Resistors in RC Circuits

Part II: Resistors in RC Circuits

The purpose of the second part of the experiment is to determine if the resistor affects the amount of charge on a capacitor in a circuit, and if so, how does it affect it? In other words, will a larger resistor allow more charge onto the capacitor, less charge onto the capacitor, or the same charge onto a capacitor as a smaller resistor does? Give students various resistors to use in

a charging circuit. The range of resistances should complement the size of the capacitors such that the charging time is less than 10–60 seconds or so. With a 100,000-microfarad capacitor, the range should be between 5–100 ohms. For a different capacitor, set the time constant to about 1/5 of the total time you want the charging to take and use the equation

to calculate an appropriate resistance range. Give students at least four to ive different resistors to choose from. If only two different resistance values are available, then they combine them in series and parallel to get different resistances. For example, two 10-ohm resistors in series create a 20-ohm resistor, and in parallel they create a 5-ohm resistor. This exercise can provide a good review for calculating equivalent resistances.

Students then should discharge the capacitor in each case through the same light bulb(s). A #40 or #50 bulb works well. They should decide how to use lighting and dimming of the light bulbs during discharging to determine if the capacitors have been charged to the same amount or different amounts with different resistors. For example, students charge the capacitor using a 20-ohm resistor. Then they discharge it through lightbulbs and observe the lighting time. Then they charge the capacitor using the 100-ohm resistor, and again discharge it through the same bulbs. Longer bulb lighting time would indicate more charge had been stored on the capacitor. Depending on the level of your students, you may want to engage the whole class in a brainstorming session about this before you break them into groups, or you may want to work individually with each group as needed.

It is unlikely that a bulb in series with a resistor and capacitor will light up signiicantly, so they cannot use the bulb lighting to determine if the capacitor is fully charged or not. So students should be instructed to connect the circuit and wait an appropriate amount of time to make sure the capacitor is fully charged. This may be 15–30 seconds, or up to a full minute, depending on the time constant of the circuit. A rule of thumb is that a capacitor is “fully” charged in about ive time constants (or

). To avoid a potential misconception,

students should allow each combination to charge for the same amount of time,

so set the suggested time as that for the largest RC combination value.

IO T

If students are having a dificult time making observations with the bulbs

A lighting, they could use an ammeter to make observations of the current. Some IG students have found this more effective and, no pun intended, enlightening.