Two positive charges of equal magnitude are fixed a short distance apart. A positive test charge is placed exactly halfway between them. What force does the test charge feel?
ANo resultant force at all
BA large force to the left
CA large force to the right
DA force straight upwards
Answer & Solution
Correct answer: A. No resultant force at all
1. The test charge is positive and both fixed charges are positive, so each one repels it.
2. One fixed charge pushes the test charge to the left and the other pushes it to the right.
3. The two fixed charges are equal in magnitude and the test charge sits at equal distances from both.
4. By Coulomb's law the two push forces are therefore equal in magnitude.
5. Equal magnitudes in opposite directions cancel, so there is no resultant force at that point.
6. Any answer naming a single sideways direction ignores the symmetry of the arrangement, which is the trap here.
_Source: Siyavula Physical Sciences Grade 11 (Everything Science, CC BY 4.0), Chapter 9: Electrostatics_
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