Make sure that we understand the difference between
Make sure that we understand the difference between Potential and Electric Potential Energy: E V (in Volts) = Potential a property of a certain position in an Electric Field with or without charges placed there EPE (in Joules) = Electric Potential Energy a property of charges placed at a certain position in an external Electric Field + - E
If a charged particle gains kinetic energy in an electric field, it loses an equivalent amount of potential energy. Point B is at a lower electric potential than is point A. Points B and C are at the same electric potential. The gain in kinetic energy depends only on the potential difference and not the path taken.
An electron acquires 7. 45 x 10 -17 J of kinetic energy when it is accelerated by an electric field from plate A to plate B. What is the potential difference and which plate is at the higher potential? A B E Plate B has a positive charge and is at a higher potential.
q x x =1. 00 cm q = 1. 60 x 10 -19 C E = 2000 N/C m = 9. 1 x 10 -31 kg m E b) Find the potential through which the charge moves. [2] a) Find the speed of the charge at the lower plate.
q x x =1. 00 cm q = 1. 60 x 10 -19 C E = 2000 N/C m = 9. 1 x 10 -31 kg m E d) Find the acceleration of the charge. c) Find the force on the charge as it moves. or
q Notes: r What is the Potential at this point? 1) Include the sign of q in your calculation! (+ or -) 2) Potential Difference can also be calculated: V = V 2 – V 1 3) The equation can also be used for a charged sphere: ++ + Total charge + + r + + Distance from center +++ 4) Electric Potential is a scalar not a vector V = V 1 + V 2 + V 3 + … (an algebraic sum, not a vector sum)
For a system of point charges Qi at distances ri from a point P: Q 1 r 1 P r 2 Q 2 r 4 Q 4 r 3 Q 3 … an algebraic sum of scalars!
Electric potential at a distance r from a positive charge Q Electric potential at a distance r from a negative charge Q
[4]What is the electric potential at a distance of 2. 5 x 10 -15 m away from a proton? +q d P d +q [5] Find the potential V at point P due to the four charges. d -q
[6] The +4 μC charge is located at 4 m on the x-axis and the -6 μC is located at +2 m on the yaxis as shown below. a) Calculate the magnitude and determine the direction of the electric fields at the origin due to the +4 μC charge and due to the -6 μC charge. b) Calculate the electric potential at the origin. c) Calculate the work done to bring a +4 μC from infinity to the origin. Ans: 13700 N/C, 100 o from +ve x axis; -18, 000 V; -0. 072 J
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