Electricity & Magnetism

How does a Van de Graaff generator make this girls hair stand on end?  Rubber belt set into motion by a plastic roller  Static 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 cover  Due to mutual repulsion, the charge is repelled to the outer surface of the spherical cover, and hence a large amount of charge will accumulate there  Human body can conduct electricity, so charge will be transferred to our body when the generator begins to operate  Because of the mutual repulsion of charge, the hair will stand upright

Electric Charge vs. Electric Current  Thunderclouds become charged due to friction between water droplets and ice crystals in the clouds as air currents move them around  The cloud becomes positively charged at the top and negatively charged at the bottom  A flash of lightning occurs when air rapidly conducts a huge number of electrons  This movement of charge constitutes an electric current

Measuring Electric Charge Through experimentation, Charles Augustin de Coulomb discovered that the magnitude of the force between two charged objects is:  Directly proportional to the product of the charges  Inversely proportional to the square of the distance between them The unit of electric charge is called the coulomb (C) in his honour Where k is a proportionality constant: 9.0 x 10 9 Nm 2 /C 2 Hi I’m Charles Coulomb’s Law

Measuring Electric Charge  Robert A. Millikan was able to determine the charge on an electron by studying the behaviour of charged oil drops  Using an apparatus where charged drops of oil fell in the presence of a strong electric field, he was able to determine that the charge on an electron was a fundamental constant of electricity  In the following exercise, you will analyze experimental evidence obtained from Millikan’s Oil drop experiment and search for patterns that yield the fundamental charge on an electron Hi I’m Robert, but you can call me Rob

Measuring Electric Charge Observations The values listed below represent the charges calculated on 12 oil drops 3.2 x C = 1.6 x C x 16.0 x C = 1.6 x C x 17.6 x C = 1.6 x C x 6.4 x C = 1.6 x C x 8.0 x C = 1.6 x C x 12.8 x C = 1.6 x C x 11.2 x C = 1.6 x C x 4.8 x C = 1.6 x C x 1.6 x C = 1.6 x C x 9.6 x C = 1.6 x C x 19.2 x C = 1.6 x C x 14.4 x C = 1.6 x C x Analysis Copy the data into your notebook, then describe & explain any patterns that arise

Measuring Electric Charge Two observations arise: 1) The smallest value for the charge on an oil drop is 1.6 x C 2) All the other values are whole-numbered multiples of 1.6 x C  Millikan called the smallest unit of charge, which is the absolute value of the charge on an electron, the elementary charge (e) The elementary charge (e) has magnitude e = 1.6 x C Therefore 1 coulomb of charge contains 6.24 x electrons 1 C = 6.24 x e Excess charge (pos sign) Deficit of charge (neg sign) The total charge on an object (Q) can be measured in coulombs by Q = Ne Where N is the number of electrons

Electric Current  The amount of electrons (charge) that move through a 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 Q - Total charge on object in coulombs (C) t – Time in seconds (s) I – Current in amperes (A), where 1 A = 1 C/s

Homework Pg 459 # 1-5, 8