The half life of something that is decaying exponentially is the time taken for the value of a decaying variable to halve.

Half Life of a Radioisotope

The most common use of half-lives is in radioactive decay. The activity is given by the equation:

${\displaystyle A_{t}=A_{0}e^{-\lambda t}}$

At t=t_½, A_t = ½A₀, so:

${\displaystyle {\frac {A_{0}}{2}}=A_{0}e^{-\lambda t_{\frac {1}{2}}}={\frac {A_{0}}{e^{\lambda t_{\frac {1}{2}}}}}}$

${\displaystyle 2=e^{\lambda t_{\frac {1}{2}}}}$

${\displaystyle \ln {2}=\lambda t_{\frac {1}{2}}}$

Therefore:

${\displaystyle t_{\frac {1}{2}}={\frac {\ln {2}}{\lambda }}}$

It is important to note that the half-life is completely unrelated to the variable which is decaying. At the end of the half-life, all decaying variables will have halved. This also means that you can start at any point in the decay, with any value of any decaying variable, and the time taken for the value of that variable to halve from that time will be the half-life.

Half-Life of a Capacitor

You can also use this formula for other forms of decay simply by replacing the decay constant λ with the constant that was in front of the t in the exponential relationship. So, for the charge on a capacitor, given by the relationship:

${\displaystyle Q_{t}=Q_{0}e^{\frac {-t}{RC}}}$

So, substitute:

${\displaystyle \lambda ={\frac {1}{RC}}}$

Therefore, the half-life of a capacitor is given by:

${\displaystyle t_{\frac {1}{2}}=RC\ln {2}}$

Time Constant of a Capacitor

However, when dealing with capacitors, it is more common to use the time constant, commonly denoted τ, where:

${\displaystyle \tau =RC={\frac {t_{\frac {1}{2}}}{\ln {2}}}}$

At t = τ:

${\displaystyle Q_{t}=Q_{0}e^{\frac {-RC}{RC}}={\frac {Q_{0}}{e}}}$

So, the time constant of a capacitor can be defined as the time taken for the charge, current or voltage from the capacitor to decay to the reciprocal of e (36.8%) of the original charge, current or voltage.

Questions

1. Radon-222 has a decay constant of 2.1μs⁻¹. What is its half-life?

2. Uranium-238 has a half-life of 4.5 billion years. How long will it take for a 5 gram sample of U-238 to decay to contain 1.25 grams of U-238?

3. How long will it be until it contains 0.5 grams of U-238?

4. Tritium, a radioisotope of Hydrogen, decays into Helium-3. After 1 year, 94.5% is left. What is the half-life of tritium (H-3)?

5. A large capacitor has capacitance 0.5F. It is placed in series with a 5Ω resistor and contains 5C of charge. What is its time constant?

6. How long will it take for the charge in the capacitor to reach 0.677C? (${\displaystyle 0.677={\frac {5}{e^{2}}}}$)

Worked Solutions