Chapter 3 · Matter and Energy
3.8Energy
10 min · two checks
Predict
Which SI unit measures energy, and how is a calorie related to it?
The idea
- Distinguish kinetic and potential energy.
- Convert between joules and calories.
Energy is the capacity to do work or to transfer heat. Kinetic energy is energy of motion and grows with mass and with the square of speed. Potential energy is stored energy of position or arrangement. A ball at the top of a ramp has gravitational potential energy. Chemical potential energy sits in the arrangement of electrons and nuclei; reactions release or absorb it when atoms rearrange.
The SI unit is the joule (J). One joule is a modest amount: lifting an apple about a meter. Chemists also inherit the calorie, now defined as exactly 4.184 J. A dietary Calorie (capital C) is a kilocalorie, 4184 J. Electrical energy is often billed in kilowatt-hours, another energy unit, not a power unit used as energy. Convert with factors; do not treat J, cal, and kcal as interchangeable names.
Keep these
- Kinetic: motion. Potential: position or chemical arrangement.
- 1 cal = 4.184 J. 1 Calorie (food) = 1 kcal = 4184 J.
- Chemical reactions rearrange chemical potential energy.
Worked path
A snack label says 180 Calories. Express that energy in kilojoules.
Observe
Food Calories are kilocalories. 180 Cal = 180 kcal.