1. Physics 16/21 electromagnetism
Electric Charge and Electric Force
MARLON FLORES SACEDON

2. electromagnetism What is electromagnetism?
A branch of physics which involves the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force usually shows electromagnetic fields, such as electric fields, magnetic fields, and light.

3. Law of electric charges
Two positive charges or two negative charges repel each other.
2. A positive charge and a negative charge attract each other.
3. Uncharge material is always attracted by a charge material.

4. Law of electric charges

5. conductors, insuLators, and induced charges

6. electric charge and the structure of Matter
Mass of electron = me = 9.11x10-31 kg
Mass of proton = mp = 1.673x10-27 kg
Mass of neutron = mn = 1.675x10-27 kg
Charge of One electron: e=1.602𝑥 10 −19 𝐶/electron
Principle of conservation of charge:
The algebraic sum of all the electric charges in any closed system is constant.

7. Atomic number 𝑚 𝑡𝑜𝑡 =nM 𝑁=𝑛 𝑁 𝐴 Where: 𝑚 𝑡𝑜𝑡 = total mass (g)
𝑛 = number of moles (mol)
𝑀 = molar mass (g/mol)
𝑁 𝐴 = x1023 atoms/mol
Abogadro’s number
𝑁 = number of atom in a substance
𝑚 𝑡𝑜𝑡 =nM
𝑁=𝑛 𝑁 𝐴

8. Coulomb’s law
Coulomb's law, or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. The law was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism.
Coulomb’s torsion balance

9. Coulomb’s law 𝐹 =𝑘 𝑞 1 𝑞 2 𝑟 2 𝑟 Electric force vector 𝑞 2 𝐹 12 - 𝐹 21+
𝑞 1
𝐹= 1 4𝜋 𝜖 𝑜 ∙ 𝑞 1 𝑞 2 𝑟 2
𝐹∝ 𝑞 1 𝑞 2 𝑟 2
𝐹=𝑘 𝑞 1 𝑞 2 𝑟 Magnitude of electric force
𝑞 2
𝐹 =𝑘 𝑞 1 𝑞 2 𝑟 2 𝑟 Electric force vector
𝐹 12
𝐹 21
𝐹 12 +
𝑞 1 =+25𝑛𝐶 -
𝑞 2 =−75𝑛𝐶
𝑟=3.0𝑐𝑚 + 𝑟
𝑞 1
Where: 𝑘 is electric constant +
𝐹 21
(Coulomb’s law: force between two point charges)
𝑞 1 𝑎𝑛𝑑 𝑞 2 are interacting charges, in 𝐶𝑜𝑢𝑙 𝑜𝑟 𝐶
𝐹 12 =9𝑥 𝑁∙ 𝑚 2 𝐶 𝑛𝐶 75𝑛𝐶 3𝑐𝑚 2
1 coul = x 109 statC
𝜖 𝑜 =8.854𝑥 10 −12 𝐶 2 𝑁∙ 𝑚 2
𝐹 12 =9𝑥 𝑁∙ 𝑚 2 𝐶 𝑥 10 −9 𝐶 75𝑥 10 −9 𝐶 3𝑥 10 −2 𝑚 2
Permittivity of free space
Problem: Two point charges, 𝑞 1 =+25𝑛𝐶 and 𝑞 2 =−75𝑛𝐶, are separated by a distance 𝑟=3.0𝑐𝑚. Find the magnitude of the electric force (a) that 𝑞 1 exerts on 𝑞 2 and (b) that 𝑞 2 exerts on 𝑞 1 .
1 4𝜋 𝜖 𝑜 =𝑘=8.988𝑥 𝑁∙ 𝑚 2 𝐶 2
𝐹 12 = 𝑁 attractive force
𝐹 21 =9𝑥 𝑁∙ 𝑚 2 𝐶 𝑥 10 −9 𝐶 +25𝑥 10 −9 𝐶 3𝑥 10 −2 𝑚 2
𝑘=9𝑥 𝑁∙ 𝑚 2 𝐶 2
𝑘=1 𝑑𝑦𝑛𝑒∙𝑐 𝑚 2 𝑠𝑡𝑎𝑡𝐶 2
𝐹 21 = 𝑁 attractive force

