Answer
The field is reduced to 1/4 of its original value. Coulomb’s law guides our answer.
Work Step by Step
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.
