Question Video: Recalling How SI Units are Defined | Nagwa Question Video: Recalling How SI Units are Defined | Nagwa

# Question Video: Recalling How SI Units are Defined Physics • First Year of Secondary School

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Which of the following SI units is defined by reference to the properties of a unique, standard object held at the International Bureau of Weights and Measures? [A] The meter [B] The kilogram [C] The mole [D] The candela [E] The steradian

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### Video Transcript

Which of the following SI units is defined by reference to the properties of a unique, standard object held at the International Bureau of Weights and Measures? (A) The meter, (B) the kilogram, (C) the mole, (D) the candela, (E) the steradian.

The question has given us five different SI units, and it’s asking us to identify which of them is defined by reference to the properties of a unique standard object. Let’s begin by recalling what physical quantity each of these units is used to measure. In option (A), we have the meter, and we can recall that this is the SI unit for length or distance. Option (B) is the kilogram, which is the SI unit for the quantity mass. Option (C) is the mole, which is the SI unit for the amount of substance or the number of equivalent parts that something consists of. It’s worth noting that whenever we’re talking about the number of moles of a substance, the chances are we’re talking about the number of atoms or molecules.

Then in option (D), we have the candela and this is the SI unit for luminous intensity. Lastly in option (E), we have the steradian and this is the SI unit of solid angle, which is used in three-dimensional geometry.

Now, let’s think about how each of these units is defined. The definition of the meter is defined by a measurement of a naturally occurring physical phenomenon. Previously, this definition was based on the light emitted from a particular kind of atom. The definition has since been updated: the distance traveled by light in vacuum in a particular interval of time. Since this definition is based on the speed of light in vacuum, then our definition of the meter is still based on a naturally occurring physical phenomenon. We know then that the meter is not defined by reference to the properties of a unique standard object, so we can eliminate answer option (A).

Let’s move on then and look at answer option (B). The kilogram was originally defined as the mass of one liter of water. However, in 1889, a specially designed object was made and this definition was updated. The kilogram was then redefined to be equal to the mass of this object, which was a cylinder made of platinum–iridium alloy. This unique object was known as the prototype kilogram and was kept at the International Bureau of Weights and Measures. We can see then that this definition of the kilogram is indeed based on the properties of a unique standard object. And so option (B) is looking like it may well be our answer.

Let’s quickly check out the remaining three options though just to be sure, starting with option (C) the mole. One mole is defined to be exactly equal to 6.02214076 times 10 to the 23 particles or parts of substance. This is a mathematical definition and not based on any unique standard object. So we can eliminate answer option (C). The SI unit in option (D), the candela, is defined as the brightness of a light source with certain properties. So in a sense we could say that this definition is based on an object, but it’s certainly not a unique object. There is no standard unique one candela object kept at the International Bureau of Weights and Measures. So we can eliminate answer option (D).

The last SI unit to consider is the one in option (E), the steradian. The definition of the steradian is a mathematical one based on the geometry of a sphere. This definition exists in the abstract sense, and there is no standard object used. That means then that we can eliminate answer option (E). We have found that neither the meter, the mole, the candela, or the steradian is defined by reference to the properties of a unique standard object. The only one of these units that is defined by reference to the properties of such an object is the kilogram, which is the SI base unit for mass.

Our answer then is that the SI unit which is defined by reference to the properties of a unique standard object held at the International Bureau of Weights and Measures is given in option (B) the kilogram.

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