If you know the current entering in the device as well as voltage across the device you will get the power used by the device. In mathematical form you can say
Above equation only tell us how much power is pumped into a circuit but this equation doesn’t say about how this power is used up. let’s take example consider two leads black box which content devices such as a resistors, lamps, transistors .using ammeter and voltmeter you can measured voltage and current. So from voltage and current you can measure power.
But it is also important to know more about the loss of power in black box. Power loss in circuit also generates heat. You can compute the heat in circuit directly. The total power pumped into this circuit gets converted into useful work and heat. As per above we notice that some of the power is wasted within the internal resistance of the battery. Internal resistance including resistance of the wires, resistance of LED. We cannot separate the heat from the light so we must apply the generalized power.
What is thermal Heat conduction and thermal resistance?
Let’s see how the energy transferred in heating. Within a gas heat transfer represents the energy between the gas molecules. Gas molecules at high temperature moved more quickly because they have a kinetic energy. As the gas introduced into other colder temperature the gas moving fast. In nonmetals heat transfer is result of the transfer of energy due to lattice vibrations.
In Solid atoms vibrating in one region to other region have less energetically vibrating atoms.
In vibrating atoms the transfer of heat can be enhanced by the cooperative motion in form of propagating lattice waves. The thermal conductivity of nonmetals depends on the lattice structure.
In terms of metals heat transfer is a result of lattice vibration effects as well as kinetic energy transfer due to mobile free electrons. At room temperature free electrons are moving quite fast. It is possible to treat these electrons like a dense gas. As you added heat it is capable of transporting this energy at lower temperature. Free electron goes down when due to increased lattice vibrations and increase in thermal velocity component. In an applied field it becomes harder to influence the free electronics with the applied field. Metals are the best thermal conductors due to the additional free electronics.
When one end of block material is placed at hot temperature heat will be conducted through the material to the colder end. The rate of heat transfer depends on the thermal resistance of the material. It also depends on the materials thermal resistivties of various materials. Thermal resistance has units of C/W.
Importance of Heat Production
Heat production has its place in electronics but most of the time heat represents power loss to be minimized whenever possible. All real circuit components contents inherent internal resistance. The internal resistance can be neglected. In some situations they cannot be ignored. Major problems arise when temperature of a circuit component increase.