Power, Conservation & Dissipation

Power

• Power is the rate at which energy is transferred, or the rate at which work is done (recall both equations):

$$P = \frac{E}{t} \qquad P = \frac{W}{t}$$

• $P$ power in watts, W; $E$ energy transferred in J; $W$ work done in J; $t$ time in seconds, s
• An energy transfer of 1 joule per second is a power of 1 watt
• Two motors lift the same weight through the same height, so they do the same work; the one that does it faster has the higher power
• Worked example: a kettle transfers 360 000 J in 2 minutes. $P = \dfrac{360\,000}{120} = 3000$ W $= 3$ kW