## Conceptual Physics (12th Edition)

Use Coulomb's Law to find the force on a test charge q from the isolated point charge. $$F = k \frac{(q_{point})(q)}{d^{2}}$$ If d is doubled, the force on the test charge q is divided by a factor of 4. The field is the force divided by q (page 418), so the field strength is also divided by a factor of 4.