The Electric Field

Some Introductory Notes The magnitude and strength of Electric fields and Gravitational fields vary inversely with the square of the distance between them.

More Notes… An electric field is very strong. A gravitational field is not strong. An electric field is caused by a charged object affecting its surroundings.

Son of More Notes… An electric field caused by a point charge exerts a force on any other charge placed in its environment. A collection of all the forces makes up an electric field.

Electric Field Lines They display the direction and magnitude of the field. They emanate from a positive point charge. They emanate towards a negative point charge.

More They show a picture of the field. If they are close the field is strong strong. If they are far apart the field is weak. An electric field is real and it can do work.

Electric Field Intensity

Defintion Electric Field intensity is the ratio of the force on a test charge to its charge.

What are the Units? The units are Newtons per Coulomb…N/C. E is the electric field intensity F is the force q’ is the test charge

More on E-Field The electric field intensity can also be calculated using the point charge, Q.

This, of course, means that the electric field intensity does not depend on the magnitude of the test charge.

Electric Potential Difference

A charge in an electric field experiences a force. If the charge moves in response to the force, work is done by the field. The system loses energy.

If the charge is moved against the field, work is done by an outside force. Energy is stored in the system.

If work is done as a charge moves from one point to another point…the points are said to differ in electric potential. This is referred to as the potential difference between the two points.

The Potential Difference The potential difference, V, between any two points in an electric field is the work done per unit charge as the charge is moved between the points.

The Equation

The Units

Sample Problem If the potential difference between two points in a field is 6.0V, what is the work required to move a charge of 300.  C between the points?

Voltage, or electric potential, can be described as electric pressure. It is analogous to water pressure in a garden hose.

The voltage of a battery is the pressure with which charge travels through a wire. As the battery dies, the pressure wanes.

Equipotential Lines

Lines of equal distance from a point charge constitute an equipotential line. The electric field intensity can be also designated as change in potential per unit distance.

Prove that a N/C is the same as a V/m.

Uniform Electric Field

Definition Two parallel conducting plates a distance, d, apart constitute a uniform electric field. The uniform field emanates from the positive plate to the negative plate.

Voltage in a Uniform E-field Voltage equals work per unit charge. That is V = Nm/C N/C is E – electric field intensity. m is distance between the plates. Therefore…

Sample Problem It requires 2.5 X N of force to move an electron from the positive to the negative plate in a uniform electric field. If the plates are separated by a distance of m, what is the electric potential between the plates?