Two point charges +3µC and –3µC are kept 20 cm apart in air. The electric field midway between the charges on the line joining them will be :42 × 105 N/C54 × 105 N/C63 × 105 N/C72 × 105 N/C

The electric field due to a point charge is given by the formula:

E = kQ/r²

where: E is the electric field, k is Coulomb's constant (9 × 10^9 N m²/C²), Q is the charge, and r is the distance from the charge.

In this case, we have two charges, +3µC and -3µC, and we want to find the electric field midway between them. The distance from each charge to the midpoint is half of the total distance, or 20 cm / 2 = 10 cm = 0.1 m.

The electric field due to the +3µC charge is:

E1 = kQ/r² = (9 × 10^9 N m²/C²) * (3 × 10^-6 C) / (0.1 m)² = 27 × 10^5 N/C

The electric field due to the -3µC charge is:

E2 = kQ/r² = (9 × 10^9 N m²/C²) * (-3 × 10^-6 C) / (0.1 m)² = -27 × 10^5 N/C

However, because the charges are of opposite sign, their electric fields at the midpoint will be in opposite directions. Therefore, the total electric field at the midpoint is the sum of the magnitudes of the two fields:

E_total = E1 + E2 = 27 × 10^5 N/C + 27 × 10^5 N/C = 54 × 10^5 N/C

So, the correct answer is 54 × 10^5 N/C.