Concept explainers
In the Millikan oil-drop experiment illustrated in Figure 15.21, an atomizer (a sprayer with a fine nozzle) is used to introduce many tiny droplets of oil between two oppositely charged parallel metal plates. Some of the droplets pick up one or more excess electrons. The charge on the plates is adjusted so that the electric force on the excess electrons exactly balances the weight of the droplet. The idea is to look for a droplet dial has the smallest electric force and assume it has only one excess electron. This strategy lets the observer measure the charge on the electron. Suppose we are using an electric field of 3 × 104 N/C. The charge on one electron is about 1.6 × 10−19 C. Estimate the radius of an oil drop of density 858 kg/m5 for which its weight could be balanced by the electric force of this field on one electron. (Problem 42 is courtesy of E.F. Redish. For more problems of this type, visit www.physics.umd.cdu/pcrg/.)
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Chapter 15 Solutions
College Physics
- A proton is located at the origin, and a second proton is located on the x-axis at x = 6.00 fm (1 fm = 10-15 m). (a) Calculate the electric potential energy associated with this configuration. (b) An alpha particle (charge = 2e, mass = 6.64 1027 kg) is now placed at (x, y) = (3.00, 3.00) fm. Calculate the electric potential energy associated with this configuration. (c) Starting with the three-particle system, find the change in electric potential energy if the alpha particle is allowed to escape to infinity while the two protons remain fixed in place. (Throughout, neglect any radiation effects.) (d) Use conservation of energy to calculate the speed of the alpha particle at infinity. (e) If the two protons are released from rest and the alpha panicle remains fixed, calculate the speed of the protons at infinity.arrow_forwardA proton is fired from very far away directly at a fixed particle with charge q = 1.28 1018 C. If the initial speed of the proton is 2.4 105 m/s, what is its distance of closest approach to the fixed particle? The mass of a proton is 1.67 1027 kg.arrow_forwardTwo metal spheres of identical mass m = 4.00 g are suspended by light strings 0.500 m in length. The left-hand sphere carries a charge of 0.800 C, and the right-hand sphere carries a charge of 1.50 C. What is the equilibrium separation between the centers of the two spheres?arrow_forward
- A Van de Graaff generator is charged so that a proton at its surface accelerates radially outward at 1.52 1012 m/s3. Find (a) the magnitude of the electric force on the proton at that instant and (b) the magnitude and direction of the electric field at the surface of the generator.arrow_forwardLightning can be studied with a Van de Graaff generator, which consists of a spherical dome on which charge is continuously deposited by a moving belt. Charge can be added until the electric field at the surface of the dome becomes equal to the dielectric strength of air. Any more charge leaks off in sparks as shown in Figure P25.52. Assume the dome has a diameter of 30.0 cm and is surrounded by dry air with a "breakdown" electric field of 3.00 106 V/m. (a) What is the maximum potential of the dome? (b) What is the maximum charge on the dome?arrow_forwardA proton and an alpha particle (charge = 2e, mass = 6.64 1027 kg) are initially at rest, separated by 4.00 1015 m. (a) If they are both released simultaneously, explain why you cant find their velocities at infinity using only conservation of energy. (b) What other conservation law can be applied in this case? (c) Find the speeds of the proton and alpha particle, respectively, at infinity.arrow_forward
- a. In Figure 23.8, why are there three plus signs on the red rod and three minus signs on the red cloth? b. Which object in Figure 23.8 has the greatest positive charge? How do you know?arrow_forwardA common demonstration involves charging a rubber balloon, which is an insulator, by rubbing it on your hair and then touching the balloon to a ceiling or wall, which is also an insulator. Because of the electrical attraction between the charged balloon and the neutral wall, the balloon sticks to the wall. Imagine now that we have two infinitely large, flat sheets of insulating material. One is charged, and the other is neutral. If these sheets are brought into contact, does an attractive force exist between them as there was for the balloon and the wall?arrow_forwardA very small ball has a mass of 5.00 103 kg and a charge of 4.00 C. What magnitude electric field directed upward will balance the weight of the ball so that the ball is suspended motionless above the ground? (a) 8.21 102 N/C (b) 1.22 104 N/C (c) 2.00 102 N/C (d) 5.11 106 N/C (e) 3.72 103 N/Carrow_forward
- A spherical balloon contains a positively charged particle at its center. As the balloon is inflated to a larger volume while the charged particle remains at the center, which of the following are true? (a) The electric potential at the surface of the balloon increases. (b) The magnitude of the electric field at the surface of the balloon increases. (c) The electric flux through the balloon remains the same. (d) None of these.arrow_forwardA Van de Graaff generator is charged so that a proton at its surface accelerates radially outward at 1.52 1012 m/s3. Find (a) the magnitude of the electric force on the proton at that instant and (b) the magnitude and direction of the electric field at the surface of the generator.arrow_forwardAn infinite line of positive charge lies along the y axis, with charge density = 2.00 C/m. A dipole is placed with its center along the x axis at x = 25.0 cm. The dipole consists of two charges 10.0 C separated by 2.00 cm. The axis of the dipole makes an angle of 35.0 with the x axis, and the positive charge is farther from the line of charge than the negative charge. Find the net force exerted on the dipole.arrow_forward
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