Understanding Vocabulary
Section 15.1
1. coulomb
2. electrically neutral
3. static electricity
4. Coulomb’s law
5. friction
6. electroscope
7. induction
Section 15.2
8. field lines
9. polarized
10. electric field
Section 15.3
11. capacitor
12. farads
Reviewing Concepts
Section 15.1
1. positively; negatively
2. repel; attract
3. An object is electrically neutral when the net charge on the object is
zero.
4. positive
5. 6.24 × 1018
6. It is the same size, but opposite in sign.
7. Because most objects are electrically neutral they exert no net force
on one another.
8. The size of the charges and the distance between the charges
determines the force between the charges.
9. The force between two charges increases by the square of the
decrease in the distance between the charges.
10. Answers are:
a. The force increases by four times.
b. The force decreases by four times.
c. The force decreases by nine times.
d. The force becomes attractive (negative).
e. The force is identical.
11. The laws are similar in form. The forces in each are inversely
proportional to the square of the distance. Replacing the mass in
Newton's law with charge in Coulomb's law and Newton's Universal
Constant with the electrostatic constant makes the laws
interchangeable.
12. The leaves will move apart. Negative charges are attracted to the ball,
leaving the leaves positively charged and causing them to move apart
by repulsion.
13. Touching an electroscope with a negatively charged object will
charge the electroscope negatively.
14. The balloon will become negatively charged and your hair will
become positively charged. This is called charging by friction.
15. Bring a charged object close to the electroscope. Ground the
electroscope. Remove the ground from the electroscope. The
electroscope will acquire a charge that is opposite to the charge of the
object that was brought close to it.
Section 15.2
16. Amperes of current measure of the number of coulombs of charge
that flow past a point in one second.
17. Positive to negative; negative to positive
18. No, only the valence electrons.
19. The voltage of the battery causes the electrons that are already in the
wire to drift through the wire. The electrons circulate through the
entire circuit, including through the battery.
20. The electrons in a conductor are free to move; they are not free to
move in an insulator.
21. joule; coulomb; watt; ampere
Section 15.3
22. A capacitor is a device that stores electrical energy. A charged
capacitor can create current in a circuit.
23. A capacitor is charged when one its plates has a positive charge and
the other plate has a negative charge.
24. The current in a circuit containing a capacitor decreases to zero as the
capacitor charges. The capacitor will stop charging and current will
stop flowing when the voltage across the plates of the capacitor
equals the voltage of the battery used to charge the capacitor.
25. The amount of charge a capacitor will hold, its capacitance, depends
upon three factors: (1) the area of the plates, (2) the insulating
material between the plates, and (3) the distance between the plates.
Solving Problems
Section 15.1
1.
2. Answers are:
a. +2
b. -1
c. 0 or neutral
3. Current = charge per time
I = (6 C)/(2 sec) = 3 amperes
4. Charge = current × time
Q = (2 amps)(10 sec) = 20 C
Section 15.2
5. Answers are:
a. I = V/R= (3 V)/2Ω = 1.5 A
b. 1.5 C
c. Charge = current × time = (1.5 A)(2 sec) = 3 C
6. Power = Voltage × Current
V = P/I = (6 W)/(2 A) = 3 V
7. Voltage = Energy/Charge
V = W/Q = (6 J)/(2 C) = 3 V
Section 15.3
8. q = CV = (3 F)(1.5 V) = 4.5 C
9. q = CV = (6 F)(1.5 V) = 9 C
10. q = CV = (6 F)(3 V) = 18 C
Applying Your Knowledge
Section 15.1
1. Synthetic materials, like nylon, seem to stick together the most and
natural fibers, like cotton, are the least.
2. When the air contains humidity, the tiny droplets of water carry
charge away making it more difficult for a net charge to be
established on an object.
3. The law of attraction was experimentally verified by Charles
Coulomb with a torsion balance he constructed himself. The torsion
balance was used to measure the amount of torsion on a fiber when an
electrostatically charged pith ball was brought close to another
charged pith ball. When the two pith balls were charged by the same
type of charge, the pith balls were repelled; when the two pith balls
had opposite charges, the force between them was attractive.
Coulomb was able to measure the torsion on the fiber and the distance
between the balls, and he derived the equation that makes up
Coulomb’s law: that the force between the two pith balls is directly
proportional to the magnitude of the charges and inversely
proportional to the square of the distance between them.
4. The stream of water is attracted to the charged object. The polar water
molecules rotate, causing the end of the water molecule closest to the
charged object to be of the opposite charge. A force is established
between the oppositely charged object and the stream of water.