Electrostatics

• “Electricity at rest”

Electric Charges

• Fundamental positive charge – proton• Fundamental negative charge – electron• If an object gains electrons – becomes

negative• If an object loses electrons – becomes

positive

Measuring the Charge

• Unit of charge is coulomb (C)• Elementary charge (e) of electron

and proton – 1.6x10-19 C. • 1 C – 6.24x1018 electrons• Braun Electroscope – device for

detecting electric charge

Conservation of Charge

• Charge cannot be created or destroyed, only transferred from one object to another.

Transferring of Charges

• (Electroscopes, Metal Spheres)

Coulomb’s Law• Force two charges have on each other• Related to Newton’s Law of Universal

Gravitation

q1 and q2 are chargesr – distance between centers

k – electrostatic constant (9.0x109 Nm2 /C2)

Fe = magnitude of force (N)

221

r

qkqFe

Force vs. DistanceF

orce

Distance

Example

• Calculate the magnitude of the force between two positive charges, q1 = 3.0 x10-6 and q2 = 6.0 x10-5 C, separated by a distance of 9.0 m.

• (Positive value of F – Repulsion)• (Negative value of F – Attraction)

Electric Field• A field exists when an electric force is

exerted on a charged particle• Vector Quantity• Idea developed by Michael Faraday• Electric Field Lines – Models to

represent an electric field–Lines ALWAYS go AWAY from

positive charges

Electric Field Lines

Electric Field Lines

• Two Equal and Opposite Charges• Two like charges (+ and -)• Parallel Plates• Hollow Sphere• Directionality of a point charge.

Electric Field Strength

• F – Force (N)• E – Electric Field Strength (N/C)• q – charge (C)

q

FE

Example

• A test charge of +2.0x10-6 coulomb experiences a force of 2.4x10-3 N when placed in an electric field. Determine the magnitude of the field.

Potential Difference

• The work required to bring a charge from infinity to that point.

• When work is done, energy is used• Scalar Quantity

Potential Difference

• V – Potential Difference (Volts, V)• W – Work (Joules, J)• q – charge (Coulombs, C)

q

WV

Problem

• When a charge of -4x10-3 C is moved between 2 points in an electric field, 0.8 J of work is done on the charge. Calculate the potential difference between the points

Electron Volt

• Electron Volt (eV) is the energy required to move one elementary charge (1.6x10-19 C)

• 1 eV = 1.6x10-19 C