SI Units - Online Tutor, Practice Problems & Exam Prep

Understanding the International System of Units (SI) is crucial for scientific calculations. The SI system is based on seven base units: the kilogram (kg) for mass, meter (m) for length, second (s) for time, kelvin (K) for temperature, mole (mol) for the amount of substance, ampere (A) for electrical current, and candela (cd) for luminous intensity. These units are foundational for various measurements such as perimeter, area, and volume. Perimeter is the sum of all sides of an object and is measured in meters (m). Area is the surface of an object, calculated using length times width, and expressed in square units like square feet (ft²). Volume represents the space occupied by a 3D object, determined by multiplying length, width, and height, resulting in cubic units such as cubic meters (m³). Mastery of these SI units and their application is essential in chemistry and other sciences for accurate measurement and problem-solving.

SI Units Concept 1

When it comes to many of the calculations you're going to see in chemistry, science related courses and math courses, you're going to see that the metric system is oftentimes used. Now the International System of Units, SI is related to the metric system and is based on seven base units. So here we have our seven images. Connected to them are their physical quantities, names, and symbols.

For the first one, we have an Advil and its physical quantity is discussing mass. For mass, the name of our SI unit is kilograms and the symbol for kilograms is kg. Next we have our ruler which represents length. The SI unit for length is meters, which is represented by m. For time the name would be seconds. Seconds is illustrated by s.

We have our thermometer, which deals with temperature. We know that there are three units per temperature, Kelvin, Celsius, and Fahrenheit. But it's Kelvin that represents our SI unit and it's capital K. And then for remaining three, we have the amount of a substance which is the mole. We'll go into greater detail about the mole later on. Its symbol is just mol.

Now you're going to see sometimes when we're doing chemistry calculations, we might write out the entire word mole or we might just use the symbol mol. They can be interchanged with one another now. Next we have electrical current, which is amperes, and that's just capital A. And then luminous intensity, the brightness of a light source. That's done by Candela, which is cd.

So these represent our seven SI units that you should commit to memory. And as we do different types of calculations, oftentimes we'll have to incorporate them within our calculations. Now that we've seen these examples, let's move on to our questions where we're going to investigate more about the SI units.

SI Units Concept 2

Now that we've talked about Sr. units, we can apply them to other areas. For a classroom, for instance, we're going to say here that we can take the SI units from up above and relate them to perimeter, area, and volume. First, let's take a look at perimeter. Perimeter can be thought of as this distance around an object. When it comes to a classroom lecture hall, all we have to do is add up all the sides of the object or figure, or in this case the room to determine its perimeter.

So if we take a look here, our typical classroom is 15 meters by 10 meters. To find the perimeter we just add up all the sides. So that'd be 15 meters plus 10 meters plus 15 meters plus 10 meters. When we add them all up together, the perimeter of the classroom is 50 meters. Next, we can look at the area of the classroom. Area can be thought of as measured surface of an object as length squared. Now it's not actually just length squared. The formal formula for area is area equals length times width.

So that same classroom. Now if we look at it in terms of feet, we could say here let's say that we re-examined it in terms of feet and we found these new measurements and let's say that the lengths were 48 feet and the widths were 32 feet. So we would take 48 feet times the 32 feet and that will give us our area. When we multiply those two together that give me 1536. And we're going to say here that if we're multiplying feet times feet, that would come out to be feet squared. Here we're not worrying too much about any greater detail. Our area here for the classroom would be 1536 feet.

Finally, we could talk about the volume of an object. If we're looking at volume, volume can be thought of as the space occupied by a 3D object as length cubed. Now, in actuality, the real formula is volume, which is V equals length times width times the height of the object. So here we have an example of a cube. We'd say here that its length is these 15 meters, its width is 10 meters, and its height is also 10 meters. So multiplying these together, 15 meters times 10 meters times another 10 meters, so we multiply that all together. That's going to give me 1500. And since it's meters times meters times meters, that becomes meters cubed.

Now notice for our area and our volume, we talked about length squared and here we have feet squared and here we talked about length cubed and here we got meters cubed. Taking the SI units that we examined up above again can be used in a lot of different ways. Here we're using it to determine the perimeter, area, and volume of different objects. Now that we've looked at this example, move on to the practice question on the bottom of the page.

Based on your knowledge of the SI base units, determine the units for the area of a typical chemistry laboratory.

• A. m²
• B. ft²
• C. km³
• D. in³

Here’s what students ask on this topic:

What are SI units?

SI units, or the International System of Units, are the standard units of measurement used worldwide in science and industry. They provide a consistent framework for measuring various physical quantities. The system is based on seven base units from which other units can be derived. These base units represent fundamental physical quantities:

• Meter (m) for length
• Kilogram (kg) for mass
• Second (s) for time
• Ampere (A) for electric current
• Kelvin (K) for temperature
• Mole (mol) for the amount of substance
• Candela (cd) for luminous intensity

Each of these units is defined by a specific, reproducible physical event or property. For example, the meter is defined by the distance light travels in a vacuum in a fraction of a second. SI units are universally accepted and used because they allow for clear communication of measurements without confusion or conversion errors.

Why do we use SI units?

SI units, or the International System of Units, are used globally because they provide a standard for measurement that transcends language and culture, allowing for clear and consistent communication in science, industry, and trade. The SI system is based on a set of seven base units which include the meter for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, mole for the amount of substance, and candela for luminous intensity. These units are universally recognized and have been defined with a high level of precision.

The adoption of SI units facilitates the comparison and exchange of data and information, ensuring that measurements are understood and replicated accurately worldwide. This consistency is crucial for scientific research, where reproducibility of experiments is essential. Moreover, it simplifies the education process, as students and professionals only need to learn one coherent system of measurement.

What are all the SI units?

The International System of Units (SI) comprises seven base units from which all other units of measurement are derived. These base units and their physical quantities are:

• Meter (m) for length
• Kilogram (kg) for mass
• Second (s) for time
• Ampere (A) for electric current
• Kelvin (K) for thermodynamic temperature
• Mole (mol) for the amount of substance
• Candela (cd) for luminous intensity

In addition to these base units, there are derived units that are made from the base units, often to measure more complex concepts like force, pressure, and energy. For example, the newton (N) is the SI unit for force, the pascal (Pa) for pressure, and the joule (J) for energy. There are also several SI prefixes used to denote multiples and submultiples of units, such as kilo- (k) for a thousand times, centi- (c) for one hundredth, and milli- (m) for one thousandth.