CHARGE AND CHARGE INTERACTIONS
The number of electrons
(e
-
)
which surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral.
● An atom is ELECTRICALLY NEUTRAL if it contains an equal number of electrons (e
-
)
and protons (p
+
)
.
● An atom is
if it contains more protons (p
+
) than electrons (e
-
) . A positively charged atom is called a cation .
● An atom is
if it contains more electrons (e
-
) than protons (p
+
) . A negatively charged atom is called an anion .
CHARGE AS A QUANTITY
The charge possessed by an object is often expressed using the unit known as the Coulomb (abbreviated C ).
Because 1 C of charge is an abnormally large quantity of charge, the unit of microcoulombs (µC) is often used.
An object would need an excess of 6.25 x 10
18
electrons to have a total charge of -1 C.
The charge on a single electron is
-19 proton is 1.6 x 10 C .
–1.6 x 10
-19
C . The charge on a single
To determine the quantity of charge on an object, the total number of electrons and protons are compared to determine their difference. This difference is then multiplied by 1.6 x 10
-19
C to determine the overall charge on the object.

1. TRUE or FALSE: An object which is positively charged contains all protons and no electrons.
FALSE – POSITIVE CHARGE MEANS MORE P
+
THAN E
-
2. TRUE or FALSE: An object which is negatively charged could contain only electrons with no accompanying protons.
FALSE – NEGATIVE CHARGE MEANS MORE E
-
THAN P
+
3. TRUE or FALSE: An object which is electrically neutral contains only neutrons.
FALSE – NEUTRAL CHARGE MEANS SAME AMOUNT OF P
+
AND E
-
4. Identify the following particles as being charged or uncharged. If charged, indicate whether they are charged positively or negatively. (n = neutron, p = proton, e = electron)
PARTICLE A: -1
PARTICLE B: NEUTRAL
PARTICLE C: +2

5. Determine the quantity and type of charge on an object which has 3.62 x 10
12 more protons than electrons.
(3.62 x 10
12
)Q = +5.792 x 10
-7
C
6. Complete the following statements: a. If an object contains 8.25749 x 10
17
protons and 5.26 x 10
14
electrons, the charge on this object is _______________ C.
(p-e) Q = (8.25749 x 10
17
- 5.26 x 10
14
)Q = +0.132 C b. If an object contains 3.12 x 10
14
protons and 4.5488 x 10
16
electrons, the charge on this object is _______________ C.
(p-e) Q = (3.12 x 10
14
- 4.5488 x 10
16
)Q = -0.007 C c. If an object contains 2.40277 x 10
19
protons and 9.88 x 10
16
electrons, the charge on this object is _______________ C.
(p-e) Q = (2.40277 x 10
19
- 9.88 x 10
16
)Q = +3.829 C

METHODS OF CHARGING
● A charging process that results in a transfer of electrons between two objects that are rubbed together.
● One object gains electrons while one object loses electrons. The number gained must always equal the number lost according to the
● Animal fur/balloon – animal hair gives up electrons and balloon gains electrons (has a greater affinity for electrons). Animal fur now has excess of protons and balloon has an excess of electrons. Since they are now oppositely charged, they are attracted to each other according to the
:

1. During a physics lab, a plastic strip was rubbed with cotton and became positively charged. The correct explanation for why the plastic strip becomes positively charged is that ... a. the plastic strip acquired electrons from the cotton. b. the plastic strip acquired extra protons during the charging process. c. protons were created as the result of the charging process. d. the plastic strip lost electrons to the cotton during the charging process.
2. A physics teacher rubs a glass object and a felt cloth together and the glass becomes positively charged. Which of the following statements are true? a. The glass gained protons during the rubbing process. b. The felt became charged negatively during this rubbing process. c. Charge is created during the rubbing process; it is grabbed by the more charge-hungry object. d. If the glass acquired a charge of +5 units, then the felt acquires a charge of -5 units. e. This event violates the law of conservation of charge. f. Electrons are transferred from glass to felt; protons are transferred from felt to glass. g. Once charged in this manner, the glass object and the felt cloth should attract each other. h. In general, glass materials must have a greater affinity for electrons than felt materials.

