Chapter 18: Electrical Potential Energy Potential & Voltage Capacitance

Potential Energy

Electrical Potential Energy potential energy associated with the force between charged particles similar to gravitational potential energy form of mechanical energy

Electrical Potential Energy Anytime a charge is moved because of an electric force, WORK is done on that charge. Remember that work is a measurement of the ENERGY it takes to do something.

Potential Energy in a Uniform Electric Field (not a point charge) as q moves away: potential energy decreases if the charge is positive increases if the charge is negative displacement in direction of field (not ) keep the sign on q SI unit is J

Electrical Potential for a Pair of Charges (a non-uniform field) don’t drop the sign on q! positive for like charges must do work to push them together negative for unlike charges

Practice Problem: An electron and a proton are displaced so that the electrical potential energy as a result from the movement is 2.110-28 J. How far apart are they?

Practice Problems: Complete problems 18A 1-4 and Section Review 1-6

Potential & Voltage

Electric Potential (also called Potential) Electric potential is the potential energy per unit charge. symbol is V SI unit is Volt scalar add potentials together include signs for absolute potential

Electric Potential (also called Potential) Just like GPE, only differences in potential can be measured.

Electric Potential (also called Potential) In a uniform electric field i.e. a parallel plate capacitor Absolute potential

Potential Difference (also called Voltage) Two points are said to differ in electric potential if work is done to move a charge from one point to another point in an electric field.

Potential Difference (also called Voltage) A difference in potential will cause our electrons to flow. As with GPE, charges seek to have the lowest potential. In other words, they will go from areas of high potential to areas of low potential. Potential difference (voltage) is the “pull” that makes electrons flow in a circuit.

Potential Difference (also called Voltage) A good source of potential that can run circuits is a battery. Most car batteries have a potential difference of 12V between terminals. Another example is an electrical outlet with a potential difference of 120 V.

Practice Problems: The electric field intensity between two parallel, charge plates is 8000 N/C. The plates are 0.05m apart. What potential difference exists between the plates? Ans: 400V

Practice Problems: A spark will jump across dry air when the electric field is larger than 1106 N/C. If two parallel plates have a potential difference of 5000 V, how far apart must they be to prevent a spark from jumping across them? Ans: 0.005m

Practice Problems: What work is done on a 5C charge when its electric potential is increased by 1.5V? Ans: 7.5J

Practice Problems: A charge of 50C is raised in potential by 110V. What work is done in raising the potential of the charge? Ans: 5500J

Capacitance

Capacitance symbol is C SI unit is F (Farad) varies with size/shape microfarad 10-6 nanofarad 10-9 picofarad 10-12 varies with size/shape Q is charge (C) C is capacitance (F) V is the potential difference (V)

Capacitance Energy is needed to charge a capacitor. Energy is released when a capacitor is discharged. hooked up to a battery to build charge discharged through a conductor (resistor) Found in: radio receivers camera flashes automobile ignition systems (eliminates sparking)

Parallel-Plate Capacitor two equal and oppositely charged metal plates separated by a small distance connected to battery terminals a potential difference exists between the plates stores charge in the electric field between them material between plates changes capacitance a dielectric (i.e. rubber, glass, waxed paper) can be used as an insulator between plates

Parallel-Plate Capacitor o = 8.8510-12 permittivity of free space A = Area of one plate d = distance between plates

Practice Problems: What is the charge on a 6mF capacitor with a potential difference of 0.60V? Ans: 3.6mC

Practice Problems: A capacitor has a charge of 3mC and a potential difference of 45 V. What is its capacitance? Ans: .067mF

Practice Problems: A 0.002F capacitor has a 6V potential difference across it. How much energy does it store? Ans: 0.036J

Practice Problems: 30J of work is done to move a +5C charge from one point to another in an electric field. What is the difference in potential between the two points?

Practice Problems: How much work is done to move an electron across a potential of 3V?

Practice Problems: Two parallel plates are separated by 0.5m. An electric field of 6000 N/C exists between the plates. What is the potential difference between the plates?

Practice Problems: A potential difference of 60V exists between two parallel plates separated by 3cm. What electric field exists between the plates?

Practice Problems: An oil drop carrying a charge of -810-19 C is suspended between two horizontal plates separated by 8mm. A 1200V potential difference exists between the plates. What is the weight of the suspended drop?