PHYS 2212 Module 10.5

Oscillations in an LC Circuit

Learning Objectives

By the end of this section, you will be able to:

• Explain why charge or current oscillates between a capacitor and inductor, respectively, when wired in series
• Describe the relationship between the charge and current oscillating between a capacitor and inductor wired in series

LC Oscillations

In the brief amount of time just after the switch is closed to complete the circuit, the capacitor begins to discharge and the current through the inductor increases at the same rate as the charge on the capacitor decreases. The charge on the capacitor provides a voltage that will drive a current when the switch is closed. However, it takes some time for the capacitor to fully discharge. The electrons on one plate of the capacitor will be attracted to the positive plate and will discharge but, depending on the capacitance and the resistance in the circuit, this will take time. The more charge there is on the capacitor, the more time it will take to discharge.

As the capacitor discharges, the current increases in this circuit. The current is the flow of electrons from the negative plate of the capacitor to the positive plate. As the rate of discharge increases, so does the current in the circuit. This increasing current in the inductor induces a back-emf in the inductor, which helps to increase the time required for the capacitor to fully discharge. The induced emf in the inductor is not strong enough to stop the flow of current though. The back-emf only reduces the rate at which the current increases.

After the switch is closed to complete the circuit, the current in the inductor is a maximum at the instant the capacitor is fully discharged. As the capacitor discharges, the current increases in this circuit. The current is the flow of electrons from the negative plate of the capacitor to the positive plate. As the rate of discharge increases, so does the current in the circuit. This increasing current in the inductor induces a back-emf in the inductor, which helps to increase the time required for the capacitor to fully discharge. The induced emf in the inductor is not strong enough to stop the flow of current though. The back-emf only reduces the rate at which the current increases. This means the current is increasing but slowing down as it does so.

At the instant that the current reaches its maximum value, all the charge has come off the capacitor such that Q = 0. At this instant the current has reached its maximum value and won’t get any larger because there isn’t a voltage on the capacitor to drive the current anymore.

After closing the switch, the capacitor will be fully charged again in a time equal to T/2, or half of the oscillation period. In this time, the voltage on the capacitor will drive a current in the circuit and this current will increase from zero to a maximum value. At the instant the current is a maximum value, the charge on the capacitor and the voltage across the capacitor will be zero. This means there is nothing to drive the current and the current will begin to decrease. The decreasing current in the inductor will induce an emf that will help to drive the current, which will charge the capacitor again. When the capacitor is recharged, it will have a polarity that is opposite to what it started with. This will occur at exactly one half of the total oscillation period, or T/2.

The capacitor will discharge again, the current will increase to a maximum value when the capacitor is fully discharged, and then the capacitor will recharge. At this point the capacitor will be fully charged with a polarity equal to what it started with and the oscillation will start all over again.

If you replaced the inductor with one having twice the inductance, the oscillation frequency would decrease by a factor of . The natural frequency of oscillation in an LC circuit is and is measured in units of Hz, or cycles per second. Doubling the inductance results in a factor of in the denominator of the calculation.

If you replaced the capacitor with one having half the capacitance, the oscillation frequency would increase by a factor of . The natural frequency of oscillation in an LC circuit is and is measured in units of Hz, or cycles per second. Halving the capacitance results in a factor of in the denominator, which is equivalent to multiplying by a factor of .