 Hello, friends. This is Dheeraj. So today also I have taken up a question. And this question is from radioactivity chapter. So let's see what this question is about. So you can see a question, which is there in your screen. A charge capacitor of capacitance C is discharged through a resistance R. So whenever you hear such thing, immediately that formula comes into mind. The charge at any point in time should be equal to q0 e to the power minus T by RC. So this is a formula for discharging of the capacitor. Then a radioactive sample decays with an average life tau. So average life is tau. So lambda is equal to what? 1 by tau. Because tau is equal to 1 by lambda. Find the value of R for which the ratio of electrostatic field energy stored in the capacitor to the activity of the radioactive sample remains constant in time. Now, energy as a function of time in the capacitor, how will we find out? Energy is q square by 2C, isn't it? And charge at any point in time is given by equation number 1. So I can use equation number 1 over here and write the sound as this will be q0 square e to the power minus 2T by RC. This is q square that divide by 2C. And the ratio of this, the energy to the activity should be constant. Now activity is what? Activity of a radioactive sample is dn by dt. And dn by dt is, if I just talk about magnitude of the activity is lambda times n. And n as a function of time is given as n0 e to the power minus lambda t. So this is what we have. And now if I take the ratio, the ratio should be independent of time. So if I take ratio of energy with activity, let's say activity just dn by dt, if I write it like this, what I'll get is q0 square by 2C lambda n0 e to the power minus 2T by RC multiplied by e to the power lambda t. So basically this entire thing, this thing is a constant. So let that be a constant k. This is k into e to the power minus of 2 by RC minus lambda times t. Now if this ratio is independent of time, then what should happen? The ratio is independent of time, then coefficient of this time should be equal to 0. So 2 divided by RC minus lambda should be equal to what? 0. We need to find what? We need to find the value of r. Now the value of r from this, you will get it by solving this equation. So 2 by RC, 2 divided by RC is equal to lambda. So r is equal to what? r is equal to 2 by C times lambda, where C is the capacitance. So this is how you solve this particular question. So I hope you have learned something today. In case you have any doubts, please feel free to get in touch with us. Thank you.