OHP RLC
EM oscillations - RLC circuit - resonance
What it shows:
The current in a circuit consisting of a capacitor, inductor and resistor
will oscillate back and forth as the capacitor charges and discharges.
How it works:
The circuit layout is shown in figure 1. Initially, the capacitor is
charged, so the knife switch links the cap to the battery. Switching to
complete the LRC circuit allows the cap to discharge. The current I increases,
as does the field B inside the inductor. When the capacitor charge is zero,
I and B are a maximum (the energy of the circuit is now stored in the inductor).
The opposite plate of the cap begins to charge, I and B decrease to zero and the
energy becomes stored in the cap. The process reverses and repeats, but energy
loss through circuit resistance damps the oscillations.
figure 1. RLC circuit layout
The amount of damping can be controlled by the resistance. Critical damping is given by
R2 = 4L/C
so for the above circuit, critical damping can be obtained for R = 54kΩ.
The period of oscillation for an under-damped circuit is given by
T = 2π(LC)½ ≈ 1.5 seconds
Setting it up:
The pieces of apparatus are a 22.5V Ever Ready® cell, 10μF
330V capacitor, resistance box, 20,000 turn 7.5kH UNILAB
1
inductor coil, knife switch and an OHP center zero milliammeter. 15μF
and 30μF capacitors can also be used to show the effect of C.
Comments:
Usually this demo is run without the external resistance, R in the circuit being
provided by the inductor's DC resistance. This will also show that you can never
create a perfect LC circuit. Rating ***
1 45kH 2x 20000 turn inductor 013.401, UNILAB Ltd., Blackburn, England BB1 3BT