Ohms Law

Ohms, Volts & Amperes.

The resistance of a conductor is measured in Ohms and the Ohm is a unit named after the German physicist George Simon Ohm (1787-1854) who was the first to show the relationship between resistance, current and voltage. In doing so he devised his law which shows the inter-relationship between the three basic electrical properties of resistance, Voltage and current. It demonstrates one of the most important relationships in electrical and electronic engineering.

Ohms Law states that: "In metallic conductors at a constant temperature and in a zero magnetic field, the current flowing is proportional to the voltage across the ends of the conductor, and is inversely proportional to the resistance of the conductor."

In simple terms, provided that the temperature is constant and the electrical circuit is not influenced by magnetic fields, then:

• With a circuit of constant resistance, the greater the voltage applied to a circuit, the more current will flow.
• With a constant voltage applied, the greater the resistance of the circuit, the less current will flow.

Notice that Ohms law states "In metallic conductors" This means that the law holds good for materials which are basically metal, but there are some materials (mainly non-metals) to which Ohms Law does not strictly apply. Here however, because we we are talking about Ohms Law shall stick to circuits which do obey it.

If we identify our three electrical properties by single letters we can write Ohms Law as a simple equation. This will allow us to work out values of current voltage and resistance in circuits:

• The property we call Resistance is indicated by the letter R and is measured in units of Ohms which have the symbol Ω (Greek capital O)
• The property we call Voltage is indicated by the letter V and is measured in units of Volts, which have the symbol V
• The property we call Current is given the letter I (we don't use C as this is used for Capacitance)and is measured in units of Amperes which have the symbol A

So we can use the letters V, I and R to express the relationships defined in Ohms Law in any one of three simple formulas;

Each of which shows us how to find the value of any one of these quantities in a circuit, provided we know the other two. For example, to find the voltage V (in Volts) across a resistor, we simply multiply the current I (in Amperes) through the resistor by the value of the resistor R (in Ohms).

Note that when we use these formulae we put the values into the formula in its BASIC UNIT i.e. VOLTS (not millivolts) Ohms (not kilohms) and AMPERES (not micro Amperes).

Briefly 15KΩ (kilohms) is entered as; 15 EXP 03 and 25mA (milliAmperes) is entered as; 25 EXP -03 etc. This is easiest to do using a scientific calculator.

How to use your calculator with the engineering notation used extensively in electronics is explained in our free booklet entitled "Maths Tips" Download it here. or from our Download page

DEFINITIONS.

1 OHM

..can be defined as "The amount of resistance that will produce a potential difference (p.d.) or voltage of 1 Volt across it when a current of 1 Ampere flowing through it."

1 AMPERE

..can be defined as "The amount of current which, when flowing through a resistance of 1 Ohm will produce a potential difference of 1 Volt across the resistance."

(Although more useful definitions of an ampere are available)

1 VOLT

..can be defined as "The difference in potential (voltage) produced across a resistance of 1 Ohm through which a current of 1 Ampere is flowing."

(Again alternative definitions using other quantities are normally used)

TRY SOME SIMPLE CALCULATIONS USING Ohms Law.