UNIT 9 Electrostatics and Currents

UNIT 9Electrostatics and Currents

Monday March 19th Electrostatics and Currents

Monday, March 19 TODAY’S AGENDA • Electric Potential Energy • Hw: Practice A (all) p599 UPCOMING… • Tues: Problem Quiz #2 (Electric Fields) • Wed: Electric Potential

Chapter 16-17 Electrostatics and Current

ConcepTest 22.1Electric Field (1)4E0 (2) 2E0 (3) E0 (4) 1/2E0 (5) 1/4E0 You are sitting a certain distance from a point charge, and you measure an electric field of E0. If the charge is doubled and your distance from the charge is also doubled, what is the electric field strength now?

ConcepTest 22.1Electric Field (1)4E0 (2) 2E0 (3) E0 (4) 1/2E0 (5) 1/4E0 You are sitting a certain distance from a point charge, and you measure an electric field of E0. If the charge is doubled and your distance from the charge is also doubled, what is the electric field strength now? Remember that the electric field is: E = kQ/r2. Doubling the charge puts a factor of 2 in the numerator, but doubling the distance puts a factor of 4 in the denominator, because it is distance squared!! Overall, that gives us a factor of 1/2. Follow-up: If your distance is doubled, what must you do to the charge to maintain the same E field at your new position?

d d +1 +1 +2 +2 +1 +1 ConcepTest 22.2aField and Force I 1) 2) 3) the same for both Between the red and the blue charge, which of them experiences the greater electric field due to the green charge?

d d +1 +1 +2 +2 +1 +1 ConcepTest 22.2aField and Force I 1) 2) 3) the same for both Between the red and the blue charge, which of them experiences the greater electric field due to the green charge? Both charges feel the same electric field due to the green charge because they are at the same point in space!

Electric Potential Energy • It is possible to define an electrical potential energy function with the electrostatic force • Work done by this force is equal to the negative of the change in potential energy • W = Fd = -q Ex (xf – xi) = -ΔPE

Work and Potential Energy • There is a uniform field between the two plates • As the charge moves from A to B, work is done on it • W = Fd=q Ex (xf – xi) • ΔPE = - W = - q Exx • Only for a uniform field

Potential Difference • The potential difference between points A and B is defined as the change in the potential energy (final value minus initial value) of a charge q moved from A to B divided by the size of the charge ΔV = VB – VA = ΔPE / q • Potential difference is not the same as potential energy

Potential Difference, cont. • Another way to relate the energy and the potential difference: ΔPE = q ΔV • Both electric potential energy and potential difference are scalar quantities • Units of potential difference V = J/C • A special case occurs when there is a uniform electric field DV = VB – VA= -ExDx • Gives more information about units: N/C = V/m

Potential Energy Compared to Potential • Electric potential is characteristic of the field only • Independent of any test charge that may be placed in the field • Electric potential energy is characteristic of the charge-field system • Due to an interaction between the field and the charge placed in the field

Energy and Charge Movements • A positive charge gains electrical potential energy when it is moved in a direction opposite the electric field • If a charge is released in the electric field, it experiences a force and accelerates, gaining kinetic energy • As it gains kinetic energy, it loses an equal amount of electrical potential energy • A negative charge loses electrical potential energy when it moves in the direction opposite the electric field

Energy and Charge Movements, cont • When the electric field is directed downward, point B is at a lower potential than point A • A positive test charge that moves from A to B loses electric potential energy • It will gain the same amount of kinetic energy as it loses in potential energy

Summary of Positive Charge Movements and Energy • When a positive charge is placed in an electric field • It moves in the direction of the field • It moves from a point of higher potential to a point of lower potential • Its electrical potential energy decreases • Its kinetic energy increases

Summary of Negative Charge Movements and Energy • When a negative charge is placed in an electric field • It moves opposite to the direction of the field • It moves from a point of lower potential to a point of higher potential • Its electrical potential energy decreases • Its kinetic energy increases • Work has to be done on the charge for it to move from point A to point B

Electric Potential of a Point Charge • The point of zero electric potential is taken to be at an infinite distance from the charge • The potential created by a point charge q at any distance r from the charge is • A potential exists at some point in space whether or not there is a test charge at that point

Electric Field and Electric Potential Depend on Distance • The electric field is proportional to 1/r2 • The electric potential is proportional to 1/r

END

UNIT 9 Electrostatics and Currents