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Resistors

In series and parallel

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Specific Instructional Objectives

At the end of the lesson, students should

be able to distinguish between series and parallel circuits,

show an awareness that the current at every point in a

series circuit is the same show an awareness that the potential difference across

resistors in parallel is the same

be able to calculate effective resistance in series and

parallel circuits.

bulb

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Theres a road obstruction

Cars traveling on a road

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How about two obstructions in series?

Each obstruction slows down the rate offlow of cars through the road by a certain

amount.

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Similar to having two resistors in

series: R1 R2

Each resistor in the circuit slows down themovement of electric charges and hence reduces

the current.

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Effective Resistance

Effective resistance of a circuit = total resistance

Effective resistance R = R1

+ R2

R1 R2

I

V V

I

R

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N Resistors in Series

Effective resistance

R = R1 + R2 + R3 + + RN

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To understand parallel resistors

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Consider a lane with one obstruction.

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What happens when two such roads are

joined together?

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Resistance in Parallel

more

resistorsconnected

in parallel

more routes

for current toflow

overall

resistancedecreases

bulb

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N resistors in parallel:

1 21 1 1 1...

N R R R R!

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R1

R2

Resistors R1 and R2 in Parallel

Effective resistance

is

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Summary

WS

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How things work

Lights on a Christmas Tree

-connected in series

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Today's standard mini-light bulbs contain a shunt

wire below the filament. If the filament burns out, the

shunt activates and keeps current running through the

bulb so that the rest of the strand stays lit.

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Brain exercise

Tammy connects a multi-headed plug to the

mains socket. He needs to provide power for his

hifi set, television, and computer.

Three outlets was not enough, so he connectedtwo more multi-headed plugs one on top of the

other.

Why did he get a scolding from his mum when

she found out?

End