Physical quantities

What is physical quantity?

Physical quantity is a property of a physical system, object, or phenomenon that can be measured and expressed using a numerical value along with a unit of measurement.

Large and small quantities are usually expressed in scientific notation, i.e. as a simple number multiplied by a power of ten. Thus, if we have a value of 0.0005, what would 0.0005 be written as?

`0.0005=5xx10^(-4)`.
Explanation: Before the whole number, there are four zeros, so we write 5 × 10^-4. The negative sign in the exponent indicates that there are four zeros before the whole number. Conversely, if there is no negative sign, there will be four zeros after the whole number.

Which of the following is the correct expression of 123000000000?

A. `12.3xx10^11`.

B. `123xx10^10`.

C. `1.23xx10^10`.

D. `1.23xx10^11`.

Answer: D

Explanation: After the whole number, there are nine zeros, so we write `123xx10^9`. The positive sign in the exponent indicates that there are nine zeros after the whole number. Conversely, if there is negative sign, there will be nine zeros before the whole number. However, to find the exponent which is expressed as answer, count the number of time you need to shift the number one digit to the right before the result is less than 10. Thus, 123000000000 needs to be shifted to the right 11 times before the result 1.23 is less than 10, so `1.23xx10^11`.

What is needed to accurately represent all physical quantities?

A. A base unit and a number.

B. A unit and a number expressed in standard form (scientific notation).

C. A unit and a numerical magnitude.

D. An SI unit and a numerical magnitude.

Answer: C

A. Incorrect: We need numerical value instead of number.

B. Incorrect: Same as A.

C. Correct: When presenting the result of a measurement of a physical quantity or the solution to a physics problem, we need a numerical value to represent the magnitude of the quantity along with its corresponding unit of measurement.

D. Incorrect: Not only SI unit could represent unit of measurement.

Which quantity is a physical quantity?

A. Kilogram.

B. Kelvin.

C. Minute.

D. Potential difference.

Answer: D

A. Incorrect: It's unit of mass.

B. Incorrect: It's unit of temperature.

C. Incorrect: It's unit of time.

D. Correct: Potential difference is measured in volts.

Which of the following is the correct expression of 0.000023?

A. `23xx10^(-4)`.

B. `230xx10^(-4)`.

C. `2.3xx10^(-4)`.

D. `2.3xx10^(-5)`.

Answer: D

Explanation: Before the whole number, there are five zeros, so we write `2.3 × 10^(-5)`. The negative sign in the exponent indicates that there are five zeros before the whole number. Conversely, if there is no negative sign, there will be five zeros after the whole number.

Which term represents a physical quantity?

A. Meter.

B. Percentage uncertainty.

C. Kilogram.

D. Spring constant.

Answer: D

A. Incorrect: Meter is unit of length.

B. Incorrect: Percentage uncertainty is the relative uncertainty multiplied by 100.

C. Incorrect: Kilogram is a unit of mass.

D. Correct: The spring constant is expressed as `k=|F|/x`. Thus, it has a unit and numerical magnitude and is a measure of spring constant.

Which pair of quantities are physical quantities?

A. Charge and ampere.

B. Efficiency and kilogram.

C. Pascal and strain.

D. Period and potential difference.

Answer: D

A. Incorrect: Ampere is unit of electric current.

B. Incorrect Kilogram is unit of mass.

C. Incorrect: Pascal is unit of pressure.

D. Correct: Physical quantity should have a unit and a numerical magnitude. 

Which rows shows what all physical quantities must have?

Answer: C

Physical quantity should have a unit and a numerical magnitude. Only vectors have direction.

What could not be a measurement of a physical quantity?

A. `10 " K"`

B. `11" J"."N"^(-1)."m"^(-1)`.

C. `17" Pa"."m"^3."N"^(-1)`

D. `25 " Tm"`

Answer: B

A. Incorrect: K is unit of temperature.

B. Correct: 

`frac("J")("N"."m")=frac("kg"."m"."s"^-2."m")("kg"."m"."s"^-2."m")=1`

It has no unit, as such, this is not a measure of any physical quantity.

C. Incorrect: `frac("Pa"."m"^3)("N")=frac(("N"/"m"^2)"m"^3)("N")="m"`

D. Incorrect: It is a measurement of a length because it has a magnitude and a corresponding unit.