Electric Potential Difference

Electric Potential Energy (PE)

Potential energy associated with a charged object due to its position relative to a source of electric force. Changing the position of the charge in the electric field changes

its PE. A larger test charge has a greater PE

Electric Potential Difference (∆V)

The work done moving a charged particle divided by the charge of the particle.As the value of a charge in a field increases,

the value of PE also increases. Electric potential difference is independent of charge at a given point.

∆V = Won q / q

Units = J/C = Volts (V)

Electric Potential Difference (∆V)

The sign of the charge and the direction of the field determines if ∆V is positive or negative. ∆V is negative if the charge is moved in the same

direction as the net force (negative work done). ∆V is positive if the charge is moved opposite the

direction of the net force (positive work done). ∆V is zero if the charge is moved perpendicular to

field lines (no work done). These points are called equipotentials.

Some Points to Remember:

Electric potential difference is also called potential difference or voltage.

The potential difference between two points can be measured using a voltmeter.

The zero point for potential can be arbitrarily assigned. Points that are grounded are usually assigned a

potential of zero.

Electric Potential Difference in a Uniform Field

V = (Vb – Va)

= Won q / q = (Fd) / q F=Eq = (Eq)d /q

V = Ed d = displacement parallel to

the field lines

+qA B

d

E

Electric Potential Difference in a Uniform Field

Charges that move parallel to the field lines experience changes in potential.

Charges that move perpendicular to the field lines do not experience changes in potential. NOTE: A potential

difference exists between points in a field even if there is no charge at those points.

A charge moves 2.0 m parallel to the direction of a uniform electric field with a field strength of 1.0 x 103 N/C. What potential difference does the charge move through?

Given:

E = 1.0 x 103 N/C

d = 2.0 m

Find: ΔV = ?

V = Ed

= (1.0 x 103 N/C)(2.0 m)

= 2.0 x 103 V

Robert A. Millikan’s Oil Drop Experiment (1909)

Millikan found that charge always occurred in multiples of 1.60 x 10-19 C (the elementary charge)

He concluded that charge is quantized

Capacitor

A device that stores electric energy and electric charge. Made of 2 conducting plates separated by some distance,

each with equal but opposite charge. Insulating material is often placed between the plates.

Capacitance (C)

The ability of a capacitor to store energy. It is the ratio of the amount of charge stored on each plate to the potential difference between the plates.

C = q / ∆V Units = farads (F) 1 F = 1 C / V Since farads are large, microfarads (F) or

picofarads (pF) are used. (1 pF = 10-12 F)

Some Uses for Capacitors

In a defibrillator, a 10. F capacitor is connected to a potential difference of 6000. V. What is the charge stored in the capacitor?

Given:

C = 10. F = 10. x 10 -6 F

∆V = 6000. V

Find: q =?

C = q /∆V

C(∆V) = q

= (10. x 10 -6 F )(6000. V)

= 0.060 C