joule

Meaning: The joule measures energy transferred or stored. Work, kinetic energy, potential energy, heat, and electrical energy are all measured in joules. Because work is force times displacement, 1 J = 1 N·m = 1 kg·m²/s². In electricity, 1 J = 1 C·V, so a charge of 1 C moved through a potential difference of 1 V gains or loses 1 J. Power is energy per time, so 1 W = 1 J/s.

Equation: W = Fd cosθ, where W is work (J), F is force (N), d is displacement (m), and θ is the angle between force and displacement (radians or degrees, dimensionless). Kinetic energy is KE = ½mv², with m in kg and v in m/s, giving J.

On the MCAT: Passages often give energy in other units, and you must convert. Common ones are calories (1 cal ≈ 4.18 J), kilocalories (nutritional Calories, 1 kcal ≈ 4.18 kJ), and the electron-volt (1 eV ≈ 1.6 × 10⁻¹⁹ J). In thermodynamics, ΔG and ΔH are often given in kJ/mol. Dimensional analysis questions may ask you to show that a quantity has units of kg·m²/s², and so is an energy.

Common confusion: Students mix up the joule with the watt. A joule is an amount of energy, while a watt is a rate, 1 J/s. Also, kilowatt-hours are a unit of energy, not power: 1 kWh = 3.6 × 10⁶ J.