How this V×I ×t came??
Firstly, we know that, Heat = V×I×t.
The equation of power is given by VI i.e. P = VI.
Therefore, the energy supplied to the circuit by the source in time t is P × t,
that is, VIt. This energy gets dissipated in the resistor as heat. Thus for a steady current I, the amount of heat H produced in time t is
H = VIt