A charge +2q moves vertically upwards with speed v, a second charge - q moves horizontally to the right with the same speed v, and a third charge + q moves horizontally to the right with the same speed v. The point P is located at a perpendicular distance a away from each charge as shown in the figure. The magnetic field at point P is

Question

G1 Physics · Accepted Answer

For a moving point charge, B⃗=μ₀q(v⃗×r̂)/(4πr²). Adding the three contributions with their charge signs and directions gives a resultant out of the page of magnitude μ₀(4qv)/(4πa²). Hence D.