Electrostatics

A little history… The word electricity comes from the Greek word for amber, ELEKTRON. 700 B.C. They found that amber would attract small particles, like dust, and generate sparks, when rubbed with fur

Ben Franklin Did famous experiments with lightening to help discover power of electricity Also discovered that charged objects come in one of two forms. Like forms repel and unlike forms attract each other. He called them “positive” and “negative” charges to differentiate them

Earnest Rutherford 1911 – Rutherford does gold-foil experiment showing that the atom is made up of mostly empty space, but with a small, positively charged center nucleus.

Electricity at the atomic level An atom is made up of a nucleus and electron orbiting. Inside the nucleus are protons (positive charged) and neutrons (no charge). Electrons orbit around the nucleus and are negatively charged.

Electrically charged atoms Atoms “want” to be neutral meaning an even number of electrons and protons (adding up to 0 charge). An atom is said to be charged” if There is an excess of electrons (the atom is negatively charged) There are more protons that electrons (the atom is positively charged)

Two ways to charge an object Conduction: Charging by contact. Charge (electrons) can be exchanged between two objects depending on their atomic properties. Some items are ready to “give up” electrons These are called conductors. Some items are ready to “accept” electrons. These are called insulators

Have an object grounded and bring a charged object near it. Induction: Charging without contact. Have an object grounded and bring a charged object near it. All opposite charges are drawn near the charged object and like charges are free to move towards the ground. Unground the object Only things left in the object are the opposite charges to the object brought near it.

Quantity of charge The unit for charge is called a Coulomb Particle Charge (C) Mass (kg) Electron -1.60x10-19 9.11x10-31 Proton +1.60x10-19 1.67x10-27 Neutron

Electrostatic Force If two charged objects are capable of attracting or repelling each other, this could cause a change in motion. Thus, electrostatics causes a force.

Electrostatic Force What variables will effect the strength of an electrostatic force? How do they effect the strength of the force?

Coulombs Law Found by Charles Coulomb in 1790 using a torsion balance

Robert Millikan 1909, an experiment was performed at the University of Chicago to determine the charge of an electron. Oil droplets were dropped between two charged parallel plates. The droplets were charged by friction before they were dropped. The negative oil drop was attracted to the positive plates and was suspended between them

Millikan oil drop continued Using force laws, he was able to find the charge of the oil drop as a multiple of an object charge, an electron. Using this information, the proportionality constant of Coulomb’s law could be found.

Coulomb’s Law

Example The electron and proton of a hydrogen atom are separated, on average, by a distance of about 5.3x10-11 m. Find the magnitude of the electric force that each particle exerts on the other.

Given:

The force is negative, which means in this case, that the electric force is attractive.

Field Forces Electrostatic force is called a “field force” because it does not need contact to exist. Can you think of any other forces that act from a distance?

Newton’s Law of Gravitation Proposed by Isaac Newton in 1660’s… Look familiar??

Gravitational Force Cavendish found the value of the proportionality constant using a torsion balance as well in 1798, and got the idea from Coulomb’s experiments in 1790! Weighing the Earth PDF.

Similarities Both a field forces relying on the inverse square of the distance the objects act on each other. Butter gun.

Differences Gravitational force is ONLY attractive Electric force can be attractive or repulsive. Electric force is usually much stronger! Proton and electron of a hydrogen atom example again…

Givens