Humans were essentially hunters and gatherers. They had developed the use of fire. B â¦ ut they did not live a settled existence, instead following herds of animals on their migrations which were their primary source of food. Men were the hunters and responsible for the group's primary food source.
Emf - is the amount of energy of any form that is changed into electrical energy per coulomb of charge. pd - is the amount of electrical energy that is changed into other forms of energy per coulomb of charge. Sources of emf: Cell, battery (a combination of cells), solar cell, generator, dynamo, thermocouple.
1 AC generator needs DC excitation of rotor to generate magnetic field. 2  The generated EMF in the generator increases its efficiency but the same EMF is the waste in the motor as it reduces its mechanical efficiency.  In motor a back EMF is generated which opposes the EMF and hence reduces its mechanical efficiency.