Electronics Basics: Trace Capacitor Charging

Read the time constant of an ideal resistor-capacitor model.

All worksheets

5 questions0m 0s

Learn the skill

For an initially uncharged ideal capacitor C charging through R from a constant source V_s, V_C(t) = V_s(1 - exp(-t/(RC))). Define tau = RC. With R = 1000 ohms and C = 0.001 F, tau = 1 s. These are diagram calculations, never capacitor-handling instructions. Use diagrams and calculations only. Never use wall outlets, short batteries, open powered equipment or handle charged capacitors.

Worked example

With a 5 V source, V_C at one time constant is about 5 x 0.632 = 3.16 V. Initially V_C = 0 and current is 5/1000 = 5 mA. At long times the ideal current approaches zero.

For a five-volt source, a 1000-ohm resistor and an initially uncharged 0.001-farad capacitor, the time constant is one second. The capacitor voltage is about 3.16 volts at one second and approaches five volts.

Would you believe it?

In an ideal resistor-capacitor charging circuit, capacitor voltage approaches its final value gradually rather than jumping there instantly.

OpenStax: RC circuits

Question 1 What is tau for the example? 1 s 1000 s 0.001 s

Question 2 At t = tau, approximately what is V_C? 5.00 V 0.00 V 3.16 V

Question 3 What is initial current in the stated model? 5000 mA 5 mA 0 mA

Question 4 As t grows very large, what happens to current? It approaches zero It stays at 5 mA It grows without limit

Question 5 Does disconnecting a capacitor guarantee that its stored energy is gone? Yes Only if it looks small No

Review your work

0correct

Complete the questions to review each answer.

View dashboard