Electrical Engineering ⇒ Topic : Energy

Energy

Energy is defined as the ability to do work. Energy is spent when work is done because energy has to be spent to maintain a force when that force acts through a distance (mechanical work). In electricity, the total electrical energy spent is the rate at which the work is done multiplied by the length of time during which work is done. In other words, electrical energy (E) is equal to the electrical power (P) multiplied by time (t)

                                                E = P . t                           ..........(1)

or                                             E = Vlt = I²Rt                   ...........(2)

In the above equation, if V is in volts, / is in amperes and t is in hours, the energy E will be in watt hours. If t is in seconds, E will be in watt seconds or joules (1 joule = 1 watt second). Electrical energy is purchased and sold in units of kilowatt hours (1 kilo watt hour = 1000 watt hours = 3.6 x 10⁶ J).

In the layman's terminology, electrical energy is always expressed in terms of units for lighting and heating in a residential house. The energy consumed in a few tens of units per month. On the other hand, in large electric generating stations,the power output is very large, say 200 Megawatts. If this power is generated for 10 hours a day, the total energy generated is 200 x 10 = 2000 Megawatthours.Thus, in practice the unit of electrical energy used depends on the magnitude of energy produced. In the case of an electricity generator, the unit of megawatt- hour is more appropriate.

ENERGY

To transfer a charge through an element, work is done. To determine the energy being supplied to the element or by the element to the rest of the circuit, we should know both the polarity of the voltage across the element and the direction of the current through the element. If a positive current enters the positive terminal of an element, an external force must drive the current.Therefore, the external force delivers energy to the element. On the other hand, if the current leaves the positive terminal or enters the negative terminal, the element is delivering energy to the external circuit.

In Figures 1(a) and 1(b), the element is absorbing energy whereas in Figures 1(c) and 1(d), the element is delivering energy.

                                                   figure (1)     Different voltage-current relationships.