Physics 101 Lawrence Weinstein Homework set 9 Solutions Project: -------- 22.2: By running the comb through your hair or rubbing it on a woolen garment, you electrically charge the comb. When you bring the charged comb close to the neutral stream of water, the charged comb polarizes the water and attracts the stream. At least one student saw the comb repel the water stream VERY strongly. The only explanation I can think of for this is if the water flowed through plastic (ie: electrically insulating) pipes. In that case, the water itself could become charged. (Normally, water flows through metal pipes. Since metal conducts electricity and the pipes are in contact with the ground at some point, any charge on the water or the pipes would flow away into the ground. This leaves the pipes and water uncharged.) Exercises: ---------- 22.6: When you wipe a CD with a dry cloth, the CD and the cloth exchange electrical charges and both become charged. The charged CD now polarizes and attracts the neutral dust. 22.8: The plastic wrap will stick better to plastic or glass containers than to metal ones. Plastic wrap sticks to things because it is charged and because charged objects attract neutral ones. The reason the wrap sticks better to plastic containers (all answers need brief explanations) is that when the charged plastic wrap contacts the glass or plastic container, the charges stay on the plastic wrap (because the plastic or glass container is an insulator). When the charged plastic wrap contacts the metal container, all the charges in contact with the metal flow onto the metal container, leaving the plastic wrap much less charged. 22.10: As you drive along, friction between the air and the car charges up the car. The rubber tires are insulators so that the charge stays on the car. When you hand your money to the toll booth attendant, charge can jump from you (since you are charged along with your car) to the attendant. This can be a real shock! The thin metal wire contacts the car and grounds it (ie: the charge flows from the car into the ground) leaving the car neutral. 22.12: When the ball of an electroscope is touched by a charged object, the charges on the object repel each other and some of them flow onto the electroscope. Since the charges still repel each other, they spread out on the electroscope and the leaves of the electroscope become charged. The charges on the leaves repel each other so that the leaves spread apart. This device is identical in function to the electroscope with the needle that I placed on the overhead projector in class. The needle deflected when I placed a charged object in contact with it or close to it. 22.14: No, you do not need to touch the charged object to the electroscope for the leaves to spread apart. If you hold a negatively charged object near the ball of the electroscope, then the electroscope will polarize. The ball will become positively charged and the leaves will become negatively charged. The leaves will then repel each other and spread apart. 22.24: When you move two charged particles half as far apart, then the distance between them is halved. Therefore, the force will quadruple since F = k(q1)(q2)/d^2. When d is half as much, 1/d^2 is four times larger. Similarly, when the distance becomes 1/3 as much, then the force will be 9 times larger. 22.32: The electric field at a distance d from a charged particle is E = kq/d^2. When d doubles from 1 m to 2 m, 1/d^2 gets 4 times smaller so that the electric field gets four times smaller. This is guided by Coulomb's Law. 22.50: The charge concentration on the corners of a charged metal file cabinet will be much larger than that on the flat parts. See figure 22.21. 22.54: It is safe to be inside a car during a lightning storm because electrical charges stay on the outside of conductors. The car's metal body is a conductor. The electric charge from the lightning stays on the outside of the body (and you stay on the inside). You can also say that the electric field inside the conductor is zero. Please note that the car's tires are irrelevant. The lightning has already crossed thousands of feet of air from the clouds to the car; the last few inches of air between the car's body and the ground are insignificant. Problems: --------- 22.2: We know that F = k(q1)(q2)/d^2 and we are told that F = 20 N, d = 6 cm and q1 = q2. We also know that k = 9*10^9 N*m^2/C^2. Since k uses units of m (meters), we need to convert d from cm to m. 100 cm = 1 m so that d = 6 cm * (1 m / 100 cm) = 0.06 m Now we can write that (replacing q1 and q2 with q): F = k q^2 / d^2 or that q^2 = d^2 F / k so that q = sqrt(d^2 F / k) = sqrt((0.06 m)^2 * 20 N / (9*10^9 N*m^2/C^2)) = 2.8*10^(-6) C Estimation: ----------- There was no estimation this week.