# Chapter 18 Electric Forces and Electric Fields. Key Concepts:

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1 Chapter 18 Lectures Monday, January 25, :33 AM Chapter 18 Electric Forces and Electric Fields Key Concepts: electric charge principle of conservation of charge charge polarization, both permanent and induced good electrical conductors vs. good electrical insulators Coulomb's law for the force exerted by one charged particle on another the electric field concept; representation of an electric field using field lines or field vectors electric field of a point charge (formula and field pattern, both for a positive point charge and a negative point charge) field pattern for a constant (i.e., uniform) electric field field pattern for an electric dipole properties of electric field lines in the vicinity of conductors (20.6) Ch18L Page 1

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18 Principle of Superposition The word "linear" is used to mean several different things in mathematics and physics. The classical theory of electricity and magnetism is a linear theory (at least in vacuum; in some materials it's a more complex story). This means that the fundamental differential equations of the theory are linear. This also means that a superposition principle is operative for electric fields. That is, suppose you wish to calculate the net electric field due to several point charges. The principle of superposition states that it's possible to calculate the electric field due to each point charge separately, and then just add the resulting fields (vector sum) to determine the net electric field. This is about as simple as it could possibly be, and makes our lives a lot easier. This is one reason why Einstein's theory of gravity is so hard to work with; it's a nonlinear theory, because its basic differential equations are nonlinear. Thus, if you use Einstein's theory of gravity to calculate the total gravitational field due to two massive objects, you can't just find the field of each object separately and then form the vector sum to determine the net field. You have to treat each configuration of masses as a new problem that you have to solve "from scratch." Ch18L Page 18

19 Newton's theory of gravity is a linear theory, so the superposition principle is valid for it. One way of looking at Newton's theory of gravity is that it's a linear approximation to Einstein's (nonlinear) theory of gravity. This perspective might appeal to all you calculus lovers out there. The following example illustrates the superposition principle in action. Ch18L Page 19

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28 OMIT Section 18.9 Ch18L Page 28

29 Example An electric field of magnitude 100,000 N/C directed towards the right causes the 5.0 g ball in the figure to hang at a 20 degree angle. Determine the charge on the ball. Solution: Draw a free-body diagram: Ch18L Page 29

30 The net force on the ball is zero, as the ball is permanently at rest. Write Newton's second law of motion for each coordinate direction in the free-body diagram: We have two independent equations for two unknowns; eliminate T and solve for q: Problem How many of the Earth's electrons would have to be transferred to the Sun so that the resulting electrostatic force of attraction between the Earth and Sun is equal to the gravitational force acting between Ch18L Page 30

31 Sun is equal to the gravitational force acting between Earth and Sun? What percentage of Earth's electrons does this represent? Try this one on your own. You'll have to look up certain information; what specifically do you need? My calculation yields a result of The mass of the Earth is about g, and Avogadro's number is about 10 23, so this means that there are about atoms (very approximately), and so there are at least that many electrons in the Earth. You can see that the fraction of the Earth's electrons that would have to be transferred is extremely small as a percentage of the total number of electrons in the Earth. You can do the calculation precisely; I'd be interested to see your solution. Selected problems and solutions: Question: What is alike when we say "two like charges?" Do they look, feel, or smell alike? Question: Plastic and glass rods that have been charged by rubbing with wool and silk, respectively, hang by threads. (a) An object repels the plastic rod. Can you predict what it will do to the glass rod? Explain. (b) Ch18L Page 31

32 predict what it will do to the glass rod? Explain. (b) Repeat part (a) for an object that attracts the plastic rod. Ch18L Page 32

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34 Question: A lightweight metal ball hangs by a thread. When a charged rod is brought near, the ball moves towards the rod, touches the rod, then quickly "flies away" from the rod. Explain. Question: Metal sphere A has 4 units of negative charge, and metal sphere B has 2 units of positive charge. The two spheres are brought into contact. What is the final charge state of each sphere? Ch18L Page 34

35 What is a coulomb? Problem: A plastic rod has been charged to 20 nc by rubbing. (a) Have electrons been added or protons been removed? Explain. (b) How many electrons have been added or protons removed? Problem: A plastic rod that has been charged to 15.0 nc touches a metal sphere. Afterward, the charge on the rod is 10.0 nc. Ch18L Page 35

36 is 10.0 nc. (a) What kind of charged particle was transferred between the rod and the sphere, and in which direction? (b) How many charged particles were transferred? Problem: Two identical metal spheres A and B are connected by a metal rod. Both are initially neutral. Then electrons are added to sphere A, then the connecting rod is removed. Afterward, what is the charge on each sphere? Problem: A metal rod A and a metal sphere B, on insulating stands, touch each other. They are originally neutral. A positively charged rod is brought near (but not Ch18L Page 36

37 Problem: A metal rod A and a metal sphere B, on insulating stands, touch each other. They are originally neutral. A positively charged rod is brought near (but not touching) the far end of A. While the charged rod is still close, A and B are separated. The charged rod is then withdrawn. Is the sphere then positively charged, negatively charged, or neutral? Explain. Question: Rank in order, from largest to smallest, noting any ties, the electric field strengths E1 to E4 at points 1 to 4 in the figure. Ch18L Page 37

38 Problem: Two small plastic spheres each have a mass of 2.0 g and a charge of 50.0 nc. They are placed 2.0 cm apart. Ch18L Page 38

39 and a charge of 50.0 nc. They are placed 2.0 cm apart. (a) What is the magnitude of the electric force between the two spheres? (b) By what factor is the electric force on a sphere larger than its weight? Problem: Determine the magnitude and direction of the force on charge A in the figure. Ch18L Page 39

