Electrical Potential Energy

1. Section 1 Electric Potential Chapter 17 Electrical Potential Energy • Electrical potential energy is potential energy associated with a charge due to its position in an electric field. • Electrical potential energy is a component of mechanical energy. ME = KE + PEgrav + PEelastic + PEelectric

2. Section 1 Electric Potential Chapter 17 Electrical Potential Energy, continued • Electrical potential energy can be associated with a charge in a uniform field. • Electrical Potential Energy in a Uniform Electric Field PEelectric = –qEd electrical potential energy = –(charge)  (electric field strength)  (displacement from the reference point in the direction of the field)

3. Section 1 Electric Potential Chapter 17 Electrical Potential Energy

4. Section 1 Electric Potential Chapter 17 Potential Difference • Electric Potentialequals the work that must be performed against electric forces to move a charge from a reference point to the point in question, divided by the charge. • The electric potential associated with a charge is the electric energy divided by the charge:

5. Section 1 Electric Potential Chapter 17 Potential Difference, continued • Potential Difference equals the work that must be performed against electric forces to move a charge between the two points in question, divided by the charge. • Potential difference is a change in electric potential.

6. Section 1 Electric Potential Chapter 17 Potential Difference

7. Section 1 Electric Potential Chapter 17 Potential Difference, continued • The potential difference in a uniform field varies with the displacement from a reference point. • Potential Difference in a Uniform Electric Field ∆V = –Ed potential difference = –(magnitude of the electric field  displacement)

8. Section 1 Electric Potential Chapter 17 Sample Problem Potential Energy and Potential Difference A charge moves a distance of 2.0 cm in the direction of a uniform electric field whose magnitude is 215 N/C.As the charge moves, its electrical potential energy decreases by 6.9  10-19 J. Find the charge on the moving particle. What is the potential difference between the two locations?

9. Section 1 Electric Potential Chapter 17 Sample Problem, continued Potential Energy and Potential Difference Given: ∆PEelectric = –6.9  10–19 J d = 0.020 m E = 215 N/C Unknown: q = ? ∆V = ?

10. Section 1 Electric Potential Chapter 17 Sample Problem, continued Potential Energy and Potential Difference Use the equation for the change in electrical potential energy. PEelectric = –qEd Rearrange to solve for q, and insert values.

11. Section 1 Electric Potential Chapter 17 Sample Problem, continued Potential Energy and Potential Difference The potential difference is the magnitude of E times the displacement.

12. Section 1 Electric Potential Chapter 17 Potential Difference, continued • At right, the electric poten-tial at point A depends on the charge at point B and the distance r. • An electric potential exists at some point in an electric field regardless of whether there is a charge at that point.

13. Section 1 Electric Potential Chapter 17 Potential Difference, continued • The reference point for potential difference near a point charge is often at infinity. • Potential Difference Between a Point at Infinity and a Point Near a Point Charge • The superposition principle can be used to calculate the electric potential for a group of charges.

14. Section 1 Electric Potential Chapter 17 Superposition Principle and Electric Potential

15. Section 2 Capacitance Chapter 17 Capacitors and Charge Storage • A capacitoris a device that is used to store electrical potential energy. • Capacitance is the ability of a conductor to store energy in the form of electrically separated charges. • The SI units for capacitance is thefarad,F, which equals a coulomb per volt (C/V)

16. Section 2 Capacitance Chapter 17 Capacitors and Charge Storage, continued • Capacitanceis the ratio of charge to potential difference.

17. Section 2 Capacitance Chapter 17 Capacitance

18. Section 2 Capacitance Chapter 17 Capacitors and Charge Storage, continued • Capacitancedepends on the size and shape of a capacitor. • Capacitance for a Parallel-Plate Capacitor in a Vacuum

19. Section 2 Capacitance Chapter 17 Capacitors and Charge Storage, continued • The material between a capacitor’s plates can change its capacitance. • The effect of a dielectric is to reduce the strength of the electric field in a capacitor.

20. Section 2 Capacitance Chapter 17 Capacitors in Keyboards

21. Section 2 Capacitance Chapter 17 Parallel-Plate Capacitor

22. Section 2 Capacitance Chapter 17 Energy and Capacitors • The potential energy stored in a charged capacitor depends on the charge and the potential difference between the capacitor’s two plates.

23. Section 2 Capacitance Chapter 17 Sample Problem Capacitance A capacitor, connected to a 12 V battery, holds 36 µC of charge on each plate. What is the capacitance of the capacitor? How much electrical potential energy is stored in the capacitor? Given: Q = 36 µC = 3.6  10–5 C ∆V = 12 V Unknown: C = ? PEelectric = ?

24. Section 2 Capacitance Chapter 17 Sample Problem, continued Capacitance To determine the capacitance, use the definition of capacitance.

25. Section 2 Capacitance Chapter 17 Sample Problem, continued Capacitance To determine the potential energy, use the alternative form of the equation for the potential energy of a charged capacitor:

26. Section 3 Current and Resistance Chapter 17 Current and Charge Movement • Electric currentis the rate at which electric charges pass through a given area.

27. Section 3 Current and Resistance Chapter 17 Conventional Current

28. Section 3 Current and Resistance Chapter 17 Drift Velocity • Drift velocityis the the net velocity of a charge carrier moving in an electric field. • Drift speeds are relatively small because of the many collisions that occur when an electron moves through a conductor.

29. Section 3 Current and Resistance Chapter 17 Drift Velocity

30. Section 3 Current and Resistance Chapter 17 Resistance to Current • Resistanceis the opposition presented to electric current by a material or device. • The SI units for resistance is the ohm (Ω) and is equal to one volt per ampere. • Resistance

31. Section 3 Current and Resistance Chapter 17 Resistance to Current, continued • For many materials resistance is constant over a range of potential differences. These materials obey Ohm’s Law and are calledohmic materials. • Ohm’s low does not hold for all materials. Such materials are callednon-ohmic. • Resistance depends on length, cross-sectional area, temperature, and material.

32. Section 3 Current and Resistance Chapter 17 Factors that Affect Resistance

33. Section 3 Current and Resistance Chapter 17 Resistance to Current, continued • Resistors can be used to control the amount of current in a conductor. • Salt water and perspiration lower the body's resistance. • Potentiometershave variable resistance.

34. Section 4 Electric Power Chapter 17 Sources and Types of Current • Batteries and generators supply energy to charge carriers. • Current can be direct or alternating. • In direct current, charges move in a single direction. • Inalternating current, the direction of charge movement continually alternates.

35. Section 4 Electric Power Chapter 17 Energy Transfer • Electric power is the rate of conversion of electrical energy. • Electric power P = I∆V Electric power = current  potential difference

36. Section 4 Electric Power Chapter 17 Energy Transfer

37. Section 4 Electric Power Chapter 17 Energy Transfer, continued • Power dissipated by a resistor • Electric companies measure energy consumed inkilowatt-hours. • Electrical energy is transferred at high potential differences to minimize energy loss.

38. Section 4 Electric Power Chapter 17 Relating Kilowatt-Hours to Joules