Electrostatics Electrostatics

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1 Electrostatics

2 You will get a charge out of these questions A combination of four electrons and two protons would have a net charge of... d) 3.2 E 19 Coulombs c) +3.2 E 19 Coulombs b) 2 a) A vulcanite rod is stroked with cat fur on a frosty night and then held securely. An electron is then placed 1 cm above the rod. Sketch a free body diagram of the forces acting on the electron. 3. How would your free body diagram change if the electron was replaced by a proton? Fe is opposite direction (downward) and Fg is greater b/c protons are 200 times more massive than electrons. 4. a) If a neutral solid object becomes positively charged, does its mass increase or decrease? b) How about if it becomes negatively charged? 5. a) If two objects are seen to electrically repel each other, can you be certain that they are both charged? YES b) How about if they are seen to attract? No, one or the other object may be neutral 6. List the steps involved in charging a pith ball by induction A proton and an electron are placed 1 m apart in an otherwise empty universe. a) What is the gravitational attraction between them? 1.03E 67 N At least my hair is more bitchin! b) What is the electrical attraction between them? 2.3E 28 N 1,000,000,000,000,000,000,000,000,000,000 Times Stronger Than Gravity!!!!!!

3 It's better to be pithed off than pithed on! Two identical negatively charged pith balls, each having a mass of 3E 2 kg hang in equilibrium as shown. 1. Complete a "componentized" free body diagram for the sphere on the left in the box below... θ 2. How many Coulombs of charge is on each pith ball? (show your work) I'm Pithed! 3. How many electrons would need to be removed from each ball to return them to a neutral state? (show your work)

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7 6 μc 4 μc

8 Like Gym Socks and Old Cheese these Problems have the POTENTIAL to be Rank!

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14 5 a) *Spherical Charge Distributions act as if all the charge is concentrated to a point at the center of the sphere. Like we did with universal gravity, we used the center of the Erf in our calculations. 4R 4R 4R

15 5 b) *Excess charge is free to move in a metal (good conductor) so the charges respond to the tiny hate for each other by moving to the extreme outer surface. The E fields inside the ball cancel out due to symmetry. This also explains why a car is a good place to be if lightning strikes Ball Bearing A = B = C = zero

16 5 c) 4R 4R 4R

17 5 d) 4R *Using the logic from part a) with same charge and same distance to the center of each charge. 4R A = B = C 4R

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21 1. More Potentially Shocking Questions Imagine an electron and a proton held midway between the plates of a charged parallel plate capacitor. If they are released, how do their accelerations and directions of travel compare? Ignore their attraction to each other. Which reaches a capacitor plate first? Both experience same force, but in opposite directions. Electron has greater acceleration b/c it is a LOT less massive. Electron would reach plate first b/c of greater acc.

22 2. Does an object with twice the electric potential of another have twice the electrical potential energy? Explain. No, unless both objects have the same amount of charge on them. Work done is equal to ΔPE Potential Charge

23 3. You are not harmed by contact with a charged balloon, even though its voltage is very high. Is the reason for this similar to why you are not harmed by the greater than 2000 C sparks from a Fourth of July type sparkler? Yes, voltage is like temperature, it has the "potential" to be dangerous but the amount of charge at that voltage determines the potential energy in the system, the same way the amount of matter at a given temperature determines how much heat energy there is in the system, and therefore how dangerous it will be to touch it. Another analogy with gravity would be... standing under a rocky cliff has the potential to be dangerous, but it depends on the mass of the rock that falls on you.

24 4. If you put in 10 joules of work to push 1 coulomb of charge against an electric field, what will be its voltage with respect to its starting position? When released, what will be its kinetic energy if it flies past its starting position? Volts KE = Work = 10J

25 5. a. Suppose that you start with a charge of C in an electric field and find that it takes 24 J of work to move the charge from point A to point B. What is the voltage difference between points A and B? b. If the charge is released, what is its kinetic energy as it flies back past point A? KE = Work = 24J

26 6. The potential difference between a storm cloud and the ground is volts. During a lightning flash, 3.0 coulombs of charge are transferred to the ground. Sydney Golden's Dad a. How much energy is transferred to the ground in this lightning flash? Work = ΔVq = (5E7V)(3 C) = 1.5E8J b. If this much energy were used to accelerate a 3500 kg truck from rest, how fast would the truck end up going? Wow! Nearly the speed of sound!!!

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