Physics homework problems

Question

1) Charges Q1 and Q2 exert repulsive forces of 17 N on each other. What is the repulsive force in Newtons when their separation is decreased so that their final separation is 63 % of their initial separation?

 

2)A positive charge of 1.3E-4 C is 20 m from a negative charge of 5.1E-4 C. What is the force of one of the charges due to the other charge in units of Newtons? 

 

3)A positive charge of 64 nano-Coulombs is 18 cm from a charge 81 nano-Coulombs. The force on one of the charges due to the other charge is____ 
Newtons.

Answer 1

1)

 

F₁ = kQ₁Q₂/d₁²

 

F₂ = kQ₁Q₂/d₂²

 

d₂ = 0.63d₁ ⇒ F₂ = kQ₁Q₂/(0.63d₁)² = 2.5195 kQ₁Q₂/d₁² = 2.5195 F₁

 

F₁ = 17 N

 

F₂ = 2.5195(17) N = 42.8315 N

 

The force increases to 42.8315 N.

 

2)

 

F = kQ₁Q₂/d²

 

F = -(9 x 10⁹ Nm²/C²)(1.3 x 10^-4 C)(5.1 x 10^-4 C) / (20 m)² = -1.492 N

 

3)

 

F = (9 x 10⁹ Nm²/C²)(64 x 10^-9 C)(81 x 10^-9 C) / (0.18 m)² = 1.44 x 10^-3 N