A point charge q is placed at the centre of a line joining two equal charges Q. The whole system is in the state of equilibrium. Can you impose some lower limit on the magnitude of the charge Q?

Explanation

To solve the problem, we need to analyze the equilibrium condition of the three charges: two equal charges \( Q \) and a point charge \( q \) placed at the center of the line joining them. Here are the steps to derive the lower limit on the magnitude of charge \( Q \):

### Step-by-Step Solution:

1. **Understanding the Configuration**:
- We have two charges, both equal to \( Q \), placed at a distance \( 2d \) apart.
- The charge \( q \) is placed at the midpoint between the two charges.

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