A resistance $R = 12\, \Omega$ is connected across a source of emf as shown in the figure . Its $emf$ changes with time as shown in the graph . What is the heat developed in the resistance in the first four seconds ? ............. $J$
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$H=\int_{0}^{4} \frac{E^{2}}{R} d t=\int_{0}^{4} \frac{(6 t)^{2}}{12} d t=64 \,J$
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