Free Electrons | Voltage Source | Constant Voltage Source

Free Electroncs

Free Electrons: The valence electrons of different materials have different energies. The larger the energy of a valence electron, the weaker it is bound to the nucleus. In some substances, particularly metals, the valence electrons possess huge energy that electrons are very loosely attached to the nucleus. These loosely attached valence electrons motion at random within the material and are known as free electrons. The valence electrons which are very loosely connected or attached to the nucleus are called free electrons.

Free Electrons

The free electrons can be easily removed or detached by applying a very little amount of external energy. As a matter of fact, these are the free electrons that determine the electrical conductivity of any matter. On this basis, conductors, insulators, and semiconductors can be explaind as under :

  • A conductor is a substance which has a huge number of free electrons. When the voltages are applied across a conductor, the free electrons flow towards the positive terminal of supply, constituting the electric current.
  • An insulator is a material that has practically no free electrons at room temperatures. Therefore, an insulator can not conduct current under the influence of voltage.
  • A semiconductor is a material that has very small free electrons at ordinary temperatures. Consequently, under the influence of voltage, a semiconductor normally conducts no current.

Voltage Source

Voltage Source

Any device that generates voltage output continuously is called voltage source. There are two main types of voltage sources, namely; direct voltage source (DC) and alternating voltage source (AC).


(i) Direct voltage source. A device that generates direct voltage output continuously is known as a direct voltage source.

Well, known examples are cells and d.c. generators. An important property of a (DC) direct voltage source is that it keeps the same polarity of the output voltage i.e. positive and negative terminals always remain the same. When load resistance RL is connected across such a source,*current passes from positive to the negative terminal via the load [See Fig. 1.8 (i)]. This is known as direct current because it has just uni-direction. The current has uni-direction as the source remains the same polarity of output voltage. The opposition or resistance to load current inside the d.c. the source is called internal resistance Ri. The equivalent circuit of a direct current source is the generated e.m.f. Eg in series having internal resistance Ri of the source as shown in Fig. 1.8 (ii). Referring to Fig. 1.8 (i), it is clear that:

direct current and free electrons

(ii) Alternating voltage source. A device that generates alternating voltage output continuously is known as alternating voltage source e.g. a.c. generator. An important property of the alternating voltage source is that it periodically changes the polarity of the output voltage. When load impedance or resistance ZL is connected across such an (AC) Source, current passes through the circuit that periodically changes in direction. This is called alternating current.

Alternating voltage source and free electrons

The opposition to load current inside the alternating current. the source is known its internal impedance Zi. The equivalent circuit of an alternating current. the source is the produced e.m.f. Eg (r.m.s.) in series with internal impedance Zi of the source as shown in Fig. 1.9 (ii). Referring to Fig. 1.9 (i), it is clear that :

numirical

Constant Voltage Source

A voltage source that has very small or low internal *impedance as compared with external load impedance is called as a constant voltage source.

Constant Voltage Source

In such a case, the output voltage approximately remains the same when load current changes. Fig. 1.10 (i) illustrates a constant voltage source. It is a direct current source of 6 V with internal resistance Ri = 0.005 Ω. If the load current changes over a wide range of 1 to 10 A, for any of these values, the internal drop across Ri (= 0.005 Ω) is blown than 0.05 volt. so that, the voltage output of the supply is between 5.995 to 5.95 volts. This can be taken as constant voltage compared with the large variations in load current. Fig. 1.10 (ii) shows the graph for a constant voltage source. It may be observed that the output voltage always remains constant in spite of the changes in load current. Thus as the load current changes from 0 to 10 A, the output voltage must remains the same (i.e.V1 = V2 ). A constant voltage source is described as shown in Fig. 1.11.

Example 1.1. A lead-acid battery fitted in a truck develops 24V and has an internal resistance of 0.01 Ω. It is used to supply current to headlights or front lights etc. If the total load is equal to 100 watts, find :
(i) voltage drop in internal resistance
(ii) terminal voltage

Solution.
Generated voltage, Eg = 24 V Internal resistance, Ri = 0.01 Ω Power supplied, P = 100 watts
(i) Let I will be the load current

Numerical  1

Comments: It is clear from the example that when the internal resistance of the supply is quite small, the voltage drop in internal resistance is very small. Therefore, the terminal voltage substantially remains fixed and constant, and the source act as a constant voltage source irrespective of load current variations.

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