Derived unit
Physical Quantities & Units - OCR A-Level Physics
Key Definition
Derived unit
A unit that is expressed as a combination of SI base units. It comes from a defining equation for the quantity.
A unit that is expressed as a combination of SI base units. It comes from a defining equation for the quantity.
- VelocityThe rate of change of displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).. A vector quantityA quantity that has both magnitude and direction.. Measured in m s⁻¹.: $m s^{-1}$ (from v = s/t)
- AccelerationThe rate of change of velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. A vector quantityA quantity that has both magnitude and direction.. Measured in m s⁻².: $m s^{-2}$ (from a = v/t)
- Force (newton, N): kg $m s^{-2}$ (from F = ma)
- EnergyThe capacity to do work. Measured in joules (J). (joule, J): kg \(m^{2}\) $s^{-2}$ (from W = Fd)
- PowerThe rate of energyThe capacity to do work. Measured in joules (J). transfer. Measured in watts (W). (watt, W): kg \(m^{2}\) $s^{-3}$ (from P = W/t)
- PressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻². (pascal, Pa): $kg m^{-1}$ $s^{-2}$ (from p = F/A)
- ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). (coulombThe SI unit of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. One coulomb is the chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). transferred by a current of 1 A in 1 second., C): A s (from $Q = It)$
- Potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). (voltThe SI unit of potential difference and EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. One volt is one joule per coulomb., V): kg \(m^{2}\) $s^{-3}$ $A^{-1}$ (from V = W/Q)
- ResistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). to currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohms (Ω). (ohm, $\Omega$): kg \(m^{2}\) $s^{-3}$ $A^{-2}$ (from R = V/I)
Examiner Tips and Tricks
- To find the base units of any quantity, substitute its defining equation step by step.
- For example, V = W/Q = (kg \(m^{2}\) $s^{-2}$) / (A s) = kg \(m^{2}\) $s^{-3}$ $A^{-1}$.
- Show every step to earn full marks.