Sim's Electrical Companion
Current, Voltage & Resistance - Ohm's Law
Ohm's law is the most fundamental equation in electrical science. The law is very simple but is a great importance. The law is defined as: The current flow through a conductor is directly proportional to the voltage and inversely proportional to the resistance. The relationship was discovered & described by George Simon Ohm in 1827.
Definitions:
Voltage is also known as potential difference or electromotive force (emf). Voltage is the force that causes the flow of current. To use a simple analogy there may be water in a pipe but it would not flow without a pump or another force e.g. gravity. Think of voltage as the pump or the force that moves the electrons.
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Current
Is the flow of electrons. To continue the analogy you can think of current as the amount of water that is flowing in a pipe. In the case of conductors the larger the conductor, the more current can flow.
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Resistance
A the name suggests is the opposition to current flow. Resistance is a combination of pure resistance and reactive elements. Resistance is measured in Ohm's but is also called impedance if there are reactive elements to the total resistance in an electrical system.
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To be more specific and link these units to the SI system of units:
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1 Volt = 1 ampere dissipating 1 watt of power in one second or 1 ampere of current passing through a 1 ohm resistor. In SI base units it is expressed as (kg x m2) / (A x s-3)
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1 ampere = One ampere of current = one coulomb of electrical charge* (6.24 x 1018 charge carriers) moving past a specific point in one second. The ampere is a base SI unit.
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1 Ohm Ω = The value of resistance experienced by a current of 1 ampere with a potential difference of 1 Volt. In SI units it can be expressed as (kg x m2) / (s-3 x A2)
Resistance & Resistivity
Resistance is a combination of pure resistance and reactive elements. Reactive elements will be covered when I have discussed power, inductors and capacitors. Pure resistance is a combination of the conductor material its cross sectional area, length & temperature. The material conductance is affected by temperature so this is taken into account in the table for resistivity.
* Annealing copper is a heat treatment that improves the ductility & maleability of the metal.
Resistance is a combination of pure resistance and reactive elements. Reactive elements will be covered when I have discussed power, inductors and capacitors. Pure resistance is a combination of the conductor material its cross sectional area, length & temperature. The material conductance is affected by temperature so this is taken into account in the table for resistivity.
The equation for pure resistance takes this all into account.
R = Total resistance in Ohms Ω
ρ = Resistivity at 20°C (Ω/m)
ι = Length in metres (m)
a = Cross sectional area in mm² (= m x10^-6)
Examples of Pure resistance equation in use:
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EXAMPLE 1
If a circuit is allowed a maximum resistance of 0.81Ω run in copper cable with a cross sectional area of 1.5mm2.
What is the maximum length of the circuit?
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EXAMPLE 2
An electrician is working in a 1960's large domestic property. It is not known whether the cable run to the cooker is of aluminium or copper construction. The resistance is 0.497Ω and the cable run is approximately 75m. The cable type is 4.0mm². Is it copper or aluminium?
