RC Circuit

Reasonable Values for RC Circuit Lab

For an RC circuit lab, especially at the 9th-grade level, you'll want to choose resistor and capacitor values that allow for observable charge and discharge times without being too fast or too slow. Here's a set of reasonable starting values:

Resistor Values:

330Ω - Low enough to see the LED light up brightly, but high enough to protect the LED.
1kΩ - A moderate value that will slow down the charging and discharging process a bit more.
10kΩ - High enough to significantly slow the process, making it easier to observe with a stopwatch.

Capacitor Values:

47µF - Small enough for relatively quick charging and discharging, which could take a few seconds.
100µF - A middle-ground value that will take longer to charge and discharge, likely on the order of 10s of seconds.
1000µF (1mF) - Large enough to have a very observable charge and discharge time, potentially taking minutes.

The exact times for charging and discharging will depend on the specifics of the circuit, the power supply voltage, and the initial charge state of the capacitor. Using these values, students can start with the smaller capacitance and lower resistance, observing the quick charge/discharge cycles. As they move to larger capacitance and higher resistance values, they'll notice longer cycles.

For a standard experiment, combining a 100µF capacitor with a 1kΩ resistor is a common starting point because it provides a middle ground with easily observable results without being too quick or too long.

It's also helpful to teach students about the time constant of an RC circuit, which is τ (tau) = R*C. The time constant represents the time it takes for the capacitor to charge to about 63% of the supply voltage or discharge to about 37% of its initial voltage. With the values provided, students can calculate the time constant and compare it to their experimental results, which can be a very enlightening exercise.