Question

Charges Q₁ and Q₂ are near each other. We have no information of their position, nature and magnitude, all we know is Q₂ experiences a force of 5N to the right. What can we infer for sure from this?

• A. $Q_1$ is a charge of -5C
• B. $Q_2$ is a charge of +5C
• C. $Q_1$ experiences a force of 5N to the left
• D. $Q_2$ and $Q_1$ are like charges

Answer 1

The correct option is C

$Q_1$ experiences a force of 5N to the left

Coulomb's force must obey Newton's third law. If there are only 2 bodies, the forces on each one due to the other body must be equal and opposite.

Now consider the case when $Q_1$ is placed leftwards with respect to $Q_2$:

From action-reaction law, one can easily infer that $Q_1$ has to experience an equal amount of force but towards left.

Suppose that $Q_1$ is now placed towards right relative to $Q_2$

Again $Q_1$ will experience a force towards left.