Electrostatics

Electrostatics - the study of electrical charges that can be collected and held in one place -

charges at rest.

Examples:

BASIC IDEAS:

Electricity begins inside the atom itself.

An atom is electrically neutral; it has the same number of protons (+) as it does electrons (-).

Objects are charged by adding or removing electrons (charged atom = ion)

Fewer electrons than protons = (+) charge occurs

More electrons than protons = (-) charge occurs

There are two types of charges: positive (+) and negative (-).

Like charges repel one another: (+) repels (+), (-) repels (-).

Opposite charges attract one another: (+) attracts (-), (-) attracts (+).

Charge is quantized

Charge is conserved

Quantization of Charge

The smallest possible amount of charge is that on an electron or proton. This amount is

called the fundamental or elementary charge.

An electron has charge: qo = -1.602 x 10-19 C

A proton has charge: qo = 1.602 x 10-19 C

Any amount of charge greater than the elementary charge is an exact integer multiple of

the elementary charge.

q = nqo , where n is an integer

For this reason, charge is said to be “quantized”. It comes in quantities of 1.602 x 10-19

C

Law of Conservation of Electric Charge

The net amount of electric charge produced in any process is zero. Net amount of charge in an

isolated system stays constant.

Insulators vs. Conductors

NOTE: Both insulators and conductors possess charge.

Conductor (Metallic Bonds - free valence e’s in outer shell)

substance that allows electrons to move easily throughout (sea of electrons) ex. Silver, copper,

Al, humid air

Insulator (Covalent Bonds - no free e’s in outer shell)

substance that does not allow electrons to move freely; electron movement is restricted ex. Glass,

rubber, plastic, dry air

Polarization is the process of separating opposite charges within an object. Object is still

electrically neutral.

Ionic bonds: bonding by the transfer of electrons. Electrons are removed from one atom and are

attached to another atom resulting in (+) and (-) ions which attract each other.

Will your electrostatic expts. work on humid days?

The neutral water molecules in the air pick up excess charge from the charged object, and are

then repelled away. Given enough time, the particles in the air will remove the excess charge

from the object leaving it neutral. On humid days there are more water molecules in the air to

steal charge more rapidly. On dry days there are fewer particles in the air to steal charges so we

accumulate charge until we touch something and get discharged (shocked).

Methods of Charging a Neutral Object: friction, conduction, induction

Charging by Friction: transfer of electrons between the two objects that are rubbed together.

Electrons with weaker bonds are “ripped” off one object and collect on the other object.

Triboelectricity (tribo = friction) - useful for charging insulators

Charge is separated due to differences in

electron affinity

Triboelectric series: shows which materials

have a greater tendency to become positive (+)

and which have a greater tendency to become

negative (−).

Ex: glass with silk or saran wrap - the glass gets (+) charge, saran wrap gets (-) charge

PVC rod with fur - PVC rod gets (-) charge, fur gets (+) charge.

Charging by Conduction: transfer of electric charge by direct contact - useful for charging

conductors

A charged object is brought near, and then ______________ a neutral object;

the amount of charge __________________ between the two objects;

(Law of Conservation of Charge)

the __________________ is acquired by each object.

Charging by Induction: charging a conductor without contact - useful for charging conductors.

Grounding: an object can be neutralized if it is allowed to conduct its charge to something larger

than itself - often Earth.

Charging an object negatively by induction

Charging an object positively by induction

A charged object is brought ______________, but _____________________, a neutral object;

the charged object attracts charges opposite to it - repels charges like it. A ground is used on the

neutral object. Electrons can flow into ground or out of it.

The neutral object obtains an _____________________ to the initially charged object.

Electrostatic force & Coulomb's Law is based on the following ideas:

Electric charge is a fundamental quantity like mass, distance, or time.

Charge is observable and measurable by the force it exerts on other charges. The force is a non-

contact force (acts over a distance).

The space around the charged object which exerts a varying non-contact force on other objects

which pass through it is defined as the field.

The variable q is used to represent an amount of charge (q may be positive or negative).

The SI unit of charge is the Coulomb.

1 C = 6.24 x 1018 electrons

1 C = ? e's

Coulomb’s Law describes the electrostatic force between two charged objects.

The force one charge, q1, exerts on another, q2, has a magnitude given by:

k - Coulomb’s constant, or k = 9 x 109 Nm2/C2

q - magnitude of each charge in coulombs (C)

d - distance of separation in meters (m)

F - electrostatic force in Newtons. (N)

The force is along the line connecting the charges, and is attractive if the charges are

opposite, and repulsive if they are the same.

Relationship between F and d:

Charges are small.

Usually expressed in non-SI units of

microcoulombs (1 µC = 10-6C)

nanocoulombs (1 nC = 10-9 C)

picocoulombs (1 pC = 10-12C).

Convert into Coulombs for calculations

Coulomb's Law Problems

Convert everything to SI units: Coulombs, Meters, Newtons

Electrostatics Sample Problems

1. A +6 µC and a +3 µC charge are separated by 0.03 m. What is the magnitude of the

electrostatic force between them?

2. A repulsive force of 220 N exists between an unknown charge and a charge of +4.7 µC.

If they are separated by 2.5 cm, what is the magnitude and sign of the unknown charge?

3. One proton and one electron are separated by 5.3 x 10-11 m. What is the magnitude of the

electrostatic force between them?

4. A pith ball with +60 µC exerts a repulsive force of 175 N on another pith ball of 50 µC.

How far apart are they?

The net force on a charge is the vector sum of all the forces acting on it.

5. Find the resultant force on the -6 µC charge. Ans: .007 N to Right

6. Find the resultant force on the +10µC charge. (Ans: .029 N to Left)

Superpostition Principle Problem

Three point charges, q1, q2, and q3, lie along the x-axis at x = 0, x = 3.0 cm and x = 5.0 cm,

respectively. Calculate the magnitude and direction of the electric force on each of the three

point charges when q1 = + 6 µC, q2 = +5 µC, and q3 = -2 µC.