Question

A kettle of water has just boiled. At that moment the water inside contains roughly 775 kJ of thermal energy. a) After three hours resting on the counter, the water in the kettle has cooled to room temperature, losing about 170 kJ of thermal energy in the process. Where did that thermal energy go, and by what process(es)?

b) Has the quality of the water’s thermal energy increased or decreased? Explain.

You have just invented a new light bulb which is 80% efficient. You install a 100 Watt

version of the light bulb above your desk. What is the rate at which it provides HEAT?

a) 80 Calories b) 80 Watts c) 20 Watts d) 20 Joules

As the temperature of an object increases, the dominant electromagnetic wavelength in its thermal glow

a) increases b) decreases

c) remains unchanged d) becomes more red

Answer 1

1. The electrical energy passed through or conducted through the coils is converted to thermal energy. The heat is getting dissipated to water and increases the kinetic energy of the water molecules. Water boils when its saturated vapor pressure equals atmospheric pressure. When the hot water is kept, its heat energy is getting dissipated to atmosphere by convection. Heat transfer continues from higher temperature to lower temperature area, until the heat energy of both the systems reach an equilibrium.

2. a. 80 calories.

If a 100Watt light bulb is used it can consume 100 Joules of electrical energy per second. If the light bulb has 80% efficiency, it means it consumes 80Joules of electrical energy per second and converts into heat.

3. b) decreases

As the temperature of an object increases, the dominant electromagnetic wavelength in its thermal glow