Aims…
2.1 use the following units: ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V), watt (W).
General Ideas…
A physical quantity is something that can be measured. For any measurement, the unit being used must be stated to give an understanding of the scale of the measurement.
For example, distance can be measured in kilometres or in miles. They are similar, but not the same and it is important to identify which was used for the measurement, to know how far the distance actually is.
Système Internationale d’Unités
The units that scientists use all over the world are standardised in the Système Internationale d’Unités – SI units. It is important to remember these six fundamental (or ‘base’) units of measurement:
- metre (m) – unit of length
- kilograms (kg) – unit of mass
- second (s) – unit of time
- ampere (A) – unit of electrical current
- kelvin (K) – unit of temperature
- mole (mol) – unit of the amount of substance
There are many quantities scientists measure that come from the base units. These derived units are very useful to quote as measurements, but they are not fundamental as they come from fundamental units.
For example, frequency is the number of times something happens per unit of time.
This is a useful quantity, but it is a division into the time unit.
The standard unit for frequency is considering the number ‘per second‘, which is called ‘hertz, Hz’, but this comes from the fundamental unit ‘second‘.
| Name | Unit | Abbreviation |
| Frequency | hertz | Hz |
| Force | newton | N |
| Energy | joule | J |
| Power | watt | W |
| Pressure | pascal | Pa |
| Electric charge | coulomb | C |
| Work Done or Work | Nm or J | W |
| Gravity or acceleration | Nkg-1ms-2 | g or a |
| Momentum | kgms-1 | p |
Specifics…
- Joule (J): the SI unit of work or energy, equal to the work done by a force of one newton when its point of application moves one metre in the direction of action of the force.
- Watt (W): the SI unit of power, equivalent to one joule per second, corresponding to the rate of consumption of energy in an electric circuit where the potential difference is one volt and the current one ampere. 1W = 1Js-1
- Second (s): the SI base unit of time which is used to determine the progression of the universe. Defined from a complex atomic transition or as a division of an Earth day.
- Resistance (W): a measure of how much a wire resists a flow of current often found by R = V / I (V = IR) measured in ohms, Ω
- Ampere (A): a unit of electric current equal to a flow of one coulomb per second. 1A = 1Cs-1
- Charge (C): an inherent fundamental property of some matter i.e. the electron has a charge of –1.6 x 10-19 C We often use the term relative charge i.e. -1 for electron and +1 for proton
- Electrical Potential Difference (V): 1 Volt means that one Coulomb of charge will gain 1 joule of potential energy when moved between two electrodes 1V = 1JC-1
- Magnetic Flux Density (T): the number of magnetic lines of flux that pass through a certain point on a surface. The SI unit is T (tesla). It tells us how strong a magnetic field is. (Y11)
Resources….
Use this PowerPoint for a quick review…2 Electricity A Units
Homogeneity of physics equations
By Cowen Physics (www.cowenphysics.com)
