Electrostatics Level 1 Physics Objectives and Essential Questions

Electrostatics Level 1 Physics

Objectives and Essential Questions OBJECTIVES At the end of this unit, the student should be able to: Define electrostatics and the nature of electric charge State Coulomb’s Law and its equation to calculate the electrostatic force between two charges Explain charging of an object by contact and induction Distinguish between conductors and isulators ESSENTIAL QUESTIONS How are objects charged? What are the principles of charging by induction? What is Coulomb’s Law?

Electric Charge – Just the facts Charge is a property of subatomic particles Facts: • Two types of charge – Positive (protons) and Negative (electrons) • LIKE charges REPEL and OPPOSITE charges ATTRACT • Similar to fluids – Can be in 2 states; STATIC and DYNAMIC

Electric Charge • The symbol for charge is q • The unit for charge is Coulomb (C) • A single charged particle (1 electron or proton) is called an elementary particle (symbol is e). The magnitude of the charge on the proton exactly equals the magnitude of the charge on the electron. Particle Charge Mass Proton +1. 6 x 10 -19 C 1. 67 x 10 -27 kg Electron -1. 6 x 10 -19 C 9. 11 x 10 -31 kg Neutron 0 1. 67 x 10 -27 kg

Charged Objects It is possible to transfer electric charge from one object to another. • Usually, electrons are transferred • The body that loses electrons has an excess of positive charge. • The body that gains electrons has an excess of negative charge. When an ebonite rod is rubbed against animal fur, electrons from atoms of the fur are transferred to the rod. This gives the rod a negative charge and leaves a positive charge on the fur

Conservation of Charge When an ebonite rod is rubbed with animal fur: Process serves to separate electrons and protons already present in the materials. Protons/Electrons are not created or destroyed. As an electron is transferred to the rod, a proton is left behind on the fur. Both have identical charges (magnitudes are the same, signs are opposite). Transfer does not change the net charge of the fur/rod system. Each material contains an equal amount of protons and electrons to begin with, net charge of the system is zero initially and will remain zero during the entire process.

Law of Conservation of Electric Charge During any process, the net electric charge of an isolated system remains constant (is conserved).

Conductors and Insulators Not only can charge exist on an object, but it can move through an object. The movement of charge is limited by the substance the charge is trying to pass through. There are generally 2 types of substances. Conductors: Substances that readily conduct electric charge Insulators: Substances that conduct electric charge poorly (resist the movement).

Charging There are three ways to charge an object: FRICTION INDUCTION CONDUCTION

Friction The only reason that we are able to use electricity in our modern world is that it is possible to separate positive and negative charges from each other. • Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths • As they pass over each other, the electrons with weaker bonds are “stripped” off of that material and collect on the other material. Example – Rub a piece of ebonite across a piece of animal fur. The fur does not hold on to its electrons as strongly as ebonite. Some of the electrons will be ripped off of the fur and stay on the ebonite. Now the fur has a slightly positive charge and the ebonite is slightly negative. NET CHARGE IS STILL ZERO CONSERVATION OF ELECTRIC CHARGE

Conduction The two objects will come into physical contact with each other (sometimes called “charging by contact”). Figure 2 a: Negatively charged metal object and an uncharged metal sphere Figure 2 b: Bring the two objects close together, separation of charge starts occur Figure 2 c: The two objects touch. Some negative charge will transfer over to uncharged metal object Figure 2 d: When negative object is removed, it will not be as negative as it was. Both objects have some negative charge.

Induction To charge a conductor without coming in direct contact with it. Figure 3 a: Metal sphere is on insulating stand has a ground wire attached. Figure 3 b: Bring a negative object nearby. Causes a separation of charge in sphere and electrons will travel down the wire. Figure 3 c: Keep negative object nearby. Either cut or remove the ground wire. The electrons have no way to travel back up the wire Figure 3 d: Now, remove the negative object. the sphere has a net positive charge

Electric Force The electric force is similar to the gravitational force. Remember

Coulomb’s Law The magnitude F of the electrostatic force exerted by one point charge q 1 on another point charge q 2, is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance r between them For calculation ease, one may use 9. 00 x 109 Nm 2/C 2 for the constant

Electric Forces and Newton’s Laws Electric Forces obey Newton’s Laws. Example: An electron is released above the surface of the Earth. A second electron directly below it exerts an electrostatic force on the first electron just great enough to cancel out the gravitational force on it. How far below the first electron is the second? Fe e mg r=? 5. 1 m e