Foundations of Physics Electrostatics

Four Fundamental Forces Gravity Cosmic and Macroscale Electromagnetic Macroscale and Microscale Weak Strong Subatomic Nuclear

Electromagnetic Force Force of attraction or repulsion due to an imbalance of electric charge Opposite Charges Attract Like Charges Repel

Electric Charge Atoms are composed of positive protons, neutral neutrons, and negative electrons Neutral atoms have equal numbers of positive protons and negative electrons

Electric Charge Protons and neutrons are locked in the nucleus and can not move freely Electrons are found outside the nucleus in energy levels and are free to move A positive or negative electric charge results from objects gaining or losing electrons

Conservation of Charge When electrons are transferred from one object to another, no electrons are created or destroyed. The electrons removed from one object equals the electrons added to another object.

Charging By Friction Results from rubbing objects together Called triboelectric charging Friction strips electrons from atoms of one object and deposits them on another object Friction transfers the electrons from the sweater to the balloon The balloon and sweater have a balance of electric charge initially Resulting in a slightly more positive sweater and a slightly more negative balloon

Electron Affinity In some materials, the electromagnetic force of attraction between the positive nucleus and the negative electrons is very strong. This attraction is called electron affinity. Electrons move from materials with low electron affinity to materials with high electron affinity.

Conductors and Insulators Conductors are materials which allow electric charge to move through them easily. Insulators are materials which do not allow electric charge to move through them easily. Iron Nail Plastic Rod Electrons strongly bound to positive nucleus Electrons are loosely bound to positive nucleus

Conduction Charging by contact Charging a neutral object by making contact with a charged object The resulting charge is always the same as the original charge. Requires a conductor

Conduction When a negatively charged object touches a neutral object, the repulsive force between the electrons pushes the mobile electrons off of the negative object and onto the neutral object. Thus, the original negative charge is spread over both objects. Objects are separated after initial touch - Charged object touches neutral object Both objects have a negative charge Some electrons move onto sphere Notice the negative charge on the original object is reduced because some electrons moved to the sphere

Conduction When a positively charged object touches a neutral object, the force of attraction pulls mobile electrons off of the neutral object onto the positively charged object Results in both objects obtaining a positive charge Objects are separated after initial touch + Charged object touches neutral object Some electrons move to positive object Both objects have a positive charge Notice the positive charge on the original object is reduced because some electrons moved onto the object from the sphere

Polarization A negatively charged balloon is brought near a neutral sphere The negatively charges repel the electrons of the sphere pushing them to the opposite side of the sphere. Results in an object with two oppositely charged ends

Van de Graaff Generator A motor at the base of the generator turns a large rubber belt which is stretched between two rollers located at each end of a long clear tube. Metal Sphere Rubber Belt Motor in Base

Van de Graaff Generator As the rubber belt moves over the rollers, electrons are removed from the rollers and begin to build up on the belt. Wire brushes at the top of the clear tube remove the excess electrons from the belt and conduct the negative charge to the metal sphere at the top of the generator.

Van de Graaff Generator The negative charge on metal sphere repels each other causing the charge to spread out evenly around the sphere. When the electromagnetic force of repulsion becomes too large, the negative charge “jumps” off the sphere to another object or the air.

Induction Charging of a neutral object by bringing a charged object near (but not touching) it. Includes two processes: Polarization and Grounding Results in two oppositely charged objects

Induction The negative charged electrons on the sphere are removed by touching it with a finger (or other object) Grounding – connecting a charged object to Earth to allow electrons to travel to or from the object depending on the situation

Induction The finger (or grounding object) is removed and the balloon and sphere are separated Notice the positive charge is equal to the negative charge on the original balloon (there Is no reduction of charge)