PHY 114 Lecture 4: Lecture 4

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16 Feb 2018

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Ue depends only on the location, not upon the path taken to get there (conservative force) For two point particles take ue = 0 at r = infinity. Electric potential energy (ue) is energy stored in the electric field. For the electric force ue = (cid:3038) (cid:3044)(cid:3117) (cid:3044)(cid:3118) (cid:3045) = (cid:3436)(cid:3038) (cid:3032)(cid:3118)(cid:3045) (cid:3440) A proton and an electron, initially separated by a distance r, are brought closer together. For these two charges, ue = (cid:3436)(cid:3038) (cid:3032)(cid:3118)(cid:3045) (cid:3440) This is like a mass falling near the surface of the earth; positive work is done by the field (work = - u) When q1 and q2 have the same algebraic sign, then ue > 0. This means that work must be done by an external agent to bring the charges closer together (work = - u) (cid:3045) = +(cid:3436)(cid:3038) (cid:3032)(cid:3118)(cid:3045) (cid:3440) Ionizing the atom will strip that atom of its electron. The charge q acts as a source of electric potential.

Related Questions

1.Two uncharged, conducting spheres, A and B, are held at rest oninsulating stands and are in contact. A positively charged rod isbrought near sphere A. While the rod is in place, the two spheresare separated. How will the spheres be charged, if at all, afterthey are separated?
Sphere A Sphere B
(a) positive positive
(b) positive negative
(c) negative positive
(d) negative negative
(e) zero zero

2. A charged conductor is brought near an uncharged insulator.Which one of the following statements is true?
(a) Both objects will repel each other.
(b) Both objects will attract each other.
(c) Neither object exerts an electrical force on the other.
(d) The objects will repel each other only if the conductor has anegative charge.
(e) The objects will attract each other only if the conductor has apositive charge.

3. An electron is moved from point A, where the electric potentialis VA = â240 V, to point B, where the electric potential is VB =â360 V. What is the change in the electric
potential energy?
(a) 1.92Ã10â17 J (b) 2.04Ã10â17 J (c) 5.75Ã10â17 J (d) 8.98Ã10â17 J(e) 1.05Ã10â16 J

4. A -2q charge is located a distance d to the right of a +4qcharge, along the x
axis. At which point (or points) along the axis does the electricfield go to zero? To
solve this problem, consider an observer at point P, which is anarbitrary distance x from the +4q charge. Find the value (orvalues) for x (in terms of the separation distance d) where thetotal electric field vanishes.
(a) The electric field is never zero in the vicinity of thesecharges.
(b) The electric field is zero at x = -0.328d.
(c) The electric field is zero at x = +4.22d.
(d) The electric field is zero at both x = -2.26d and x =+2.26d.
(e) The electric field is zero at x = +3.41d.

5. point A is a distance L away from a point charge Q. Point B is adistance 4L away from Q. What is the ratio of the electrostaticpotential at B to that at A, i.e., what is VB/VA?
(a) 1/16
(b) 1/9
(c) 1/4
(d) 1/3
(e) can not be determined without specifying Q and L

6. Which one of the following statements best explains why it ispossible to define an electric potential
in a region of space that contains an electric field?
(a) Work must be done to bring two positive charges closertogether.
(b) Like charges repel one another and unlike charges attract oneanother.
(c) A positive charge will gain kinetic energy as it approaches anegative charge.
(d) The work required to bring two charges together is independentof the path taken.
(e) A negative charge will gain kinetic energy as it moves awayfrom another negative charge.

PHY 114 Lecture 4: Lecture 4

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