Capacitors let go slowly

Science · Energy · ages 17-18

Name ______________________ Date ____________

A capacitor discharges through a resistor. Where has the voltage fallen after one time constant?
- Exactly half
- About 37 percent
- About 63 percent
What is the time constant of an RC circuit?
- R times C, in seconds
- R over C, in volts
- C over R, in amps
After five time constants a discharging capacitor is effectively empty.
Circle one: True False
A 10 V capacitor with RC of 1 s discharges for one time constant. What voltage in volts is left?
Answer: ______________
A 10 microfarad capacitor discharges through a 100 kilohm resistor. What is RC in seconds?
Answer: ______________
What does the gradient of log voltage against time give?
- RC itself
- The starting voltage
- Minus one over RC
Log voltage against time falls with gradient minus 0.5 per second. What is RC in seconds?
Answer: ______________
A 10 V capacitor with RC of 1 s falls to 3.7 V. How many seconds passed?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

About 37 percent · One constant leaves about 37 percent of the start behind.
R times C, in seconds · Tau equals R times C, and ohms times farads come out in seconds.
True · Five constants leave a tiny leftover, so the discharge counts as done.
3.7 · One constant leaves 37 percent: 37 percent of 10 is 3.7 volts.
1 · Multiply 100000 ohms by 0.00001 farads to get 1 second.
Minus one over RC · Logging straightens the curve into a line whose gradient is minus one over RC.
2 · Gradient is minus one over RC, so RC is 1 over 0.5, which is 2 seconds.
1 · 3.7 V is 37 percent of 10 V, which marks exactly one constant: 1 second.