Question

A main difference between gravitational and electric forces is that electrical forces

A) attract.
B) repel or attract.
C) obey the inverse-square law.
D) act over shorter distances.
E) are weaker.

Answer 1

B) repel or attract

Step-by-step explanation:

Gravitational forces and electric forces are very similar in many aspects, however they also have some differences.

Concerning similarities, we can see that their formula is quite similar:

`F_G = G(m_1 m_2)/(r^2)\\F_E = k (q_1 q_2)/(r^2)`

where G is the gravitational constant, m1 and m2 the masses of the two objects, r the separation between the two objects, k the Coulomb's constant, q1 and q2 the charges of the two objects. We see that both forces obey an inverse-square law, since they both depend on
`1/r^2`.

However, there is one big difference between the two forces. In the formula, m (the mass) can only be positive, while q (the charge) can be either positive or negative. As a result, we have:

- The gravitational force can only have one sign --> in fact, it is always attractive

- The electric force can have 2 different signs --> in fact, it can be both attractive or repulsive (in particular, it is attractive when the charges have opposite signs, and repulsive when the charges have same sign).