1. Jan 25, 2010

### henryc09

1. The problem statement, all variables and given/known data
A radioactive source emits electrons. The number of electrons emitted per second
N(t) decreases with time according to the equation:

dN/dt = −1.8 × 10−5N(t) ,
where t is expressed in seconds. If N = 12 electrons/sec at t = 0, after what time t
has N fallen to the value 4.0 electrons/sec? [7]
What is the half-life of the radioactive source in seconds? [3]

2. Relevant equations

3. The attempt at a solution
I have the first part done, but am confused about the second part. How can you tell the half life of the material from the information given? N is just the amount of electrons given off per second, and the half life is the amount of time taken for the amount of substance to halve right? I don't see how they can be directly related. Any ideas?

2. Jan 25, 2010

### Stonebridge

One way is to just work out how long it takes for the number of electrons given off per second to drop from 12 to 6. That represents the half-life time.

3. Jan 25, 2010

### henryc09

OK I was thinking that must be the only way. Thanks :)