1. From which particle does an atom get most of its mass?
P
+
and N
0 m e-
= 9.109 x 10
-31
kg m n0
= 1.672 x 10
-27
kg m p+
= 1.672 x 10
-27
kg
2. TRUE or FALSE: The charge on an electron and proton are of equal magnitude.
TRUE
3. TRUE or FALSE: If an electron is removed from an atom, it is called a cation .
TRUE
4. TRUE or FALSE: To become positively charged, an object must gain protons.
FALSE
5.
TRUE or FALSE: When you rub your feet on carpet, you acquire a negative charge and the carpet acquires a positive charge.
TRUE
6.
An object has 3.5 x 10
12 electrons and 4.0 x 10
13
protons. What is the total charge on this object?
(p-e) Q = (4.0 x 10
13
- 3.5 x 10
12
)Q = +5.84 x 10
-6
C or +5.84 µC

● A charging process used to charge a conductor without actually touching with a charged object.
● A separation of charge must occur in order to cause the attraction/repulsion.
● A charging process that starts by the process of induction .
● The negative charge is added/removed through the ground.
● A charging process transfers a charge by direct contact between two objects.

+ +
CAN X CAN Y
CAN X CAN Y
CAN X CAN Y
1. Two neutral conducting soda cans are touching each other. A positively charged balloon is brought near one of the cans as shown above. The cans are separated while the balloon is nearby, as shown. After the balloon is removed the cans are brought back together. When touching again, can X is ____.
NEUTRAL
2. Two neutral conducting soda cans are touching each other. A positively charged glass rod is brought near Can X as shown. Which of the following occur as the glass rod approaches Can X? a. Electrons jump from the glass rod to can X. b. Electrons jump from the glass rod to can Y.
+
+
+ c. Electrons jump from can X to the glass rod. d. Electrons jump from can Y to the glass rod. e. Protons jump from the glass rod to can X. f. Protons jump from can X to the glass rod.
g. No selection above correctly describes this situation.
CAN X CAN Y

3. TRUE or FALSE?
Two neutral conducting soda cans are touching each other.
-
A negatively charged balloon is brought near Can X as shown. As the balloon approaches Can X, there is a movement of electrons between the balloon and can X
(in one direction or the other).
CAN X
FALSE
4. A positively charged balloon is brought near a neutral conducting sphere as shown below. While the balloon is near, the sphere is touched (grounded).
+ +
At this point, there is a movement of electrons. Electrons move ____. a. into the sphere from the ground (hand) b. out of the sphere into the ground (hand) c. into the sphere from the balloon d. out of the sphere into the balloon e. from the ground through the sphere to the balloon f. from the balloon through the sphere to the ground
CAN Y

F
12
= kq
1 q
2 r
2
● k = 8.99 x 109 Nm2/C2
● q
1
and q
2
represent the charges on particles 1 and 2
● r represents the radius, or distance between the two particles
Coulomb’s Law is known as an
● According to Coulomb’s Law, the force is inversely proportional to the square of the distance separating them.
● To determine what happens to the force for a given change in distance, perform the following:
1. square the radius
2. take the inverse
3. multiply the force by this factor
EXAMPLE : Let’s assume that the distance between two electrons is increased by a factor of 4. By what factor will the force change for this change in distance?
1. square the radius 4
2
= 16
2. take the inverse
3. multiply the force by this factor
1/16
Force will be 1/16 as big

SAMPLE PROBLEM A
Two charged particles are spaced 1 m apart. One of them has a charge of +2 C and the other has a charge of +3 C. What is the electrostatic force between them? Is this an attractive force or a repulsive force?
1 m q
1 q
2
F
12
F
21
F
12
= (kq
1 q
2
) / r
2
= (k(2)(3)) / 1
2
= 5.394 x 10
10
N.
The force is repulsive since they both have a positive charge.
SAMPLE PROBLEM B
A distance of 0.5 m separates a proton and an electron. What is the electrostatic force between these two particles? Is this an attractive force or a repulsive force? q
1
0.5 m q
2
F
12
F
21
F
12
= (kq
1 q
2
) / r
2
= (k(q e-
)(q p+
)) / 0.5
2
= 9.206 x 10
-28
N.
The force is attractive since they have opposite charges.

SAMPLE PROBLEM C
The figure below shows three point charges that lie along the x-axis. Determine the magnitude and direction of the net electrostatic force on q
1
.
0.20 m 0.15 m
q
1
q
2 q
3
+4.0
µ
C +3.0
µ
C -7.0
µ
C
F
F
12
= (kq
1 q
2
) / r
2
= (k(4
13
= (kq
1 q
2
) / r
2
= (k(4
E
-6
E
-6
)(3
)(7
E
-6
E
-6
)) / 0.2
2
= 2.697 N.
)) / 0.35
2
= 2.055 N.
F
1
= -F
12
+ F
13
= -2.697N + 2.055 N = -0.642 N to the left