What Is the SI Unit of Power?
In Physics, work is done when a force acting on an object results in its displacement. Work done is not determined by the amount of time the force took to cause the displacement. Sometimes the work is done quickly, and other times it might be slow and take longer. If a bodybuilder and a sixth-grader carry a weight of 10 kgs up a flight of stairs, then the bodybuilder would most likely take less time than the sixth-grader to finish the same job. Here we can say that the bodybuilder has a higher power rating than the sixth-grader.
So we see it is the power that has to do with the rate at which work is done. We will look into a few concepts around power, work, and energy in this article and discuss the unit of power in si unit and provide you with units of power list in different measurement systems.
Formulas for Work and Power
The word “work” has a different meaning in the world of Physics. In Physics, work done is given by force applied on an object to move a distance (d), and its formula is:
W = F * d.
Even after applying a lot of force, if an object does not move, then in the eyes of a physicist, no work is done.
The word “power” is also used differently by physicists. Power is the ratio of work and time, i.e. the time taken to complete a work gives the power of an entity. It is expressed mathematically as:
Power = \[\frac{Work}{Time} \] or \[P = \frac{W}{t}\]
Power is the rate at which energy (heat and electrical) is transferred. While studying electricity, you would find the power is defined as:
Power = V * I, where V is the voltage within the circuit, and I is the current through the circuit.
We could derive another formula for power from the formula for work. Since Work = force * distance and power =worktime, then power = force * distancetime.
Now we know that velocity = distance/time hence:
Power = Force * Velocity = F * V
What is the SI Unit of Power in Physics?
To define units of power let us look into how the SI system works. The SI or metric system has only 7 base units, and all the other units are derived from these base units. In the metric system, mass is measured in kilograms, length in metres, and time in seconds. Unit of force (which is mass times acceleration) is kg.m/s2. From this, we can get the unit of work as kg.m2/s2. But this unit gets cumbersome to write in every equation, so scientists have defined another unit Joule (J), which is named after the physicist James Prescott Joule. I Joule = 1 kg.m2/s2and it is the unit of energy in the SI system.
The unit of power is the watt, which is abbreviated by W. It has an SI base unit and several derived units that are used to measure other physical quantities associated with energy or work. The International System (SI) states that one joule per second equals one watt. One kilowatt-hour represents 3600 joules, while a megawatt-hour stands for 1000000 watts/3600000 seconds - meaning it's equivalent to roughly 12 million horsepower! Here are some additional facts about the unit of power:
When measuring electrical voltage in volts, amperage in amps, or resistance in ohms you're using three different forms of this fundamental measurement
Power can be calculated knowing only two of the three aforementioned electrical values
Power is work divided by time; hence the SI unit of power is joules/second. Scientists have given a name for this unit, and the unit of power in the SI unit is Watt (named after James Watt, the Scottish inventor of steam engines that helped launch the Industrial revolution.).
1 Watt = 1 Joule /second = 1 kg m2 s-3
Submultiples and Multiples of Watt
The SI multiples and submultiples of Watt are listed in the table below:
Units of Power in Other Systems and their Equivalent in Watts
You can find below units of power list with formula on how to convert them into Watt
How Can Varying Power Be Measured?
In certain setups like houses, the power is used to varying degrees. For example, we use less electricity during the day than at night when all the lights are on. In these situations, power is expressed in three ways:
Instantaneous Power - This is the power measured at any given instant of time.
Average Power - When you measure power over long periods of time, you can get average power. This can be calculated by plotting a graph of power vs time and finding the area under the curve, which gives the total work done, and then dividing it by the total time.
Peak Power - The maximum value of instantaneous power in a specific system over a long period of time is peak power. In systems like car engines and stereo systems, peak powers occur much higher than the average power. But peak power is maintained only for a very short duration to avoid damages.
