Electrostatics Part II

Objectives 1. Define electric potential2. Compare electric potential and electric field3. Find the potential due to a point charge V= Q/4πεo

r4. Find the potential due to a charged sphere5. Find the potential at point due to several charges6. Discuss the hazards of charging by friction7. Explain the use of lightning rods to protect buildings8. Distinguish between electrical conductors and

insulators using electron arrangement

Electric Potential V• Electric potential is the potential energy per unit

charge, i.e. V=PE/q• The electric potential tells you how much

potential energy a single point charge at a given location will have.

Electric Potential of a Point Charge• The electric potential V of a point charge is

• Thus V for a point charge decreases with distance, whereas E for a point charge decreases with distance squared

• The electric potential is a scalar while the electric field is a vector.

Problem

• What is the electric potential 25cm away from a +6.0uC charge?

• V= +6.0 x 10-6/ 4π (8.85x10-12)(0.25m) = +2.16 x 105 V

Electric Potential due to Multiple Charges

• Recall that electric potential is scalar.

• The total or net electric potential is the scalar sum of the electric potential due to each of the individual point charges qi.

V due to Multiple Charges• Find a potential energy of a system of two

point charges shown in the figure.

• Find a potential energy of a system of three point charges.

Problems

• Find an electric potential at point P due to a system of three point charges as shown in the figure.

V on a Charged Sphere

• If the charge is symmetrically distributed, the potential outside a sphere of charge Q is the same as that from a point charge

Charging

• Charging means gaining or losing electron.• Matters can be charged with three ways– charging by friction, – charging by contact– charging by induction.

Ways of Charging

Friction

Induction: the charged object will have opposite sign as the charger

Conduction: the charged object will have same sign as the charger

Earthing/ Grounding

• Grounding is the process of removing the excess charge on an object by means of the transfer of electrons between it and another object of substantial size.

Negative sphere: Electrons move from sphere to the ground to neutralise it

Positive sphere: Electrons move from ground to sphere to neutralise it

Lightning Rod

• lightning rods provide a low-resistance path to ground that can be used to conduct the enormous electrical currents when lightning strikes occur.