In an oscillating LC circuit, the maximum charge on the capacitor is qm. Determine the charge on the capacitor, q, and the current through the inductor, I, when energy is shared equally between the electric and magnetic fields. Express your answers in terms of qm, L, and C.

a. q = qm/√2 b. qm/√(2LC)

Explanation:

a. Charge on the capacitor

Let U₁ = energy in inductor and U₂ = energy in capacitor and U = total energy in circuit.

So, U₁ + U₂ = U.

Since the energy is shared equally between the capacitor and inductor, U₁= U₂,so

2U₂ = U

and U₂ = U/2

Now U₂ = q²/2C where q is the charge on the capacitor with capacitance C and

U = (qm)²/2C where qm is the maximum charge on the capacitor.

Since U₂ = U/2,

Substituting the values for U₂ and U, we have

q²/2C = [(qm)²/2C]/2

q² = (qm)²/2

taking square-root of both sides, we have

q = qm/√2

b. The current in the inductor

Since the energy in the capacitor equals the energy in the inductor,

1/2LI² = 1/2q²/C where L is the inductance of the inductor and I the current through it.

I² = q²/LC

taking square-root of both sides, we have

I = q/√LC

substituting the value of q from above, we have

I = qm/√2 ÷ √LC

I = qm/√(2LC)

the velocity you are looking for is made up of two parts: horizontally velocity, vh, and its vertical velocity, vv. the actual velocity of the ball will be these two parts added together, and because velocity is a vector then they will need to be added vectorially.  

neglecting air resistance etc etc.,  

the horizontal velocity component will be constant (because there is no accelerating force in the horizontal direction.  

vh = 8 ms-1  

the vertical velocity component has an accelerating force: 'g'.  

using v = u +at where a = g = 9.81 ms-2.  

the vertical component of velocity, vv = 0 + gt = vv =9.81 x 2  

vv = 19.62 ms-1 . the zero appears because the virtical component of velocity at time t=0 is zero. remember, the ball was thrown horizontally.  

how fast it will be moving (it actual path (being a parabola )) will be the vector sum of these two.  

adding vectorally gives: -  

v = sqr root (8^2 + 19.62^2) = sqr root(64+384.9) = 21.18 or 21.2 ms-1

less than one you can't create energy