Atoms are composed of

1. Electrons – fundamental negatively charged (-) particle of matter (-1.6 x10-19C)

2. Protons – fundamental positively charged (+) particle of matter (+1.6 x10-19C)

3. Neutrons – neutral (no charge)

• The magnitude of the charge of an electron and a proton are the same

• An object can become electrically charged by gaining or losing electrons. When the object gains electrons it becomes negatively charged and when it loses electrons it becomes positively charged.

• When an atom gains or loses electrons, it becomes a charged particle known as an ion.

• Protons and neutrons *cannot* be removed from the atom because they are tightly bound within the nucleus.

• Like charges repel each other and unlike charges attract each other. Charged objects can also attract neutral objects.

• The only proof that an object is charged is repulsion. If two objects are repelled, they have the same charge.

• Elementary charges (e) - equal in magnitude to the charge on an electron (-e) or the charge on a proton (+e)

• The SI unit of charge is the coulomb

• One coulomb is equal to 6.25x1018 elementary charges.

• The charge on an electron (-e) is -1.6x10-19 coulomb and the charge on a proton (+e) is +1.6x10-19 coulomb.

• The net charge on a charge object is always quantized, which means that the charge is always an integral multiple of e.

• For example, the charge could be +e, -2e, -3e, etc. but it cannot be +0.5e, -1.3e, etc.

• Example: An object possessing an excess of 6.0x106 electrons has a net charge of?

• Answer: (6.0x106) x (-1.6x10-19C) = -9.6 x 10-13C

• Example: A metal sphere has a charge of -1.1x10-6C. Approximately how many more electrons than protons are on the sphere?

• Answer: (1.1x10-6C) / (1.6x10-19C)

= 6.9x1012

• Law of conservation of charge – charges within the system may be transferred from one object to another, but charge is neither created nor destroyed

Example: Two identical metal spheres have charges of -2e and -4e respectively. The spheres touch and are then pulled apart. What is the charge on each sphere?

(-2e) + (-4e) = (-6e)/2 = -3e

Electroscope• Used to detect the presence of an

electric charge on an object.

• Two types: gold-leaf electroscope and needle electroscope.

• Gold leaf: thin, movable leaves diverge when charged. If no charge is present, the leaves point straight down.

• Needle electroscope: the presence of a charge will deflect the needle from its usual upright position.

• Objects may be charged through conduction and induction

• Charging by friction – rub neutral objects together such as fur and rubber and each will become oppositely charged as the other.

• Charging by Contact – touch a charged object to a neutral one and half of the charge from the original will transfer to the object leaving both with identical charges.

• Charging by Induction – A charged object causes another object to become oppositely charged by using a ground and the concept of redistribution of charges.

•When you bring a charged object (rod) near a conductor (electroscope) the side next to the charged object becomes oppositely charged. The other end of the conductor would have the same charge as the object. We say the charges were redistributed.

• Coulomb’s law – the size or magnitude of the electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

Example: Calculate the electrostatic force between two +1.0x10-6C charged spheres, 4.0x10-1m apart?

Answer: + 5.6x10-2NPositive sign indicates repulsion

Fe = kq1q2/r2

•Fe is the electrostatic force in newtons•q1 and q2 are the charges in coulombs•r is the distance of separation in meters•k is the electrostatic constant, 8.99x109 N*m2/C2

The force that q1 exerts on q2 is equal in magnitude but opposite in direction to the force that q2 exerts on q1.