10. Coulomb’s law Superposition of electric charge 𝑦 𝑥
𝐹= 1 4𝜋 𝜖 𝑜 ∙ 𝑞 1 𝑞 2 𝑟 2 -
−𝑞 3
𝑟 13
𝐹 21 = 1 4𝜋 𝜖 𝑜 ∙ 𝑞 1 𝑞 𝑟 +
𝑞 2
𝐹 31 = 1 4𝜋 𝜖 𝑜 ∙ 𝑞 1 𝑞 𝑟 𝐹
𝐹 31
𝑟 12 +
𝑞 1
𝐹 21
𝐹 = 𝐹 𝐹 31
Note: vector sum or geometric sum not arithmetic sum

11. Coulomb’s law Example. 𝐹 = 𝐹 21 + 𝐹 31 = 𝐹 𝑥 𝑖 + 𝐹 𝑦 𝑗
Calculate the net electric force on charge 1 due to charge 2 & charge 3. 𝑦 𝑥
𝐹= 1 4𝜋 𝜖 𝑜 ∙ 𝑞 1 𝑞 2 𝑟 2
1 4𝜋 𝜖 𝑜 =𝑘=9𝑥 𝑁∙ 𝑚 2 𝐶 2 +
𝑞 3 =7𝑛𝐶
3cm
𝐹 21 =9𝑥 𝑁∙ 𝑚 2 𝐶 2 ∙ (3𝑛𝐶)(9𝑛𝐶) (6𝑐𝑚) 2
𝑟 13 = =5𝑐𝑚
𝑟 13
4cm
𝜃= 𝑇𝑎𝑛 − = 53.1 𝑜
=9𝑥 𝑁∙ 𝑚 2 𝐶 2 ∙ 3𝑥 10 −9 𝐶 9𝑥 10 −9 𝐶 6𝑥 10 −2 𝑚 2
=6.75𝑥 10 −5 𝑁 +
𝑞 1 =3𝑛𝐶 𝜃
𝑟 12 -
𝑞 2 =−9𝑛𝐶
𝐹 31 =9𝑥 𝑁∙ 𝑚 2 𝐶 2 ∙ (3𝑥 10 −9 𝐶)(7𝑥 10 −9 𝐶) (5𝑥 10 −2 𝑚) 2 𝜃 𝛽
=7.56𝑥 10 −5 𝑁
6cm
𝐹 31
𝐹 31
𝐹 = ?
Using component method:
=2.21𝑥 10 −5 𝑁
Σ 𝐹 𝑥 =6.75𝑥 10 −5 𝐶−7.56𝑥 10 −5 𝐶(cos 𝑜 )
=−6.06𝑥 10 −5 𝑁
Σ 𝐹 𝑦 =0−7.56𝑥 10 −5 𝐶(sin 𝑜 )
vector sum
𝐹= (2.21𝑥 10 −5 ) 2 + (6.06𝑥 10 −5 ) 2
=6.45𝑥 10 −5 𝑁
𝐹 = 𝐹 𝐹 31 = 𝐹 𝑥 𝑖 + 𝐹 𝑦 𝑗
Magnitude
𝛽= 𝑇𝑎𝑛 −1 Σ 𝐹 𝑦 Σ 𝐹 𝑥
= 𝑇𝑎𝑛 − 𝑥 10 − 𝑥 10 −5
𝐹 =(2.21𝑥 10 −5 𝑁) 𝑖 +(−6.06𝑥 10 −5 𝑁) 𝑗
𝐹 = 2.21𝑥 10 −5 𝑁 𝑖 − 6.06𝑥 10 −5 𝑁 𝑗 ANSWER
= 70 𝑜 from +x-axis in QIV
Direction

12. Coulomb’s law Assignment 1 Assignment 2
Two point charges are located on the +x-axis of a coordinate system: 𝑞 1 =1.0𝑛𝐶 is at 𝑥=+2.0𝑐𝑚, and 𝑞 2 =−3.0𝑛𝐶 is at 𝑥=+4.0𝑐𝑚. What is the total electric force exerted by 𝑞 1 and 𝑞 2 on a charge 𝑞 3 =5.0 𝑛𝐶 at 𝑥=0?
Assignment 2
Calculate the net electric force on charge 1 due to charge 2 & charge 3. Calculate also the magnitude and direction of the net force. 𝑦 𝑥 -
𝑞 4 =4𝜇𝐶 +
𝑞 3 =−6𝜇𝐶
5cm
4cm
𝑟 13
6cm
6cm +
𝑞 1 =3𝜇𝐶 𝜃
𝑟 12 -
𝑞 2 =9𝜇𝐶 𝜃
12cm

13. Problems/ assignment

14. Problems/ assignment

15. Problems/ assignment

16. Problems/ assignment

17. eNd