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Edexcel GCSE Physics · 1PH0
Edexcel 1PH0 · 1.1 Check the specification (PDF) (opens in a new tab)
Physics deals with measurable properties called physical quantities, such as mass, time and temperature. A measurement needs both a numerical value and a : “20” alone is incomplete, whereas is a time measured in seconds.
A quantity symbol used in an equation is not necessarily its unit symbol. For example, may represent the quantity mass, but the SI unit of mass is the kilogram, with the symbol kg.
The International System of Units, or SI, gives scientists an agreed set of measurement standards. A time recorded in seconds in one laboratory can therefore be compared directly with a time recorded in seconds elsewhere. Consistent units also allow values to be used correctly in equations.
For this course, the six main SI base quantities and units to recall are:
| Physical quantity | SI unit | Symbol |
|---|---|---|
| length | metre | m |
| mass | kilogram | kg |
| time | second | s |
| electric current | ampere | A |
| temperature | kelvin | K |
| amount of substance | mole | mol |
The kilogram—not the gram—is the SI base unit of mass. Kelvin temperatures do not use a degree sign, so write 300 K rather than 300 °K.
Other quantities have derived units, produced by combining base units. Speed, for example, is distance divided by time. Measuring distance in metres and time in seconds gives the unit metres per second, m/s.
Some derived units have special names. The nine named derived units identified for this course are:
| Physical quantity | SI unit | Symbol |
|---|---|---|
| frequency | hertz | Hz |
| force | newton | N |
| energy | joule | J |
| power | watt | W |
| pressure | pascal | Pa |
| electric charge | coulomb | C |
| electric potential difference | volt | V |
| electric resistance | ohm | Ω |
| magnetic flux density | tesla | T |
For example, Newton’s second law is . Using mass in kilograms and acceleration in metres per second squared shows that
The special name “newton” makes this combined unit shorter to write. Other compound units used in physics include m/s² for acceleration and N/kg for gravitational field strength.
Unit symbols are case-sensitive. The ampere is A and the kelvin is K, while named derived units include N, J, W and Pa. Full unit names normally begin with a lower-case letter, even when named after a person: newton, joule and watt.
Leave a space between the value and its unit, as in 8 m, 25 A or 300 K. Unit symbols do not take plural endings: both one kilogram and five kilograms use the symbol kg.
Units also help to check calculations. Distance divided by time should give m/s; mass multiplied by acceleration should give kg m/s², or N. An unexpected unit can reveal that values have been substituted into an equation incorrectly.
Also recognise compound units such as m/s, m/s² and N/kg.
A quantity is what is measured; a unit is its agreed standard. Write the value and unit together, preserve capitalisation and never pluralise a unit symbol. Remember: mass is in kg, but weight is in N.
Write the unit with every numerical answer unless it is already supplied in the answer space.
Learn unit symbols exactly. Capitalisation matters: A, K, N and W are not interchangeable with a, k, n and w.
Do not add an “s” to make a unit symbol plural: write 5 kg, not 5 kgs.
Do not confuse mass in kilograms (kg) with weight in newtons (N).
Use Ω, the Greek capital letter omega, as the symbol for the ohm.
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Last updated
Physical quantity
A measurable property written as a numerical value and a unit, such as a time of 12 s.
Unit
An agreed standard used to express the measurement of a physical quantity.
SI
The International System of Units: the internationally agreed system of measurement used in science.
Base unit
A fundamental SI unit from which other units can be formed.
Derived unit
A unit formed by combining base units through multiplication or division, sometimes given a special name such as the newton.
Put your knowledge into practice — try past paper questions for Physics
Physical quantity
A measurable property written as a numerical value and a unit, such as a time of 12 s.
Unit
An agreed standard used to express the measurement of a physical quantity.
SI
The International System of Units: the internationally agreed system of measurement used in science.
Base unit
A fundamental SI unit from which other units can be formed.
Derived unit
A unit formed by combining base units through multiplication or division, sometimes given a special name such as the newton.