Welcome to the 4th Marking Period

• Unit V– Ch 32, Electrostatics– Ch 33, Electric Fields and Potential– Ch 34, Electric Current– Ch 35, Electric Circuits– Ch 36, Magnetism– Ch 37, Electromagnetic Induction

ElectrostaticsChapter 32

Charge

• Protons and electrons each have a fundamental property called electric charge.

• A proton charge is perfectly equal to an electron, but opposite sign.

Charge

• Atoms normally have the same number of electrons as protons.

• This means that the atom is electrically neutral – from outside, the atom appears neither positive nor negative.

Charge

• You already know this stuff!– Like charges repel.– Opposite charges attract.– Electrical charge is conserved.– Objects can be charged by friction (given a

“net charge”); like when you are walking across the carpet, or sliding out of your car seat. When this happens, charge is moved, but still not created or destroyed.

Charging by Friction

Benjamin Franklin

• Discovered the principle of conservation of charge.

• Created the labels + and –• Used a cloud to create

sparks on the ground – the famous kite experiment.

• Just turned 300 years old.

Charles Coulomb

• French researcher who studied friction and electricity.

• Developed a law to explain the force between two charges.

Coulomb’s Law

• If the amount of charge at point q1 doubles, the force doubles.

• If the amount of charge at point q2 doubles, the force doubles.

• If the distance between the charges doubles, the force becomes one quarter of what it was.

Conduction / Insulation

• Some materials allow electrical charge to move freely from one place to another.These materials are called conductors.

• Other materials resist the movement of electrical charge through the material.These materials are called insulators.

• Some materials have a behavior between that of a conductor and an insulator.These materials are called semiconductors.

Charge Movement

• If an excess of charge is placed on the surface of a conductor, the charges spread out to get away from one another.

• If an excess of charge is placed on the surface of an insulator, the charges cannot move away from one another.

3 Ways to Charge an Object

1) Friction

2) Conduction – if an object has an excess of charge, it can transfer that charge to other objects through contact.

3) Induction – if an object has an excess of charge, it can cause charge separation in other objects due to proximity, without contact.

Induction

• Induction is how electrical storms and lightning strikes happen. We will discuss this more next class, and in the homework.

Polarization

• The charges in an atom can be separated because of outside forces.

• This induced charge separation in an atom is called polarization of the atom.

• Polarizing the atoms in an object does not involve charging the object. It just means that the charges pull apart slightly.

Polarization

Electroscope

Homework

• Read ch 32.

• P505 #1-25, due by Friday, 4/7.