Electrostatic force and Gravitational force
Electrostatic force is a type of non-contact force occurs between objects with a charge.
𝑸𝟏 𝑸𝟐
𝑭𝒆 = 𝒌 𝟐
𝒓
𝑘 𝑖𝑠 𝑘𝑛𝑜𝑤𝑛 𝑎𝑠 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑡𝑛𝑡 (𝐶𝑜𝑢𝑙𝑜𝑚𝑏 ′ 𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡)
𝑁𝑚2
𝑘 = 9𝑥109
𝐶2
Gravitational force is the force by which the earth attracts other objects by mass.
𝒎𝟏 𝒎𝟐
𝑭𝒈 = 𝑮
𝒓𝟐
𝐺 𝑖𝑠 𝑔𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
The value of G in the laboratory was first determined by Cavendish using the torsional balance.
−11
𝑁𝑚2
𝐺 = 6.67𝑥10
𝑘𝑔2
The following table compares electric force vs. gravitational force
Electric force Gravitational force
Force occurs between charged objects Force occurs between any object with mass
Can involve attraction (pull towards) or Only involves attraction
repulsion (push away)
Force originates from the object’s charge and Force originates from the object’s mass and
can vary depending on the medium does not vary depending on the medium
Stronger than gravitational force Weaker than electric force
Example 1: Compare the magnitude of the electric force between two charges of each charge 1C
separated by a distance of 1m with the magnitude of the gravitational force if each have a mass
of 1kg and placed 1m apart
Solution
𝑄1 𝑄2 9
𝑁𝑚2 (1𝐶)(1𝐶)
𝐹𝑒 = 𝑘 2 = 9𝑥10 = 9𝑥109 𝑁
𝑟 𝐶2 (1𝑚)2
𝑚1 𝑚2 −11
𝑁𝑚2 (1𝑘𝑔)(1𝑘𝑔)
𝐹𝑔 = 𝐺 = 6.67𝑥10 = 6.67𝑥10−11 𝑁
𝑟2 𝑘𝑔2 (1)2
𝐹 9𝑥109 𝑁
Taking the ratio: 𝐹𝑒 = 6.67𝑥10−11 𝑁 = 1.3𝑥1020 ⟹ 𝑭𝒆 = 1.3𝑥1020 𝑭𝒈
𝑔
The electrostatic force is about 1020 times the gravitational force
EXERCISE
1. What is the similarity between gravitational force and electric force?
2. The gravitational force between two objects does not depend on
A. Sum of the masses C. Gravitational constant
B. Product of the masses D. Distance between the masse
3. Mass M is divided into two parts xM and (1 – x)M. For a given separation, the value of x for
which the gravitational attraction between the two pieces becomes maximum is
1 3
A. 1 B. 2 C. 2 D. 5
4. The mass of the moon is about 1.2% of the mass of the earth. Compared to the gravitational
force the earth exerts on the moon, the gravitational force the moon exerts on earth
A. Is the same B. Is smaller C. Is greater D. Varies with its phase
1
Prepared by Yadesa Dabala Telegram: https://t.me/Physicsforall123456
