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Chapter 23: Q. 8 (page 652)
A circular disk has a surface charge density . What will the surface charge density be if the radius of the disk is doubled?
The surface charge density if the radius of the disk is doubled from .
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A problem of practical interest is to make a beam of electrons turn a corner. This can be done with the parallel-plate capacitor shown in FIGURE. An electron with kinetic energy enters through a small hole in the bottom plate of the capacitor.
a. Should the bottom plate be charged positive or negative relative to the top plate if you want the electron to turn to the right? Explain.
b. What strength electric field is needed if the electron is to emerge from an exit hole away from the entrance hole, traveling at right angles to its original direction?
Hint: The difficulty of this problem depends on how you choose your coordinate system.
c. What minimum separation must the capacitor plates have?
The two parallel plates in FIGURE Pare apart and the electric field strength between them is . An electron is launched at a angle from the positive plate. What is the maximum initial speed the electron can have without hitting the negative plate?
One type of ink-jet printer, called an electrostatic ink-jet printer, forms the letters by using deflecting electrodes to steer charged ink drops up and down vertically as the ink jet sweeps horizontally across the page. The ink jet forms diameter drops of ink, charges them by spraying electrons on the surface, and shoots them toward the page at a speed of . Along the way, the drops pass through two horizontal, parallel electrodes that are wide, and spaced apart. The distance from the center of the electrodes to the paper is . To form the tallest letters, which have a height of , the drops need to be deflected upward (or downward) by . What electric field strength is needed between the electrodes to achieve this deflection? Ink, which consists of dye particles suspended in alcohol, has a density of .
The electric field strengthfrom the surface of adiameter metal ball is. What is the charge on the ball?
An electric field can induce an electric dipole in a neutral atom or molecule by pushing the positive and negative charges in opposite directions. The dipole moment of an induced dipole is directly proportional to the electric field. That is, , where is called the polarizability of the molecule. A bigger field stretches the molecule farther and causes a larger dipole moment.
a. What are the units of ?
b. An ion with charge is distancefrom a molecule with polarizability . Find an expression for the force .
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