# Question Video: Identifying the SI Unit for Energy Chemistry • 10th Grade

What is the SI unit for energy? [A] Watt (W) [B] Joule (J) [C] Mole (mol) [D] Kilogram (kg) [E] Electron Volt (ev)

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### Video Transcript

What is the SI unit for energy? (A) Watt, (B) joule, (C) mole, (D) kilogram, or (E) electron volt.

SI units make up the International System of Units. These units are the fundamental units in the metric system. They are a set of standardized base units derived from fundamental constants. We want to know the SI unit for energy, which is a system’s capacity for doing work. Let’s run through the units in our answer choices to see which ones measure energy.

The watt measures power, which describes how much energy is transferred per unit time. While power is related to energy, the watt does not measure energy, so this is not the correct answer. The joule, however, does measure energy. The mole measures the amount of substance, so we can eliminate this option. We can also eliminate the kilogram, which is the unit for mass. Finally, the electron volt also measures energy. So, we have two units here that measure energy. Which one is the SI unit? Why, that’s the joule. The joule is the energy needed to raise the temperature of one gram of water by one over 4.184 degrees Celsius.

The electron volt is defined as the energy gained by an electron when it is accelerated through a potential difference of one volt. We can convert between the electron volt and the joule using this expression, which shows the amount of energy described by the electron volt is incredibly small compared to the joule. This makes the electron volt very useful for describing the energy changes involved in atomic and nuclear processes. As the joule is larger, it’s more useful for describing a wider range of processes, which is why the joule is the SI unit instead of the electron volt. So, what is the SI unit for energy? It’s answer choice (B), the joule.