### Extended Response

## 18.1 Electrical Charges, Conservation of Charge, and Transfer of Charge

True or false—Conservation of charge is like balancing a budget.

- true
- false

True or false—Although wood is an insulator, lightning can travel through a tree to reach Earth.

- true
- false

True or false—An eccentric inventor attempts to levitate by first placing a large negative charge on himself and then putting a large positive charge on the ceiling of his workshop. Instead, while he attempts to place a large negative charge on himself, his clothes fly off.

- true
- false

## 18.2 Coulomb's law

Electrostatic forces are enormous compared to gravitational force. Why do you not notice electrostatic forces in everyday life, whereas you do notice the force due to gravity?

- Because there are two types of charge, but only one type of mass exists.
- Because there is only one type of charge, but two types of mass exist.
- Because opposite charges cancel each other, while gravity does not cancel out.
- Because opposite charges do not cancel each other, while gravity cancels out.

A small metal sphere with a net charge of 3.0 nC is touched to a second small metal sphere that is initially neutral. The spheres are then placed 20 cm apart. What is the force between the spheres?

- 1.02 × 10
^{−7}N - 2.55 × 10
^{−7}N - 5.1 × 10
^{−7}N - 20.4 × 10
^{−7}N

## 18.3 Electric Field

Point charges are located at each corner of a square with sides of 5.0 cm . The top-left charge is *q*_{1} = 8.0 nC The top right charge is *q*_{2} = 4.0 nC. The bottom-right charge is *q*_{3} = 4.0 nC. The bottom-left charge is *q*_{4} = 8.0 nC. What is the electric field at the point midway between charges *q*_{2} and *q*_{3}?

- $(\u20132.1\phantom{\rule{0.25em}{0ex}}\text{\xd7}\phantom{\rule{0.25em}{0ex}}{10}^{4}\text{N/C})\hat{x}$
- $(2.3\phantom{\rule{0.25em}{0ex}}\text{\xd7}\phantom{\rule{0.25em}{0ex}}{10}^{4}\text{N/C})\hat{x}$
- $(4.1\phantom{\rule{0.25em}{0ex}}\text{\xd7}\phantom{\rule{0.25em}{0ex}}{10}^{4}\text{N/C})\hat{x}$
- $(4.6\phantom{\rule{0.25em}{0ex}}\text{\xd7}\phantom{\rule{0.25em}{0ex}}{10}^{4}\text{N/C})\hat{x}$

A long straight wire carries a uniform positive charge distribution. Draw the electric field lines in a plane containing the wire at a location far from the ends of the wire. Do not worry about the magnitude of the charge on the wire.

- Take the wire on the x-axis, and draw electric-field lines perpendicular to it.
- Take the wire on the
*x*-axis, and draw electric-field lines parallel to it. - Take the wire on the y-axis, and draw electric-field lines along it.
- Take the wire on the z-axis, and draw electric-field lines along it.

## 18.4 Electric Potential

A square grid has charges of *Q* = 10 nC are each corner. The sides of the square at 10 cm . How much energy does it require to bring a *q* = 1.0 nC charge from very far away to the point at the center of this square?

- 1.3 × 10
^{−6}J - 2.5 × 10
^{−6}J - 3.8 × 10
^{−6}J - 5.1 × 10
^{−6}J

How are potential difference and electric-field strength related for a constant electric field?

- The magnitude of electric-field strength is equivalent to the potential divided by the distance.
- The magnitude of electric-field strength is equivalent to the product of the electric potential and the distance.
- The magnitude of electric-field strength is equivalent to the difference between magnitude of the electric potential and the distance.
- The magnitude of electric-field strength is equivalent to the sum of the magnitude of the electric potential and the distance.

## 18.5 Capacitors and Dielectrics

A 12 μF air-filled capacitor has 12 V across it. If the surface charge on each capacitor plate is *σ* = 7.2 mC / m^{2}, what is the attractive force of one capacitor plate toward the other?

- 0.81 × 10
^{5}N - 0.81 × 10
^{6}N - 1.2 × 10
^{5}N - 1.2 × 10
^{6}N

Explain why capacitance should be inversely proportional to the separation between the plates of a capacitor.

- Capacitance is directly proportional to the electric field, which is inversely proportional to the distance between the capacitor plates.
- Capacitance is inversely proportional to the electric field, which is inversely proportional to the distance between the capacitor plates.
- Capacitance is inversely proportional to the electric field, which is directly proportional to the distance between the capacitor plates.
- Capacitance is directly proportional to the electric field, which is directly proportional to the distance between the capacitor plates.