The Unit of Power is used in Various Areas:
Electrical Measurements
One watt is equal to 0.74549 horses or 550 foot-pounds per second
When measuring electrical voltage in volts, amperage in amps, or resistance in ohms you're using three different forms of this fundamental measurement
Power can be calculated knowing only two of the three aforementioned electrical values
Watts and horsepower are not interchangeable, as one watt is equal to 0.74549 horses or 550 foot-pounds per second.
Voltage - Unit is power is used in measuring voltage
Voltage is the electrical potential difference between two points in a circuit.
It is measured in volts (symbolised by V).
One watt is equal to one volt applied across a resistance of one ohm.
Ampere - Unit is power is used in measuring amperes
The rate at which electrical energy flows through a circuit or part of an electronic system, defined as one coulomb per second.
One watt is equal to 0.001335 ampere
The watt is most commonly used when measuring electrical power, though it can be applied to other forms of power as well. In the United States, we use watts to measure our electricity usage on our monthly energy bills. We also rate motors in horsepower, which is a unit of power derived from the watt. Many appliances list their wattage or horsepower rating in order for you to know how much power they consume. For example, if you have a large air conditioner that consumes 5000 watts, your electric bill will likely be higher than if you had a small air conditioner that consumed only 1000 watts.
Horsepower- Unit is power is used in measuring Horsepower
A unit of power derived from the watt, typically used to rate motors.
One horsepower is equivalent to approximately 0.74549 kilowatts or 550 foot pounds per second
As you can see, the watt is a very versatile and important unit of measurement! Hopefully this article has helped you become more familiar with it. If you have any additional questions, please feel free to ask in the comments section below.
FAQs on Unit of Power in Physics
1. What is power in Physics, and what is its standard unit?
In Physics, power is defined as the rate at which work is done or energy is transferred. It essentially measures how quickly energy is used or converted. The standard SI unit for measuring power is the Watt (W), named in honour of the engineer James Watt.
2. How is the unit of power, the Watt, calculated?
A Watt is directly related to energy and time. One Watt is defined as the transfer of one Joule of energy per second. Therefore, if an appliance uses 10 Joules of energy in 2 seconds, its power is 5 Watts. The basic formula is Power (P) = Energy (E) / Time (t).
3. What are some other common units used for power besides the Watt?
Yes, while the Watt is the standard unit, other units are frequently used in different applications. These include:
- Kilowatt (kW): Equal to 1,000 Watts. It's commonly used to measure the power of household appliances, electric motors, and tools.
- Horsepower (hp): An older unit still widely used in the automotive industry to describe the power of engines. One mechanical horsepower is approximately equal to 746 Watts.
4. What is the main difference between the unit of power (Watt) and the unit of energy (Joule)?
The key difference is that a Joule (J) measures the total amount of energy, while a Watt (W) measures how fast that energy is being used. Think of it like water: Joules represent the total volume of water in a tank, whereas Watts represent the flow rate of water coming out of the tap.
5. My electricity bill is measured in kilowatt-hours (kWh). Is that a unit of power?
This is a common point of confusion. A kilowatt-hour (kWh) is actually a unit of energy, not power. It represents the total amount of energy consumed over a period. It is calculated by multiplying power (in kilowatts) by time (in hours). For example, a 1 kW device running for one hour consumes 1 kWh of energy.
6. Why do we still use horsepower (hp) for car engines instead of just using Watts?
The use of horsepower is largely due to tradition and consumer familiarity. It was originally created to help people understand the power of a steam engine in terms of a familiar source—a horse. Today, it remains embedded in the automotive industry because customers have a better intuitive feel for what 200 hp means for a car's performance compared to its equivalent in kilowatts (about 149 kW).
7. Is the 'power' of a lens, measured in Dioptres, related to the unit of power in Physics?
No, they are completely unrelated concepts, despite using the same word. The power of a lens, measured in Dioptres, describes its ability to converge or diverge light and is related to its focal length. The unit of power in Physics, the Watt, measures the rate of energy transfer. They are two different physical quantities.
