watt
Summary
The watt (W) is the SI unit of power, the rate at which energy is transferred or work is done. One watt is one joule per second. The testable point is that a watt measures a rate, not an amount of energy.
Detail
Meaning: The watt is the SI unit of power. 1 W = 1 J/s, so a 60 W bulb converts 60 J of electrical energy every second. In SI base units, 1 W = 1 kg·m²/s³. A larger power means the same energy is transferred in less time.
Equation: P = W/t = ΔE/t, where P is power in watts (W), W is work in joules (J), ΔE is energy transferred in joules (J), and t is time in seconds (s). For a force on a moving object, P = Fv cosθ, where F is force in newtons (N), v is speed in m/s, and θ is the angle between them. In circuits, P = IV = I²R = V²/R, where I is current in amperes (A), V is potential difference in volts (V), and R is resistance in ohms (Ω).
On the MCAT: Typical questions give a person climbing stairs, a motor lifting a load, or a resistor heating, and ask for power. Find the work or energy, then divide by time. Circuit questions use P = IV or P = I²R. Expect to convert between W, kW, and horsepower-style data.
Common confusion: A watt is a rate, not an amount of energy. The kilowatt-hour on an electric bill is a unit of energy, with 1 kWh = 3.6 × 10⁶ J. Doing the same work in half the time doubles the power, but the work stays the same.
Sources
- University Physics
- Fundamentals of Physics
- Physics for Scientists and Engineers
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.