Title: Advancing Physics A2 Teacher Edition
Author: The Institute of Physics

Discharge and time constants

Question 70S: Short Answer

^Teaching ^Notes   ^|   ^Key ^Terms   ^|   ^Answers

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Practical Advice
These questions require familiarity with time constants and the exponential function.
Level
Practice

Key Terms
exponential decay processes, resistance, capacitance
Supplementary Terms
time constant

Answers and Worked Solutions
1.	25 s 
2.	30 μA
3.	For example, C = 10 μF and R = 100 kΩ;  for example, C = 200 μF and R = 100 kΩ.
4.	69 s 
5.	8.6 × 10^2 s
6.	8.6 MΩ

A 250 μF capacitor is charged through a 100 kΩ  resistor.
1.	Calculate the time constant of the circuit.

2.	The initial current is 100 μA. What is the current after 30 s?

3.	Suggest values of R and C which would produce RC circuits with time constants of 1.0 s and 20 s.

4.	A 10 μF capacitor is charged to a potential difference of 20 V and then isolated. If its leakage resistance is 10 MΩ, how long will it take for the charge to fall to 100 μC?

A 100 μF capacitor is charged and connected to a digital voltmeter (which has effectively infinite resistance). The p.d. measured across the capacitor falls to half its initial value in 600 s. 
5.	Calculate the time constant of the discharge process.

6.	Calculate the resistance of the capacitor insulation.

Advancing Physics