40 Problem: What magnitude charge creates a 1.0 N/C electric field at a point 1.0 m away? Ch18L Page 40

41 Problem: A 30 nc charged particle and a 50 nc charged particle are near each other. There are no other charges nearby. The electric force on the 30 nc particle is N. The 50 nc particle is then moved very far away. Afterward, what is the magnitude of the electric field at its original position? Problem: A +10 nc charge is located at the origin. (a) What are the strengths of the electric field vectors at the positions (x, y) = Ch18L Page 41

42 the strengths of the electric field vectors at the positions (x, y) = (5.0 cm, 0.0 cm), ( 5.0 cm, 5.0 cm), and ( 5.0 cm, 5.0 cm)? (b) Draw a diagram showing the electric field vectors at these points. Solution: Problem: What is the strength of an electric field that will balance the weight of a 1.0 g plastic sphere that has been charged to 3.0 nc? Solution: Ch18L Page 42

43 Problem: A parallel-plate capacitor is formed from two 4.0 cm 4.0 cm electrodes spaced 2.0 mm apart. The electric field strength inside the capacitor is N/C. What is the charge in nc on each electrode? Solution: Problem: A 2.0-mm-diameter copper ball is charged to +50 nc. What fraction of its electrons have been removed? Copper has density 8900 kg/m 3, atomic mass 63.5 g/mole, and atomic number 29. Solution: Ch18L Page 43

44 Problem: Two protons are 2.0 fm apart. Determine the magnitude of the (a) electric force and (b) gravitational force of one proton on the other. (c) Calculate the ratio of the electric force to the gravitational force and comment. Solution: Ch18L Page 44

45 Problem: An electric dipole is formed from 1.0 nc point charges spaced 2.0 mm apart. The dipole is centred at the origin, oriented along the y-axis. Determine the electric field strengths at the points (a) (x, y) = (10 mm, 0 mm) and (b) (x, y) = (0 mm, 10 mm). Solution: Ch18L Page 45

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47 Problem: The figure shows four charges at the corners of a square of side L. Assume that q and Q are positive. (a) Draw a diagram showing the three forces on charge q due to the other charges. (b) Obtain an expression for the magnitude of the net force on q. Solution: Ch18L Page 47

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49 Problem: Two 2.0-cm-diameter disks face each other, 1.0 mm apart. They are charged to ±10 nc. (a) Determine the electric field strength between the disks. (b) A proton is shot from the negative disk towards the positive disk. What launch speed must the proton have to just barely reach the positive disk? Solution:. Ch18L Page 49

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51 Problem: A small bead with a positive charge q is free to slide on a horizontal wire of length 4.0 cm. At the left end of the wire is a fixed charge q, and at the right end is a fixed charge 4q. How far from the left end of the wire does the bead come to rest? Solution: Ch18L Page 51

52 Problem: An electric field E = (200,000 N/C, right) causes the 2.0 g ball in the figure to hang at an angle. Calculate the angle θ. Solution: Ch18L Page 52

53 Problem 49: An electron is released from rest at the negative plate of a parallel plate capacitor. The magnitude of the charge per unit area on each plate is C/m 2, and the plates are separated by a distance of m. How fast is the electron moving just before it reaches the positive plate? Solution: Assume that the electric field inside the capacitor is uniform (standard assumption for a parallel-plate capacitor). The force exerted by the capacitor's electric field on the electric field is therefore constant, and the electron moves in a straight line across the capacitor. Ch18L Page 53

54 The magnitude of the capacitor's uniform electric field is: By Newton's second law, the constant acceleration of the electron is: Using the following kinematics equation, we can determine the speed of the electron just before it hits the positive plate. Ch18L Page 54

55 Problem 50: Two particles are in a uniform electric field whose magnitude is 2500 N/C. The mass and charge of particle 1 are m 1 = kg and q 1 = C, whereas the corresponding values for particle 2 are m 2 = kg and q 2 = C. Initially the particles are at rest. The particles are both located on the same electric field line but are separated from each other by a distance d. When released, they accelerate, but always remain at this same distance from each other. Find d. Solution: Draw a free-body diagram for each particle. Then use the condition that the acceleration of each particle is the same, in conjunction with Newton's second law. You will be left with one equation with only one unknown, d, which can be solved for d. Ch18L Page 55

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57 (That's a very big capacitor!) Do you understand why the values for the electric charges of the two particles were inserted into the equation as positive numbers, even though one of the charges was negative? Remember that when we draw free-body diagrams, we put the directions of the forces in "by hand" and then input the values into the resulting equations as magnitudes only. Problem 52: The drawing shows an electron entering the lower left side of a parallel plate capacitor and exiting at the upper right side. The initial speed of the electron is m/s. The capacitor is 2.00 cm long, and its plates are separated by cm. Assume that the electric field between the plates is uniform everywhere and find its magnitude. Ch18L Page 57

58 Solution: Doesn't this problem remind you of projectile motion? Yes! It's the same idea. The path of the electron, which is a curve, might be intimidating at first, but when you realize that you can treat the components of the motion of the electron separately, it is less intimidating. Set up your coordinate system so that the positive x-axis is to the right and the positive y-axis is upward. Then the force in the x-direction is zero, which makes that component easy. The force in the y-direction is constant, so that component is also pretty easy. Here's my strategy for solving the problem: Use the dimensions given and kinematics to determine the acceleration of the electron. Then use Newton's second law to determine the force acting on the electron, which can then be connected to the electric field. Ch18L Page 58

59 The magnitude of the electric field is 2100 N/C. The direction of the electric field is downward (i.e., in the negative y-direction). Ch18L Page 59

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