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Magazine Problem 13
\(\mathrm{A}+2.0\) -nC point charge is \(3.0 \mathrm{cm}\) away from a $-3.0
\mathrm{-nC}$ point charge. (a) What are the magnitude and direction of the
electric force acting on the +2.0 -nC charge?
(b) What are the magnitude and direction of the electric force acting on the
-3.0 -nC charge?
Problem 14
Two metal spheres separated by a distance much greater than either sphere's
radius have equal mass \(m\) and equal electric charge \(q .\) What is the ratio
of charge to mass \(q / m\) in C/kg if the electrical and gravitational forces
balance?
Problem 16
Two point charges are separated by a distance \(r\) and repel each other with a force \(F\). If their separation is reduced to 0.25 times the original value, what is the magnitude of the force of repulsion between them?
Problem 17
\(\mathrm{A} \mathrm{K}^{+}\) ion and a \(\mathrm{Cl}^{-}\) ion are directly
across from each other on opposite sides of a membrane 9.0 nm thick. What is
the electric force on the \(\mathrm{K}^{+}\) ion due to the \(\mathrm{Cl}^{-}\)
ion? Ignore the presence of other charges.
Problem 21
A total charge of \(7.50 \times 10^{-6} \mathrm{C}\) is distributed on two
different small metal spheres. When the spheres are \(6.00 \mathrm{cm}\) apart,
they each feel a repulsive force of \(20.0 \mathrm{N} .\) How much charge is on
each sphere?
Problem 22
Two Styrofoam balls with the same mass \(m=9.0 \times 10^{-8} \mathrm{kg}\) and
the same positive charge \(Q\) are suspended from the same point by insulating
threads of length \(L=0.98 \mathrm{m} .\) The separation of the balls is
\(d=0.020 \mathrm{m} .\) What is the charge \(Q ?\)
Problem 24
An equilateral triangle has a point charge \(+q\) at each of the three vertices \((A, B, C) .\) Another point charge \(Q\) is placed at \(D\), the midpoint of the side \(B C\). Solve for \(Q\) if the total electric force on the charge at \(A\) due to the charges at \(B, C,\) and \(D\) is zero.
Problem 25
A small sphere with a charge of \(-0.60 \mu \mathrm{C}\) is placed in a uniform
electric field of magnitude \(1.2 \times 10^{6} \mathrm{N} / \mathrm{C}\)
pointing to the west. What is the magnitude and direction of the force on the
sphere due to the electric field?
Problem 26
The electric field across a cellular membrane is $1.0 \times 10^{7} \mathrm{N}
/ \mathrm{C}$ directed into the cell. (a) If a pore opens, which way do sodium
ions (Na") flow- into the cell or out of the cell? (b) What is the magnitude
of the electric force on the sodium ion? The charge on the sodium ion is \(+e\)
Problem 27
What are the magnitude and direction of the acceleration of a proton at a
point where the electric field has magnitude \(33 \mathrm{kN} / \mathrm{C}\) and
is directed straight up?
