Lecture 5 - Electric Charges

Static Electricity

• objects can be charged by rubbing

• plastic ruler rubbed with a cloth becomes negatively charged

• when two unlike materials are rubbed together, they become charged

  • combing hair, sliding in car seat, walking on carpets

what is charging

transfer of electrons from one material to another material

what is a conductor

materials in which free electron can move within the material

what is an insulator

materials in which electron cannot move within the material

two types of electric charges

• positive

  • possessed by protons

• negative

  • possessed by electrons

• like charges repel, opposite charges attract

electron transfer equation

number of electrons transferred = Amount of charge/charge of one electron

Coulomb’s Law (1780)

Electric force between two point charges is given by Coloumb’s law

F = k (q1 * q2) / r²,

• k is the electrostatic constant

• q1 and q2 are the magnitudes of the charges

• r is the distance between the charges.

An electron and a proton are held at a distance of 0.5m, if released which partial would have greater acceleration at any one instant

both experience the same force but the mass of electron is much smaller than a protons and therefore to compensate, it has greater acceleration

Electric Field: contact forces

charges apply force without any physical contact to push or pull the other charges - force at a distance

Electric Field Lines

Electric field around a charge can be represented by field lines. They start on a positive charge and end on a negative charge.

When another charged object called the test charge enters the electric field, electric force acts upon it

Electric field at a point equation

E = F/q

• F = force on a small charge

• q = charge of the small charge

Electric Shielding

• the charges induced on the two surfaces of the box produce an electric field inside the box

• the field opposes the external field, causing a zero net field in the box