A couple of questions. 1. A capacitor of capacitance C is discharging through a resistor of resistance R. When will the energy stored be equal to half its initial value? My logic went like this: Energy in a capacitor (U) = Q^2/2C so: Q = (2CU)^0.5 so therefore the charge when the energy is halved = (2C(U/2))^0.5 = (CU)^0.5 so from Q=Q0*exp(-t/RC) (CU)^0.5 = (2CU)^0.5*exp(-t/RC) (CU)^0.5 = (2)^0.5*(CU)^0.5*exp(-t/RC) t = -RCln(1/(2)^0.5) and this second question I don't really follow at all. A particle of mass 6.0g moves at 4.0km/s in an xy plane, in a region of space with uniform magnetic field given by 5.0imT. At one instant when the particles velocity is directed at 37degrees counter clockwise from the positive x direction, the magnetic force on the particle is 0.48kN. What is the particles charge? I had a go using: F=qvBsin(x) (where x = angle) and got a charge of 0.039C. Is this the right way to go about it? Seems quite a large charge to me.