A styrofoam coffee cup is so light and it is such a good insulator, that its mass and heat capacity may often be neglected in measurements like this one, thus making the analysis much simplier.
a. If 150 grams of water at room temperature, 20°C is placed in a Styrofoam cup, how long will it take to raise the temperature of the water to 30°C? Assume the the power supply supplies a current of 2 amperes at a voltage of 10 volts.