Discharging capacitors

Now let's take a look at what happens when we discharge the capacitor.

Here's the circuit as we left it.

The charge curve after 10 seconds (10 time constants).

Once the capacitor is fully charged the voltage will remain at 10 volts and the current at 0 amps. This condition will continue as long as the switch remains in the charge position.

Before we move on, let's take a look at the voltages and currents while the circuit is in charge mode.

The demonstration circuit in charge mode.

Notice the direction of conventional current and the voltage polarities. In this mode the resistor and capacitor are both acting as impedances. As such the voltage will be positive where conventional current enters the impedance and negative were conventional current exits the impedance. The battery is a current source so the voltages are reversed (see Voltage polarity in a series circuit above).

Now let's flip the switch to the discharge position. Just for convenience let's do that right at the 10 second mark. Let's also freeze time and see what happens at that moment.

At the moment the switch is moved to the discharge circuit, the capacitor looks like a battery.

Remember that, even though the capacitor was acting like an open circuit, it also has an energy store of electricity under pressure. The moment the battery is taken out of the circuit the capacitor looks like a battery. As a current source conventional current now exits the positive terminal. The current now flows through the resistor in the opposite direction. Notice that the voltage polarity has reversed across the resistor.

The current has reversed direction and the voltage polarity across the resistor has reversed.

Since the current has reversed direction we will call that a negative current. We need to expand the graph vertically to show this.

Complete charge and discharge curves for a capacitor. Notice that when the voltage is at 0 the current is at maximum. As the voltage rises the current decreases. After charging the voltage is at maximum and current is at zero.  When discharging the current jumps to the maximum value. It also becomes a negative value because it reverses direction through the circuit. If the switch is flipped back to the charge position the current will jump back to its maximum positive value and the cycle will repeat. Remember this graph. You will see something like it again when we discuss square waves and capacitors in AC circuits.

The moment the switch is moved to the discharge position current instantly starts to flow in the opposite direction to that when charging. Notice the sharp negative spike in the current at the 10 second mark. This is not an imaginary event. If you place the probes of an oscilloscope (a voltmeter that displays a graph of voltage as it changes over time) across the resistor, it will display a curve that looks like the red current line above. This would be the voltage across the resistor as the current flowed through it.

Now the circuit consists of a 10 volt source (the capacitor) and a 1 ohm resistor. The graph shows that we will have 10 volts across the capacitor and 10 amps of current flowing through the circuit. Again, the current is shown as a negative current simply because it is flowing in the opposite direction from when the capacitor was charging.

Now let's move forward to 1 time constant. Looking at the graph you can see that both the voltage and the current decreased by 63.2%. The energy store of the capacitor is quickly flowing away like air escaping from a compressed air tank. After 5 time constants the voltage is only 0.07% of where it started and the current is at the same point. The capacitor is essentially discharged and current is no longer flowing in the circuit.