Question

Unit of power is :

• A. Joule
• B. Newton
• C. Watt
• D. Pascal

Hint:

Think of Energy (Joule) like the total "gas in a tank," while Power (Watt) is how fast you are "burning that gas" as you drive. Fast driving requires more Watts (power) even if the total Energy (Joule) used is the same.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question is asking for the standard SI unit of "Power," which is defined as the rate at which work is done or energy is transferred.
Step 2: Key Formula or Approach:
The formula for power relates work (or energy) and time:
\[ \text{Power} (P) = \frac{\text{Work} (W)}{\text{Time} (t)} = \frac{\text{Energy} (E)}{\text{Time} (t)} \]
Step 3: Detailed Explanation:

• Definition of Watt: The SI unit for power is the Watt (W), named in honor of the 18th-century inventor James Watt. One Watt is defined as the rate of energy transfer of one Joule per second.
  
• Relating Units: This relationship is expressed as \( 1 \text{ W} = 1 \text{ J/s} \). In terms of fundamental SI base units, a Watt is \( 1 \text{ kg} \cdot \text{m}^2 \cdot \text{s}^{-3} \).
  
• Comparison with Other Units:
  - Joule (A): This is the unit for energy or work, representing a quantity, not a rate.
  - Newton (B): This is the unit for force.
  - Pascal (D): This is the unit for pressure.
  
• Practical Context: We encounter the Watt in everyday life, such as in the power rating of a light bulb. For example, a 60W bulb converts 60 Joules of electrical energy into light and heat every second. A higher wattage signifies a faster rate of energy conversion.
  
• Larger Units: For larger-scale applications, such as power plants or engines, power is often measured in Kilowatts (kW), Megawatts (MW), or Horsepower (hp), with \( 1 \text{ hp} \) being approximately \( 746 \text{ W} \).
  

Step 4: Final Answer:
The correct SI unit for power, which quantifies the rate of energy transfer, is the Watt.

Answer 2

Imagine a small machine that performs \(200\text{ J}\) of work in \(4\text{ s}\). Working out what that means numerically for each of these units checks which one is actually built to describe this rate.

1. Joule: \(200\text{ J}\) is simply the total energy used, a fixed quantity, it says nothing about how quickly that energy was delivered, so Joule alone cannot describe a rate.
2. Newton: Newtons describe force, a push or pull, and have no built-in relationship to how much energy is used over what time, so it cannot represent this machine's rate of working either.
3. Watt: Dividing the energy by the time, \(200\text{ J}/4\text{ s} = 50\text{ J/s}\), and one Joule per second is, by definition, one Watt. So this machine's power output is \(50\text{ W}\), the unit built exactly to express energy per unit time.
4. Pascal: Pascals measure pressure, force spread over an area, a quantity that has nothing to do with how fast the machine is doing work, so it doesn't fit this scenario at all.

Only the Watt is defined as, and numerically equals, energy delivered divided by time taken, which is exactly what power means.

Therefore, the correct answer is Watt.