Electric Fields and Capacitance

Electric Fields

Conductors have free electrons.

Insulators don’t have free electrons as they are attached to the atoms.

Like charges repel, unlike charges attract.

Equipotentials are lines of equal potential.

Equipotential surfaces are perpendicular to field lines.

Any electrical conductor is an equipotential surface.

Coulomb’ law states that any 2 charged particles exert a force on each other which is proportional to their 

charges and inversely proportional to the square of their distance apart.

Air is an insulator but during a thunderstorm, the insulating properties break down due to the electric field 

between the clouds and the ground.

ε₀ is the measure of how well a medium will permit an electric field to pass through it.

Electrical potential is the work done per unit charge on a small positively charged object to move it from 

infinity to that point in the field.

Electric fields can be attractive and repulsive, gravitational fields are only attractive. Both fields can be 

represented by inverse square laws.

Electric field strength at a certain point in an electric field is defined as the force per unit charge on a positive test charge placed at that point.

Electric field is equal to the negative of the potential gradient.

No work is done when a charged particle moves along a line of constant potential.

Electric potential is a scalar quantity.

Electric field strength is a vector quantity.

Potential gradient is a point in a field where the change in potential per unit change of distance along the field 

line at that point.       

Capacitance

Capacitors store electrical energy.

Charge a capacitor by connecting it to a battery.

Discharge a capacitor by disconnecting from the battery and attaching it to the circuit.

The time taken to charge or discharge depends on the capacitance (which affects the charge) and the 

resistance (which affects the current).

Dielectric is an electrical insulator which can be a solid (ceramic, mica, glass, plastics and oxides of various metals), liquid (distilled water), gas (dry air) or a vacuum.

©2011 Grant Dwyer