# Characteristic of DC source

### Physical background

The simple physical background representative a simple circuit in which a battery of emf ε and internal resistance Ri drives a current I through an external resistor of resistance. The external resistor is usually referred to as the load resistor R. It could stand for either an electric light, an electric heating element, or, maybe, an electric motor. The basic purpose of the circuit is to transfer energy from the battery to the load. If we connect the source to the load R, the current I is

Figure 1 Equivalent circuit of the DC source

Figure 2 Electric circuit of DC source with the load R

The internal resistance Ri causes the terminal voltage USV = V1V2 between points 1 and 2 (Fig. 2) to depend on the current

We can use eq. (2) to determine the electromotive voltage ε and internal resistance Ri measuring the loading characteristics USV = f(I). After rearrangement eq. (1), we obtain another useful expression

Eq. (3) makes it possible to determine the electromotive voltage ε and internal resistance Ri measuring the loading characteristics 1/I = f(R).

Figure 3 Dependence of the terminal voltage USV on the current I, with the linear fit

Figure 4 Dependence of the inverted value of the current 1/I on loading resistance R

#### References

[1] HALLIDAY, D., RESNICK, R., KRANE, K.S.: Physics : extended version. Volume two – 4th ed. New York : John Wiley & Sons, Inc., 1992. 1216 p. ISBN 0-471-54804-9.

[2] SCHAUER, F., KREMPASKÝ, J.: Fyzika I a II, Zlín : Univerzita T. Bati, 2004. 611 s.