Resistors are the absolute most basic component of an electric circuit. They get their name from have “resistance” or the physical ability to resist current. Resistance is typically represented by the symbol R.
The value of the resistance of a particular material is represented by R=p*(L/A), where p is known as the resistivity of the material in ohm-meters. A is the cross-sectional area of the resistor and L is its length. Each materials has a known value of resistivity. Materials with a high resistivity make good resistors, while materials with a low resistivity make good conductors (materials that allow current to flow freely).
Georg Simon Ohm, a German physicist, discovered the most basic relationship between current through a resistor and the resulting voltage. He correctly stated that V=IR, which means the voltage across the resistor is equal to the current through that resistor multiplied by the value of its resistance. For example, if a 10 A current is flowing through a 3 Ohm resistor, the voltage across said resistor is 30V.
From V=IR, we can deduce that the ohm is equivalent to V/A. By dividing the right-hand side of the equation by I, we see that R=V/I, so an ohm is equivalent to Volts/Amps.
1. An electric iron draws 1 A at 60 V. What is its resistance?
From Ohm’s law: R=V/I=60/1=60 Ohms.
2. A 10k Ohm resistor draws 2.5 mA of current. What is the voltage drop across the resistor?
From Ohm’s law: V=IR=2.5mA*10K Ohms=25V.