Electric field – between parallel plate

 If we take two parallel plates of metal and give them equal and opposite charge, what does the electric field look like between the plates?

SOLUTION:

Just remember: Field lines point away from (+) charge and toward (-) charge.

PRACTICE: Justify the statement “the electric field strength is uniform between two parallel plates.”
SOLUTION:

∙Sketch the electric field lines between two parallel plates.
∙Now demonstrate that the electric field lines have equal density everywhere between the plates.

Potential difference

∙Because electric charges experience the electric force, when one charge is moved in the vicinity of another, work W is done (recall that work is a force times a displacement).

∙We define the potential difference V between two points A and B as the amount of work W done per unit charge in moving a point charge from A to B.

Note that the units of V are JC^-1 which are volts V

Potential difference – the electronvolt

When speaking of energies of individual charges (like electrons in atoms), rather than large groups of charges (like currents through wire), Joules are too large and awkward.  

We define the electronvolt eV as the work done when an elementary charge e is moved through a potential difference V.

From W = qV we see that
1 eV = eV = (1.60 x 10^-19 C)(1V) = 1.60 x 10^-19 J.  

FYI – We will use electronvolts exclusively in Topic E when we study atomic physics.

Potential difference – path independence

EXAMPLE:

A charge of q = +15.0 µC is moved from point A, having a voltage (potential) of 25.0 V to point B, having a voltage (potential) of 18.0 V, in three different ways. What is the work done in each case?