Question Detail
Question
Two point charges (-25 uC and +50 uC) are 10 cm apart. Find the electric field at a point 2 cm from the negative charge (on the line joining them, between the charges).
Correct Answer
Option D is the correct answer.
Detailed Explanation
Point P is 2 cm from -25uC and (10-2)=8 cm from +50uC. Field from -25uC (E1): k q/r2 = 9x109 x 25x10-6 / (0.02)2 = 2.25x105/4x10-4 = 5.625x108 N/C, pointing TOWARDS the negative charge (field lines point into negative charges). Field from +50uC (E2): 9x109 x 50x10-6 / (0.08)2 = 4.5x105/6.4x10-3 = 7.03x107 N/C, pointing AWAY from the positive charge - which, at P (between the charges), also points toward the negative charge. Both fields point the same way (toward the negative charge), so they ADD: E_total = 5.625x108 + 0.703x108 = 6.33x108 N/C, directed towards the negative charge.
Hint
Point P is 2 cm from -25uC and (10-2)=8 cm from +50uC.