1. Electric Potential Difference

2. 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

3. 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)

4. 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.

5. 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.

6. V = (Vb – Va) = Won q / q = (Fd) / qF=Eq = (Eq)d /q V = Ed d = displacement parallel to the field lines A +q B Electric Potential Difference in a Uniform Field E d

7. 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.

8. 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 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?

9. 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

10. 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.

11. 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)

12. Some Uses for Capacitors

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