The inner and outer surfaces of a cell membrane carry a negative and positive charge, respectively. Bccause of these chargex a potential difference of about0.078 Vexists across the membrane. The thickness of the membrane is7.8×107 m. What is the magnitude of the electric field in the membrane?

Each of the following statements is related to conductors in electrostatic equilibrium. Choose the words that make each statement correct. (i) The net charge is always zero [(a) inside; (b) on] the surface of an isolated conductor. (ii) The electric field is always zero [(c) inside; (d) just outside] a perfect conductor. (iii) The charge density on the surface of an isolated, charged conductor is highest where the surface is [(e) sharpest; (f) smoothest].

Problems 72, 73, and 74 are grouped. 72. A Figure P26.72 shows a source consisting of two identical parallel disks of radius R. The x axis runs through the center of each disk. Each disk carries an excess charge uniformly distributed on its surface. The disk on the left has a total positive charge Q, and the disk on the right has a total negative charge Q. The distance between the disks is 3R, and point A is 2R from the positively charged disk. Find an expression for the electric potential at point A between the disks on the x axis. Approximate any square roots to three significant figures. FIGURE P26.72 Problems 72, 73, and 74.

(a) What magnitude point charge creates a 10,000 N/C electric field at a distance of 0.250 m? (b) How large is the field at 10.0 m?

A thin conducing plate 2.0 m on a side is given a total charge of 10.0C . (a) What is the electric field 1.0 cm above the plate? (b) What is the force on an electron at this point? (c) Repeat these calculations for a point 2.0 cm above the plate. (d) When the electron moves from 1.0 to 2.0 cm above the plate, how much work is done on it by the electric field?

(a) Would life be different if the electron were positively charged and the proton were negatively charged? (b) Does the choice of signs have any bearing on physical and chemical interactions? Explain your answers.

(a) Calculate the number of electrons in a small, electrically neutral silver pin that has a mass of 10.0 g. Silver has 47 electrons per atom, and its molar mass is 107.87 g/mol. (b) Imagine adding electrons to the pin until the negative charge has the very large value 1.00 mC. How many electrons are added for every 109 electrons already present?

A thin, square, conducting plate 50.0 cm on a side lies in the xy plane. A total charge of 4.00 108 C is placed on the plate. Find (a) the charge density on each face of the plate, (b) the electric field just above the plate, and (c) the electric field just below the plate. You may assume the charge density is uniform.

(a) What is the direction of the total Coulomb force on q in Figure 18.46 if q is negative, qa= qcand both are negative, and qb= qcand both are positive? (b) What is the direction of the electric field at the center of the square in this situation?

(a) What is the final speed of an electron accelerated from rest through a voltage of 25.0 MV by a negatively charged Van de Graff terminal? (b) What is unreasonable about this result? (c) Which assumptions are responsible?