhoff’s loop rule: The algebraic sum of the potential differences in any loop must be zero.

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Resistors in parallel:

Resistors in series:

21 RRR

21

111

RRR

Circuit with capacitors

Joule’s Law

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22

Why does a light bulb burn out when you switch the light on and never when you turn it off?

The amount of current drawn by a given device is determined by its power inputi

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The current in a 100-W light bulb is

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W

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120

100

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120

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i

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A kitchen circuit

An 1800-W toaster, a 1.3-kW electric frying pan, and a 100-W lamp are plugged into the same 20-A, 120-V circuit.

a) What current is drawn by each device, and what is the resistance of each device?

b) Will this combination blow the fuse?

To prevent the fuse from blowing, you replace the fuse with one rated at 40 A.Is this a reasonable thing to do?

RiP 2

Wheatstone bridge

British scientist and inventor1802-1875

Charles Wheatstone

Samuel Hunter Christie, British scientist and mathematician

Wheatstone English concertina

Electromotive force