In the S.I. system, the unit of energy is-

Options

A. electron - volt
B. joule
C. calorie
D. erg

Answer

To determine the unit of energy in the S.I. system, we can analyze the given options: joule, calorie, erg, and electron volt.

### Step-by-Step Solution:

1. **Identify the S.I. unit of energy**:
The S.I. unit of energy is defined as the joule (J).

2. **Review other units of energy**:
- **Calorie**: 1 calorie is defined as the amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is equivalent to approximately 4.18 joules.
- **Erg**: 1 erg is a smaller unit of energy, defined as 10^-7 joules.
- **Electron Volt**: 1 electron volt (eV) is the energy gained by an electron when it is accelerated through a potential difference of 1 volt. It is equivalent to approximately 1.6 x 10^-19 joules.

3. **Compare the units**:
- Joule (1 J) is the standard unit of energy in the S.I. system.
- Calorie (4.18 J) is a larger unit than a joule, but it is not the S.I. unit.
- Erg (10^-7 J) is much smaller than a joule.
- Electron Volt (1.6 x 10^-19 J) is also much smaller than a joule.

4. **Conclusion**:
Since the question asks for the unit of energy in the S.I. system, the correct answer is **joule**.

### Final Answer:
The unit of energy in the S.I. system is **joule**.

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