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 = W on 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. Electric Potential Difference in a Uniform Field  V = (V b – V a ) = W on q / q = (Fd) / qF=Eq = (Eq)d /q  V = Ed d = displacement parallel to the field lines +q AB d E

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. A charge moves 2.0 m parallel to the direction of a uniform electric field with a field strength of 1.0 x 10 3 N/C. What potential difference does the charge move through? Given: E = 1.0 x 10 3 N/C d = 2.0 m Find: ΔV = ?  V = Ed = (1.0 x 10 3 N/C)(2.0 m) = 2.0 x 10 3 V

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. 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