They’re Attractive & Repulsive at the Same Time

When two charges are brought together they either push away from each other or they pull

towards each other.

+ +

- -

Since the charges will change their motion, Newton’s First Law tells us they must have a force applied to them!!!!

Newton’s Third Law tells us that the force on each charge must be equal in size but opposite in direction. They are an action-reaction pair!!!!

If there is no net force acting on an object, then

Electric Charge

We know that there are two types of charge – positive and negative.

We know that protons are 2000 times bigger than electrons in terms of mass.

We know that even though protons are positively charged & electrons are negatively charged, protons and electrons carry the same size charge.

The size of the charge on a proton & an electron is defined in units called a Coulomb (C).

A Coulomb (C) is the unit used to measure electric charge.

Electric Charge

A Coulomb of charge is very large.

It would take 6.25 X 1018 protons to make 1-C of charge.

For those of you who avoid scientific notation like the plague: It takes 6,250,000,000,000,000,000 protons to make 1-C of charge.

A proton has a charge of 1.6 X 10-19-C, an electron has a charge of -1.6X 10-19-C.

For you anti-plague people:1.6 X 10-19-C = 0.000 000 000 000 000 000 16-C

Particle

Charge (C)

Mass (kg)

Proton 1.60 x 10-19 1.673 x 10-

27

Electron -1.60 x 10-19 9.109 x 10-

31

Neutron 01.675 x 10-

27

Coulomb’s LawIn 1875, Charles Coulomb did some experiments on the forces between electric charges.

He discovered two things:

1) The size of the force depends directly with the product of each charge..

FE ~ Q1∙Q2

Where Q is the size of the charge in Coulombs.

2) The size of the force depends inversely with the square of the distance between the charges.

FE ~ 1/r2

Where r is the distance between the charges.

Example

Find the electrical force between a 1-C charge and a 3-C charge that are 6-m apart.

Know Formula

Work

Q1 = 1-CQ2 = 3-Cr = 6-m

The (+) answer

means the force is

repulsive – LIKE

CHARGES REPEL

Example 2

Find the electrical force between a 5-C charge and a -5-C charge that are 100-m apart.

Know Formula

Work

Q1 = 5-CQ2 = -5-Cr = 100-m

The (-) answer

means the force is

attractive – opposite CHARGES

attract