ELECTRIC FIELDS
Static electricity or electrostatics
Ebonite rod rubbed against animal fur.
Friction between the rod and fur detaches electrons from the atoms of fur and they transfer to the rod.
Rod is said to be electrified or to have become electrically charged (negatively).
A perspex rubbed against dry cloth becomes positively charged.
Positive and negative electric charges occur in nature
Inside the atom
Positively charged nucleus surrounded by much lighter negatively charged electrons.
Experiments in electrostatics
Like charges repel one another and unlike charges attract one another.
The gold leaf electroscope is design to detect static electricity.
To determine the type of charge, charge the electroscope with a known charge.
Charging by induction
Conductors and insulators
Electric charges cannot move freely in electrical insulators. When E is applied no current.
In electrical conductors electric charges move freely, E produces current.
Charges continue to move until E = 0.
When no currents flows, E = 0 in conductor, i.e. it is an equipotential object.
Semiconductors – their electrical properties are between those of insulators and those of conductors.
A conductor connected to the Earth is said to be grounded.
The Earth can then be considered an infinite "sink" to which electric charges can easily migrate.
Coulomb’s Law
Electric field force between two stationary charges
k is a constant called the Coulomb constant.
In SI unit of charge is the Coulomb (C) and k = 8.9875 x 109 Nm2/C2.
This constant is also written as k = 1/(4pe0) where the constant e0 (lowercase Greek epsilon) is known as the permittivity of free space e0 = 8.8542 x 10-12C2/Nm2 .
The smallest unit of charge known in nature is the charge of electron and proton.
e = 1.60219 x 10-19 C
Therefore, 1C of charge is approximately equal to the charge of 6.24 x 1018 electrons or protons.
Force is a vector quantity.
In Coulomb’s law electric force exerted by charge Q on charge q is
where r is a unit vector directed from Q to q.
Because the electric force obeys Newton’s third law the electric force exerted by Q on q is equal in magnitude to the force exerted by q on Q and in the opposite direction;
F21 = - F12
Forces due to several charges add vectorially.
Superposition of forces
The total force on a test charge due to several other charges is the vector sum of the forces between individual charge pairs.
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Total force F = F1 + F2 + F3 …
Example: Consider the arrangement of charges below. Find the net force (magnitude and direction) on the charge +2q.
Coulomb’s force exerted by Q depends only on position in space, and the charge q. We can therefore define an electric field E.
At any point, E created by Q is the force per unit charge (F/q) exerted on a small charge q placed at that point
SI unit of E: NC-1
Electric field is also a vector quantity.
If Q is positive, the electric field is directed radially outward from it.
If Q is negative, the field is directed toward it.
Note: positive charge experiences force in direction of field;
negative charge experiences force in opposite direction.
Electric field lines
To visualize electric field patterns we draw lines that follow the same direction as the electric field vector at any point.
The electric field vector E is tangent to the electric field line at each point.
E is great when the field lines are close together and small when they are far apart.
The lines must begin on a positive charge and terminate on a negative charge.
No two field lines can cross.
When a particle of charge q and mass m is placed in an electric field E, the electric force exerted on the charge is qE.
If this is the only force the particle will accelerate
F = q E = m a
The acceleration of the particle
If E is uniform acceleration is constant.
Electron in the TV tube
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Uniformly charged plates set up a uniform
electric field E. 
Electron experiences a force in the upward direction
F = qE = -eE
Electron undergoes constant acceleration
If the electron has an initial horizontal velocity vo as it enters the plates, what is the equation of its trajectory? Measure x and y from where it enters the plates. After time t:
Horizontal acceleration = 0
x = vot
Vertical acceleration 
The trajectory is a parabola.
Calculation of the electric field due to several charges
Similarly to calculation of force, use superposition:
Example: calculate the electric field at P due to electric dipole
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Field due to a conductor
Charges within a conductor are free to move. At equilibrium, when all charges are stationary, the field due to a conducting body will be
Note: within the conductor, E = 0
(otherwise charges would move)
At the surface of the conductor, the electric field is perpendicular to the surface.
(otherwise, charges would move along the surface)