Lab 2: Ohm’s LawOnly 10 more labs to go!!

When trying to understanding electricity it is helpful to draw analogies to the classicalmechanics.

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flow, current

GravitationalField

In this example we have flow of water (current) from the storage tank because the gravitational field (force) is pushing the water downwards (in order to decrease the water’s potential.

Electric Field

+

+

++

+

PE

Electric current flow

Analogous to the water flow due to thegravitational field, charge will flow fromit’s storage device (a battery) whenin the presences of an electric field.

When charges flow we can do work, in fact batteries are characterized by the amount of work they can do.

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PEV

Electric potential- the amount of work each Coulomb of charge delivers, measured in VOLTS, V

So 110 V means that 110 Joules of work can be deliveredper 1 Coulomb of charge

The current, I in a circuit is defined as the amount of charge that flows persecond.

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QI

Current, I, is measured in Amperes or Amps, A.

1 A of current means that 1 Coulomb of charge has flowed pasta point in a second.

We electrical current is not free to flow we call this Electrical Resistance

As charge flows through something that has resistance, some of the charge’senergy is dissipated. We call this type of device a resistor. Resistance is measured in ohms, .

a bad, circuit diagram of a resistor

Electric potential, current and resistance are related through Ohm’s law:

RIV As current flows through a resistor, the charges dissipate energy. Each Coulomb ofcharge will deposit one joule of energy per volt. If one Coulomb of charge flows per second, then one Watt of power is dissipated. Remember:

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t

VqPVqE

q

EV

Also remember

from before:

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qIthatrecall

t

VqP

We have several variations of the power equation:

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VV

R

VIVP

alsoRIIRIIVP2

2

What if I gave you two light bulbs, one rated at 60 W and the other at 100 W at120 Volts. Which bulbl will have the largest resistance? From the equation above:

P

VR

2

144100

14400

100

)120(

24060

14400

60

)120(

22

100

2

100

22

60

2

60

W

V

W

V

P

VR

W

V

W

V

P

VR

The first thing you are going to do today:

A

V

Construct a graph of current vs. voltage. Increment the voltage by 5 to 7 Volts ,when the voltage exceeds 40 Volts, increment by 10 to 15 Volts.

0 2 4 6 8 10 12 140

2

4

6

8

10

12

14

Current vs. Voltage for an Ideal Resistor

I Cu

rre

nt (

A)

V Voltage (Volts)

What’s the slope of this line?From Ohm’s law:

RIV

VR

I1

slopeR

Rslope

11

The next procedure you’re going to examine the lamp circuit:

A

V

Construct a graph of current vs. voltage. Increment the voltage by 5 to 7 Volts ,when the voltage exceeds 40 Volts, increment by 10 to 15 Volts.

V Voltage (Volts)

I C

urr

ent

(Am

ps)

Current vs. Voltage for a LampNotice how the slope of the tangentline is changing.(For smaller voltages the slope is highertherefore the resistance is lower)

In this case the temperature of the bulbis increasing with increasing power.

As the temperature increases theresistance also increases

The next procedure you’re going to examine a light emitting diode (LED) circuit.An LED is a special semiconductor device.

A

V

ecI According to device physics:

a

VcI )ln()ln(

If I take the natural logof both sides:

Construct a graph of the natural log of current vs. voltage. Increment the voltage by 5 to 7 Volts for all data points.

I

Voltage

On a log graph

I

Voltage

On a regular graph

slopeR

1