Electricity & Electricity & MagnetismMagnetism

How does a Van de Graaff generator make this girls hair stand on end? Rubber belt set into motion by a plastic rollerStatic electricity is generated by the friction or by a high voltage at a pointed electrode The rolling rubber belt then carries the charge to the inner surface of the spherical metal coverDue to mutual repulsion, the charge is repelled to the outer surface of the spherical cover, and hence a large amount of charge will accumulate thereHuman body can conduct electricity, so charge will be transferred to our body when the generator begins to operateBecause of the mutual repulsion of charge, the hair will stand upright

Electric Charge vs. Electric Electric Charge vs. Electric CurrentCurrent

Thunderclouds become Thunderclouds become charged due to friction charged due to friction between water droplets and between water droplets and ice crystals in the clouds as air ice crystals in the clouds as air currents move them aroundcurrents move them around

The cloud becomes positively The cloud becomes positively charged at the top and charged at the top and negatively charged at the negatively charged at the bottombottom

A flash of lightning occurs A flash of lightning occurs when air rapidly conducts a when air rapidly conducts a huge number of electronshuge number of electrons

This movement of charge This movement of charge constitutes an electric currentconstitutes an electric current

Measuring Electric Measuring Electric ChargeChargeThrough experimentation, Charles Through experimentation, Charles Augustin de Coulomb discovered Augustin de Coulomb discovered that the magnitude of the force that the magnitude of the force between two charged objects is:between two charged objects is:Directly proportional to the product Directly proportional to the product of the chargesof the chargesInversely proportional to the square Inversely proportional to the square of the distance between themof the distance between themThe unit of electric charge is called The unit of electric charge is called the coulomb (C) in his honourthe coulomb (C) in his honour

221

dQkQF Where k is a proportionality constant: 9.0 x 109

Nm2/C2

Hi I’m Charles

Coulomb’s Law

Measuring Electric Measuring Electric ChargeCharge

Robert A. Millikan was able to determine Robert A. Millikan was able to determine the charge on an electron by studying the charge on an electron by studying the behaviour of charged oil dropsthe behaviour of charged oil drops

Using an apparatus where charged Using an apparatus where charged drops of oil fell in the presence of a drops of oil fell in the presence of a strong electric field, he was able to strong electric field, he was able to determine that the charge on an determine that the charge on an electron was a fundamental constant of electron was a fundamental constant of electricityelectricity

In the following exercise, you will In the following exercise, you will analyze experimental evidence obtained analyze experimental evidence obtained from Millikan’s Oil drop experiment and from Millikan’s Oil drop experiment and search for patterns that yield the search for patterns that yield the fundamental charge on an electronfundamental charge on an electron

Hi I’m Robert, but you can call me Rob

Measuring Electric Measuring Electric ChargeCharge

ObservationsObservationsThe values listed below represent the The values listed below represent the charges calculated on 12 oil dropscharges calculated on 12 oil drops3.2 x 103.2 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x16.0 x 1016.0 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x17.6 x 1017.6 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x6.4 x 106.4 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x8.0 x 108.0 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x12.8 x 1012.8 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x11.2 x 1011.2 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x4.8 x 104.8 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x1.6 x 101.6 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x9.6 x 109.6 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x19.2 x 1019.2 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x14.4 x 1014.4 x 10-19-19 C = 1.6 x 10 C = 1.6 x 10-19-19 C x C x

2

3

9

1612

411

85

7

10

AnalysisCopy the data into your notebook, then describe & explain any patterns that arise

Measuring Electric Measuring Electric ChargeChargeTwo observations arise:Two observations arise:1) The smallest value for the charge on an oil drop is 1.6 x 101) The smallest value for the charge on an oil drop is 1.6 x 10 -19-19 C C2) All the other values are whole-numbered multiples of 1.6 x 102) All the other values are whole-numbered multiples of 1.6 x 10 -19-19

CC

Millikan called the smallest unit of charge, which is the absolute Millikan called the smallest unit of charge, which is the absolute value of the charge on an electron, the elementary charge (e)value of the charge on an electron, the elementary charge (e)The elementary charge (e) has magnitude e = 1.6 x 10-19

CTherefore 1 coulomb of charge contains 6.24 x 1018 electrons

1 C = 6.24 x 1018 e

Excess charge (pos sign)Deficit of charge (neg sign)

The total charge on an object (Q) can be measured in coulombs by Q =

NeWhere N is the number of electrons

Electric CurrentElectric Current The amount of electrons The amount of electrons

(charge) that move through (charge) that move through a given area per seconda given area per second

When an energy source is connected between the ends of a conductor, electrons move through a conductor from atom to atom in an ordered way, pushed by the excess of electrons at the negative end of the battery

tQI

Q - Total charge on object in coulombs (C)

t – Time in seconds (s)

I – Current in amperes (A), where 1 A = 1 C/s

HomeworkHomework

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