What is the relationship between electric current and voltage?

Basically voltage is directly proportional to current (V = I*R) but in case of continuous power sourcing we find that any change in the amount of supplied current affects the supplied voltage. This means that if a transformer is supplying constant power (P) to an electrical system, changing the electrical load (motors, actuators) will affect the supplied voltage (P = V*I). So, in case of high load, we get system voltage drop and in case of low load, system voltage increases as P = V*I.