ELECTRIC POTENTIAL AND

POTENTIAL DIFFERENCE

It was indicated that positive charged moves from points of lower electric potential and negative charges move from points of lower to points of higher electric potential.

However, electric potential and potential differences were not defined for electric fields around point charges.

Electric potential was defined as the potential energy per unit. Ex. V=Ue/q

The unit of electric potential was given as the volt (1 volt= 1 joule per coulomb)

For a fixed positive charge, V=Ue/q becomes V=kQ/r

Where Q is the magnitude of the fixed charged

If the fixed charge is negative then the electric potential is negative .

Thus the potential around a positive charge is positive, while it is negative around a negative charge.

Electric potential is a scalar, that is, it does not have a direction.

EXAMPLE Two charges X and Y of size +4.0x10 -9C and

-3.0x10 -9 C are placed 20cm apart. What is the electrical potential at A, the midpoint of the line joining the two charges

EXAMPLE Consider a fixed positive charges, the

electric potential at A, a distance r1 from the charge Q, is given by: Va=kQ/r1 While the potential at B, a distance r2 away from

Q, is given by:

Vn=kQ/r2

EQUIPOTENTIAS

Lines joining points of the same potential are called EQUIPOTENTIALS

The positive plate is another equipotential and so is the negative plate.

The picture above shows that the equipotentials are perpendicular to the lines of force.

The electric potential at a distance r from a charge of +Q on a spherical conductor is given by V=kQ/r.

The equipotential lines are perpendicular to the electric field lines for a spherical charge as well as for two parallel plates.

The electric field exists in space around a fived known, hence in three dimensions we have equipotential surfaces.

Since the potential difference between two points on an equipotential is zero, no energy is gained or lost by charge in moving along an euipotential.

No potential difference exists between any points on the surface since the movement of charge occurs on the surface after equilibrium is reached.

ELECTRONS IN METAL

The movement or flow of electric charge constitutes an ELECTRIC CURRENT

When an electroscope is earthed the electric current is transient, i.e. it only exists for a very short time.

This transient current is due to the movement of negative charges wire.

The negatively- charged electrons that move in wires is demonstrated by the following experiment.

The main part of the apparatus consist of a thin metal wire filament which passes along the axis of a hollow metal cylinder.

The filament can be heated by connecting it to battery A.

An electric field is created between the filament and the cylinder by battery B.

The positive terminal of battery B is connected to the cylinder while the negative terminal is connected to the filament via an ammeter, used to measure any current that flows.

The filament and cylinder are enclosed in an evacuated glass envelope so that no gas molecules can interfere with the experiment.

If the connections to battery B is reversed so that the filament is positive and the cylinder is negative then no current is detected even if the temperature of the filament incresed.

If positive particles were emitted then we would expected current to flow.

These negative particles are accelerated towards the positive cylinder by the electric field created between the filament and the cylinder.

No changed occurs in the nature of the filament and no metal is deposited on the cylinder if the experiment is continued for some time

These negative particles are ELECTRONS It is described as the “boiling” out of

electrons from a hot metal wire. It is basis upon which cathode ray tubes operate. The process is called “THERMIONIC EMISSION”