If the change in energy (ΔE) is measured in Joules and the time interval (Δt) is in seconds, then the power (P) would be in units of Watts. If you pick a textbook off the ground and put it on a table over an interval of 5 seconds, that would be a power of about 2 Watts.
What about thermal energy? When you heat up water, the water increases in thermal energy. The change in thermal energy can be calculated with the following equation.
In this expression, m is the mass of the water (in grams) and C is the specific heat capacity in Joules per gram per degree Celsius. Don’t confuse this ΔT with the time interval. The capital T stands for temperature. The last part of this energy calculation is the decrease in energy in the battery. You can’t really measure the actual battery energy—instead you have to calculate it. If you find both the electric current and the voltage across the battery, you can find the power.
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