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Circuit Elements

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Objectives

Describe the basic properties of electric current

and solve problems relating current, charge,and time.

Distinguish between the drift speed of a charge

carrier and the average speed of a charge carrier

between collisions.

See how circuits affect our lives everyday.

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What Are Circuits Used For?

Every electronic device uses a combination of 

circuits!!

Circuits are the pathway for electrical energy to

flow.

Circuits are a closed loop of conductors giving a

clear path for the electrons and therefore

current to flow.

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Examples of Circuits

I-Pod Circuit Board

Computer Circuit Board

Cell Phone Circuit Board

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Electric Current

Electric Current is the rate at whichelectric charges Pass through a givenarea.

Electric Current is represented by I(Capital i) and it has the unit of Amperesor Amp

I= Q/ t

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Drift Speed Drift Speed is the net

speed of a charge carrier

moving in an electric field

Drift speeds are relatively small because of the

many collisions.

If charge moves so slow how do the light turnon almost instantly?

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Average speed betweencollisions

Unlike drift speed the speed between collisions

can be in any direction not just in the directionof the electric field.

This is not an average speed but just the speed

between on collision and the next.

The fact that so many collisions happen in such

a short amount of make electricity seem to flowinstantaneously.

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Example Problem

How long did it take 6 coulombs of 

charge to pass by in order for the

current to have been 9 amperes?

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Equation for Resistance

R = V / I this equation isknow as Ohm’s Law 

Resistance is Potential

difference divided by the

current.

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Ohmic Vs. Non-Ohmic

Ohmic materials follow Ohm’s Law this

means that they have the sameresistance over a wide rang of potential

differences.

Non-Ohmic materials have variableresistance when potential differencechanges.

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Resistance Cont’d  Resistance depends on the length of the

material, the cross sectional area, thetemperature and the material.

Resistors are objects with knowresistance that are used to control theamount of current flowing through acircuit.

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Checking for Understanding

If a charge is moving at 10 m/s betweencollisions, but the collisions make itchange direction a lot and its average

velocity is 3 m/s. What is the drift speedand what is the speed between collisions?

If the Potential difference in a battery is

8V and the current in the circuit is 2Amps, what is the resistance in thecircuit?