Questions: Electric Field

Question 1

A positive test charge of 2 μC placed at a point experiences a force of 8 N to the right. If the test charge is replaced with a 4 μC charge at the same point, what is the electric field at that point?

• A. 16 N/C to the right
• B. 4 N/C to the right
• C. 4 × 10⁶ N/C to the right
• D. The field doubles to 8 × 10⁶ N/C

Answer: (B)

E = F/q = 8 N / (2 × 10⁻⁶ C) = 4 × 10⁶ N/C. With the 4 μC charge, F = 16 N, but E = 16 N / (4 × 10⁻⁶ C) = 4 × 10⁶ N/C — unchanged. The electric field is a property of the space created by source charges, not of the test charge placed there. Doubling the test charge doubles the force but the ratio F/q stays constant.

Question 2

Removing most test charges from a region of space eliminates the electric field that existed there.

• T. True
• F. False

Answer: False

The electric field exists independently of any test charge — it is a property of space created by the source charges. The test charge is only a conceptual probe to measure the field; its absence does not change the field. This is one of the most important conceptual shifts in moving from Coulomb's law to the field concept.

Question 3

Two equal positive charges are placed 10 cm apart. Describe the direction of the electric field at the exact midpoint between them.

Model answer: The electric field at the midpoint is zero. The field from the left charge points to the right (away from positive), and the field from the right charge points to the left (away from positive). The two contributions are equal in magnitude and opposite in direction, so they cancel by vector superposition.

This question tests whether students can apply the superposition principle as a vector sum rather than a scalar sum. Each charge contributes a field at the midpoint, but because the charges are equal and the distances are equal, the magnitudes are the same while the directions are opposite — they sum to zero.