In words, the voltage has value `1` up until time `t = a`. Then it is turned off.

We have a "rectangular pulse" situation and need to use this formula:

`V(t) = u(t − a) − u(t − b)`

In our example, the pulse starts at `t = 0`, so we use `u(t)`, and finishes at `t = a`, so we use `u(t − a)`.

So the required function is:

`V(t)=1·[u(t)-u(t-a)]` `=u(t)-u(t-a)`

Graph of `V(t)=u(t)-u(t-a)`, a shifted unit step function.