Electricity and Magnetism Module 5

Electricity and Magnetism  Electric forces hold atoms and molecules together.  Electricity controls our thinking, feeling, muscles and metabolic processes.  Electricity and magnetism underpin much of our current technology (e.g. computers).  Electricity and magnetism are linked on a fundamental level.

Part One Static Electricity

What is electricity? The collection or flow of electrons in the form of an electric charge

What is static electricity? The stationary buildup of an electric charge due to an excess or a shortage of electrons. Like the shoes rubbing against the carpet. Electrons are transferred from the carpet to the shoes.

As electrons collect on an object, it becomes negatively charged. As electrons leave an object it attains a positive charges. Charges interact with each other: Often when you remove clothes from the clothes dryer, they seem to stick together. This is because some of the clothes have gained electrons by rubbing against other clothes. The clothes losing electrons become positive. The negative clothes are attracted to the positive clothes.

Lines of force diagrams for (A) a negative charge and (B) a positive charge when the charges have the same magnitude as the test charge.

The charge on an ion is called an electrostatic charge. An object becomes electrostatically charged by –Friction,which transfers electrons between two objects in contact, –Contact with a charged body which results in the transfer of electrons, –Induction which produces a charge redistribution of electrons in a material.

The Tribolectric Series Rub two substances together … Plasticwill become negatively charged Sulphur Gold Nickel. Copper Hard rubber (ebonite) Wood, amber, resin Cotton Paper Silk Lead Wool Glasswill become positively charged

The van de Graaf generator (large silver ball) deposits electrons on the ball. When a person places their hand on the ball and the machine is turned on, electrons are transferred to and collected on the person touching the silver ball. Why do you think this machine affects the hair of the children in the picture? How did the children become electrostatically charged?

What causes you to be shocked when you rub your feet across carpet? How did you get charged? An electrical discharge is the passing of an electric current through the air from a negatively charged object to a positively charge object. This is what causes lightning !

Charging by induction: The comb has become charged by friction, acquiring an excess of electrons. The paper (A) normally has a random distribution of (+) and (-) charges. (B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges. This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb.

How are static charges detected? An electroscope.

The charge of one electron is called the Elementary Charge It has a value of x C C is the symbol for the Coulomb, the unit of measurement for electrical charge. One Coulomb is equal to the charge of 6.25 x electrons or protons.

A Question for you.. During a storm, impressive bolts of lightning form jagged lines across the sky. The lightning is caused by a brief but powerful electrical discharge. If the electrical discharge of a lightning bolt is equal to 20 C, How many electrons were involved in the neutralization process?

Answer 20 x 6.25 x = 1.25 x electrons

Question 2 After charging a piece of fabric, Jonathan calculates that it has lost 2 x electrons. a) what is its charge in coulombs? b) Is this charge positive or negative?

Answer a) 2 x = 3.2 x or C 6.25 x b) Positive charge

Electric Force and Coulomb’s Law If two point charges q 1 and q 2 are separated by a distance r, the magnitude of the electric force F e between them is: where k = 9 x 10 9 Nm 2 /C 2 is the Coulomb constant, q 1 and q 2 are in Coulombs ( C ), r is in meters ( m ) and F e is in Newtons ( N ).

Question #3 Two positively charged particles with respective charges of 0.02 C and 0.05 C are placed 2 cm apart. What is the intensity of the electrical force each particle exerts on the other?

Answer F e = 9x10 9 x.02 x.05 = 2.25 x N.02 2