Honors Physics
Work Sheet #1
Electric Charge and Electrostatic Charging
Electric Charge
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
MC A combination of two electrons and three protons would have a net charge of (a) 1, (b) 1, (c)
1.6  1019 C, (d) 1.6 1019 C. (c)
MC An electron is just above a fixed proton. The direction of the electric force on the proton is (a) up, (b)
down, (c) zero. (a)
MC In Exercise 2, which one feels the bigger size force: (a) The electron, (b) proton, or (c) both feel the
same size force? (c)
CQ (a) How do we know that there are two types of electric charge? (b) What would be the effect of
designating the charge on the electron as positive and the charge on the proton as negative? see ISM
CQ An electrically neutral object can be given a net charge by several means. Does this violate the
conservation of charge? Explain. no, see ISM
CQ If a solid neutral object becomes positively charged, does its mass increase or decrease? What if it
becomes negatively charged? decrease; increase
CQ How can you determine the type of charge on an object using an electroscope that has a net charge of a
known sign? Explain. see ISM
CQ If two objects electrically repel each other, are both necessarily charged? What if they attract each
other? yes; not necessarily (polarization)
 What is the net charge of an object that has 1.0 million excess electrons? 1.6 1013 C
 In walking across a carpet, you acquire a net negative charge of 50 C. How many excess electrons do
you have? 3.11014 electrons
 An alpha particle is the nucleus of a helium atom with no electrons. What would be the charge on two
alpha particles? 6.40  1019 C
IE  A glass rod rubbed with silk acquires a charge of 8.0  1010 C. (a) Is the charge on the silk (1)
positive, (2) zero, or (3) negative? Why? (b) What is the charge on the silk, and how many electrons have
been transferred to the silk? (c) How much mass has the glass rod lost? see ISM
IE  A rubber rod rubbed with fur acquires a charge of 4.8  109 C. (a) Is the charge on the fur (1)
positive, (2) zero, or (3) negative? Why? (b) What is the charge on the fur, and how much mass is
transferred to the rod? (c) How much mass has the rubber rod gained? (a) (1) positive (b)
4.8  109 C, 2.7  1020 kg,
20
(c) 2.7  10 kg
15.2 Electrostatic Charging
14. MC A rubber rod is rubbed with fur. The fur is then quickly brought near the bulb of an uncharged
electroscope. The sign of the charge on the leaves of the electroscope is (a) positive, (b) negative, (c)
zero. (a)
15. MC A stream of water is deflected toward a nearby electrically charged object that is brought close to it. The
sign of the charge on the object (a) is positive, (b) is negative, (c) is zero, (d) can’t be determined by the data
given. (d)
16. MC A balloon is charged and then clings to a wall. The sign of the charge on the balloon (a) is positive, (b)
is negative, (c) is zero, (d) can’t be determined by the data given. (d)
17. CQ Fuel trucks often have metal chains reaching from their frames to the ground. Why is this important? to
remove excess charge due to friction of rubber on road
18. CQ Is there a gain or loss of electrons when an object is electrically polarized? Explain. no, charges simply
reorient themselves
19. CQ Explain carefully the steps you would use to create an electroscope that is positively charged by
induction. After you are done, how can you verify that the electroscope is positively (and thus not
negatively) charged? see ISM
20. CQ Two metal spheres mounted on insulated supports are in contact. Bringing a negatively charged object
close to the right-hand sphere would enable you to temporarily charge both spheres by induction. Explain
clearly how this would work and what the sign of the charge on each sphere would be. see ISM
Answers
4.
(a) We know there are two types of charges, because attractive and repulsive forces can be produced by
different combinations of just two types of charges.
(b) There would be no effect as it is simply a sign convention.
5.
6.
No . Charges are simply moved from the object to another object.
If an object is positively charged, its mass decreases , because it loses electrons. If an object is
negatively charged, its mass increases , because it gains electrons.
7.
Bring the charged object near the electroscope and observe how the leaves move. If the repulsion
between the leaves increases, the charge on the object has the same sign as the one on the electroscope; if the
repulsion between the leaves decreases, then the charge on the object has opposite sign as the one on the
electroscope.
8.
Yes , both objects are charged with the same type of charge if they repel each other.
However, if two objects attract each other, both are not necessarily charged. For example, through
polarization and polarization by induction, a charged object can attract a neutral object, but the neutral object
has a non-uniform charge distribution.
9.
q = ne = (106)(1.6  1019 C) = 1.6  1013 C .
10.
q = ne,
11.
There are two protons in each  particle. So q = +ne = (2)(2)(1.60  1019 C) = +6.40  1019 C .
12 .

q
–50  106 C
n= e =
= 3.1  1014 electrons .
1.60  1019 C
(a) The charge on the silk must be (3) negative because of the conservation of charge. When one
object becomes positively charged, it loses electrons. These same electrons must be gained by another object,
and therefore it is negatively charged.
(b) 8.0  1010 C according to charge conservation.
q = ne,

q 8.0  1010 C
n= e =
= 5.0  109 electrons .
1.6  1019 C
(c) The mass is m = (9.11  1031 kg/electron)(5.0  109 electrons) = 4.6  1021 kg .
13.
(a) The charge on the fur must be (1) positive because of the conservation of charge. When one object
becomes negatively charged, it gains electrons. These same electrons must be lost by another object, and
therefore it is positively charged.
(b) +4.8  109 C according to charge conservation. From q = +ne,
q
4.8  109 C
n= e =
= 3.0  1010 electrons.
1.6  1019 C
The mass is (3.0  1010)(9.11  1031 kg) = 2.7  1020 kg .
(c) The electrons moved from fur to the rubber rod, so the mass is still 2.7  1020 kg .
14.
(a). The fur is positively charged (see Exercise 15-13). When the positively charged fur is brought near
an electroscope, the leaves are charged by polarization. So the charges on the leaves are positive.
15.
(d). Water will be deflected towards the object regardless of being positively or negatively charged. The
water is still neutral but polarized.
16.
(d). The balloon clings to the wall regardless of being positively or negatively charged. The wall is still
neutral but polarized.
17.
This is to remove excess charge due to friction of rubber on road . If the excess charge is not removed,
this could result in a spark, causing a gasoline explosion.
18 .
No charges simply reorient themselves . There is no gain or loss of electrons.
19.
If you bring a negatively charged object near the electroscope, the induction process will charge the
electroscope with positive charges. You can prove the charges are positive by bringing the negatively charged
object near the leaves and see if the leaves are attracted by the negatively charged object.
20.
The spheres can be charged through polarization by induction. For example, if you bring a positively
charged object near one of the two spheres, the sphere near the charged object will have a net negative charge,
and the other sphere will have a net positive charge (polarization by induction). Then you separate the two
spheres (while keeping the positively charged object nearby), and the spheres will have opposite charges
according to charge conservation.