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Two point charges q and 4q are kept 30 cm apart. At a distance ......., on the straight line joining them, the intensity of the electric field is zero.

Asked in RS Academy GUJCET booklet · Null point

• (1) 20 cm from 4q
• (2) 7.5 cm from q
• (3) 15 cm from 4q
• (4) 5 cm from q

Step-by-step solution

Why the other options are wrong

• (2) No single slip gives 7.5 cm. Check: there (with 22.5 cm to 4q) the field of q is (kq)/(0.075²)≈178kq and that of 4q is (4kq)/(0.225²)≈79kq, which are not equal.
• (3) Takes the midpoint. That works only for equal charges; the larger charge 4q pushes the null point towards the smaller one.
• (4) No single slip gives 5 cm. Check: there the field of q is (kq)/(0.05²)=400kq and that of 4q is (4kq)/(0.25²)=64kq, which are not equal.

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