Suppose a 6-volt battery is connected across lamp whose resistance is `20 Omega` through a variable resistor as shown in (Fig. 3.11). If the current in the circuit is `0.25 A`, calculate the value of the resistance from the resistor which must be used ?
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Explanation

Here, supply voltage, `V = 6 V`
resistance of the lamp, `r = 20 Omega`
current in the circuit, `I = 0.25 A`
Let `R_1` be the resistance rerquired from the variable resistance (a rheostat, Rh) to be palced in series with the lamp.
Total resistance in the circuit, `R = R_1 + r = R_1 + 20 Omega`
Applying Ohm's law, `I = (V)/(R)` or `0.25 A = (6 V)/(R_1 + 20 Omega)`
Thus, `R_1 + 20 Omega = (6 V)/(0.25 A) = 24 Omega or R_1 = 4 Omega`.