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Jetzt kostenlos anmeldenA physical quantity is a property of an object, something we can measure with instruments or even by using our senses.
Two simple examples of physical quantities are the mass of an object or its temperature. We can measure both with instruments, but we can also sense them using our hands by lifting the object or touching it.
There is a range of physical properties that we can measure. All these properties are related to an object’s dimensions or its constitution. The seven elemental physical quantities are:
People confuse weight and mass all the time. The best way to explain the difference is by using an example featuring a ball.
A ball has a different weight on Mars than it does on Earth. However, the matter that composes the ball remains the same. And if the matter does not change, then neither does the mass.
Weight is the amount of force that gravity exerts on mass; it is force per mass. A scale, therefore, measures the gravitational force that pulls down the mass of an object.
This can also be explained using the gravity force formula that determines the weight of an object:
\[\text{weight} = \text{mass} \cdot \text{gravity}\]
The amount of matter in the ball does not change, so mass is a constant. The main difference is the gravity because gravity on Earth is higher than gravity on Mars:
\[\text{gravity (Earth)} > \text{gravity(Mars)}\]
Therefore, the weight on Earth will be higher than on Mars:
\[\text{mass} \cdot \text{gravity (Earth)} > \text{mass} \cdot \text{gravity(Mars)}\]
Physical quantities have two categories: extensive quantities and intensive quantities. This classification is related to an object’s mass. Extensive quantities depend on an object’s mass or size, while intensive quantities do not.
Mass and electrical charge are examples of extensive physical quantities.
Mass depends on the size of the object. If you have two weights made of steel and one is double the size of the other, the larger one will have double the mass.
Another example concerns electrical charge. If the particles of an object have some electrical charge, their number tells us how much electrical charge the object has. If the object increases its mass, thus increasing its number of particles, the electrical charge will be larger.
Intensive physical quantities do not depend on the object’s mass or size. Simple examples of this are time and temperature.
We can measure the time it takes for two objects of different mass to move from position A to position B. In both cases, time flows in the same way, independent of the composition or size of the objects.
Imagine we have an object with a temperature of 100 Kelvin, which we divide in half. In ideal circumstances where there is no heat transfer, the two halves will each still have the same temperature of 100 K.
Derived physical quantities are the properties of an object that result from two elemental physical quantities. Derived quantities can result from a relationship of the same physical quantity (e.g. area) or by relating two different ones (e.g. velocity). See below for some examples of derived physical quantities.
Area and volume: related to length:
\[Area = length \cdot width; \space Volume = length \cdot width \cdot height\]
Velocity and acceleration: related to length and time:
\[Velocity = \frac{length}{time}; \space Acceleration = \frac{length}{time^2}\]
Density: related to length and mass:
\[Density = \frac{mass}{length^3}\]
Weight: related to acceleration and mass (in a planet, acceleration is its gravitational acceleration):
\[Weight = gravity \cdot mass\]
Pressure: related to force and length (for pressure, the force can be the weight exerted by an object, and the area over which this force acts is related to length):
\[Pressure = \frac{force}{length^2}\]
Physical quantities have several characteristics related to their properties, some of which are listed below.
Temperatures below zero are the result of taking the temperature at which water freezes as a zero (0) value. In Celsius, every temperature below the freezing point of water is negative.
Physical quantities are important because they allow us to describe an object. Objects have a certain mass, a certain length, and a certain amount of atoms. The units are the values of reference we use to measure the properties of objects.
Imagine measuring the weight of two rocks. You can tell by holding them in your hands that one is heavier than the other. However, to determine their precise weight, you need to compare them against a standard value (unit), in this case, the kilogram.
A physical quantity is a quantity that is used to describe the properties of an object.
A vector quantity is a physical quantity with a value and a direction. An example of this is velocity. You need the velocity value and its direction to know what is happening.
A fundamental physical quantity is one of the seven elemental quantities that describe the properties of an object. They are temperature, mass, length, electrical charge, mole, luminosity, and time.
Physical quantities can be extensive or intensive. They are extensive if they depend on the mass or size of the object and intensive if they do not.
What is a physical quantity?
A property that we can measure.
How are physical quantities and units related?
One is the property we measure, while the other is the amount of that property.
Can time and mass as physical quantities be negative?
No, they cannot.
Physical quantities can be extensive or …?
Intensive.
Does an extensive property depend on the mass and size of the object?
Yes, it does.
Do intensive properties depend on an object’s mass and size?
No, they don’t.
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