The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another (example: from light energy to chemical energy in food in photosynthesis)

The second law of thermodynamics says that the entropy of any isolated system always increases. Entropy is a gradual decline to disorder. A simple way to think of the second law of thermodynamics is that a room, if not cleaned and tidied, will invariably become more messy and disorderly with time. Heat is a less ordered form of energy – this is why in most energy transformations, some energy is “lost” as heat.

Example: A 100 Watt bulb uses 100 J/sec of electrical energy. If it is 20% efficient, then the bulb converts 20% of the electrical energy into light and 80% is wasted by being transformed into heat (ever felt a hot light bulb?).

Explain how the First Law of Thermodynamics (conservation of energy) occurs in this example.

Explain how the Second Law of Thermodynamics (law of entropy) occurs in this example.

Energy Conversions

Energy: The basic unit of energy is a Joule (J). Other units are calorie, kilojoule, British Thermal Unit (BTU), and therm.
1000J = 1 kJ
1000 cal = 1kcal
1 cal = 4.184 J
1 BTU = 1.05 kJ
1 therm = 100,000 BTU

Suppose a TV uses 200 J of energy. How many BTUs is this?

Sandy went to the gym and burned 500 kcal of energy. How many joules is this?

Killowatt-hours: kWh is kilowatt-hour. This refers to the amount of energy in kW consumed in one hour. This is actually a unit of energy, not time.

1 kWh = 1 kW x 1 hr = 3.6 x 106 J

Example: What is the energy consumed when using 2kW for 3 hours?

2 kW x 3 hours = 6 kWh

What is the energy consumed when using 5W for 2 hours?

Suppose a hair dryer uses 20 kJ of energy. How many kWh is this?