Physics 132: Lecture e 5 Elements of Physics II Agenda for Today Conductors and Insulators Movement of charges Conservation of charge Static electricity Electroscope Physics 201: Lecture 1, Pg 1
Problem It is difficult to describe the motion of this electron using forces p e F = ma x = x o v 0 t ½at 2 v= v 0 at 0 Must have constant acceleration, or else we have to use calculus Physics 201: Lecture 1, Pg 2
Work done by gravity Lets compute the work done by the gravitational force. final h f By moving the mg block up a distance h, it gains potential initial energy of mgh! mg h i U = -W C U = -(-mg(h f - h i ))= mg( h) Physics 201: Lecture 1, Pg 3
Equipotential lines The potential energy also increases if we move a positively charged ball against an electric ec c field. E-Field - -- - - - Physics 201: Lecture 1, Pg 4
Clicker Question 1: What happens to a negatively charged ball s potential energy if it moves down from point A to B? ()Iti (a) increases (b) It decreases (c) It stays the same - A B - - - -- - Physics 201: Lecture 1, Pg 5
Clicker Question 2: A negatively charged ball moves down from point A to B. What does the electric force do? (a) Positive work (b) Zero work (c) Negative work - A B - - - -- - Physics 201: Lecture 1, Pg 6
Electric Potential It is often convenient to work with the electrostatic potential energy per unit charge This is called the Electric Potential V PE q We will only deal with the difference in electric potential V PE q Unit is J/C or the Volt (V) Physics 201: Lecture 1, Pg 7
Clicker Question 3: The electric potential at point A is that at point B (a) greater than (b) Equal to (c) Less than Student: The electric potential at point A is greater than that at point B, no matter the sign of charge?? A B - - - -- - Physics 201: Lecture 1, Pg 8
Electric Potential Energy (U) vs. Electric Potential (V) Electric Potential Energy (U) - the energy of a charge at some location. Electric Potential (V) - found for a location only tells what the Potential Energy would be if a charge were located there (usually we talk about potential ti differences between two locations): U = qv Neither has direction, just location. Sign matters! Physics 201: Lecture 1, Pg 9
Electric Potential We generally deal with differences in electric potential Voltage = difference in electric potential Like height in U = mgh By definition Positive charge will feel a force from high to low electric potential Negative charge will feel a force from low to high electric potential If there is a potential difference a charge will feel a force. Physics 201: Lecture 1, Pg 10
QuickCheck 28.7 Clicker Question 4: If a negative charge is released from rest, it moves in the direction of A. A stronger electric field. B. A weaker electric field. C. Higher electric potential. D. Lower electric potential. E. Both A and C. Physics 201: Lecture 1, Pg 11
ΔU Electric Potential q( V) V=V = V f -V i ΔU mg( h) h = h f -h i Electric potential is like the height in mgh equation!! Physics 201: Lecture 1, Pg 12
Clicker Question 5: Two protons are launched with the same speed from point 1 inside the parallel-plate plate capacitor of the figure. Points 2 and 3 are the same distance from the negative plate. Is U 1 2, the change in potential energy along the path 1 2, larger than, smaller than, or equal to U 1 3? ()ΔU (a) 1 2 = ΔU 1 3 (b) ΔU 1 2 > ΔU 1 3 (c) ΔU 1 2 < ΔU 1 3 K f qv f K i qv i Physics 201: Lecture 1, Pg 13
Van de Graaf Positive charge is pulled off belt Belt pulls electrons off metal sphere Metal sphere becomes charged About 100, 000 volts!!! Air will ionize (electrical breakdown) at an electric field of 3 10 6 N/C Similar il to lightning i Physics 201: Lecture 1, Pg 14
Sparks - - - - G Air will ionize (electrical breakdown) at an electric field of 3 10 6 N/C Similar to lightning Spark caused by recombination o of atoms Physics 201: Lecture 1, Pg 15
Electric Potential due to a point charge Electric potential due to a point charge at a point a distance r away. k is our constant from before kq V r q is the charge of the point charge Remember potential add like a scalar!!! Physics 201: Lecture 1, Pg 16
Clicker Question 6: Four point charges are arranged at the corners of a square. Find the electric field E and the potential V at the center of the square. a) E = 0 V = 0 -Q Q b) E = 0 V 0 c) E 0 V 0 d) E 0 V = 0 e) E = V regardless of the value -Q Q Physics 201: Lecture 1, Pg 17
Clicker Question 7: An electron moves along the trajectory t of the figure from i to f. Does the electric potential energy increase, decrease, or stay the same? (a) The electric potential energy will remain the same. (b) The electric potential energy will decrease. (c) The electric potential energy will increase. Physics 201: Lecture 1, Pg 18
Work You must be careful which charge to use W e-field Charge that is moving q( V) W outside q( V) Charge that is already there causes the V Physics 201: Lecture 1, Pg 19
Clicker Question 8: A charge q 1 =3 C is brought in from a great distance away to a point.5 m from a charge q 2 =20 C. The work done by the electric field is: (a) Positive (b) Negative (c) zero q 2 =20 C r=.5m q 1 =3 C Point a Physics 201: Lecture 1, Pg 20
Example 1: How much work does the electric field do when a charge of q 1 =3 C is brought from a great distance away to a point 0.5 m from a charge q 2 =20 C? Physics 201: Lecture 1, Pg 21
Battery A battery is an example of a potential difference Positive side is 12 V higher potential than the negative side Charge wants to go from one side to another - Physics 201: Lecture 1, Pg 22