# Electricity: Resistance

## Understanding Resistance

• Resistance is the opposition that a substance offers to the flow of electric current. It’s measured in ohms (Ω).
• In conductors such as metal wire, resistance increases with an increase in length and decreases with an increase in cross-sectional area.
• Resistance depends on the type of material. Different materials resist the flow of electrical current to different degrees.
• Higher temperature usually increases resistance as it causes the ions in the conductor to vibrate more, thus increasing the collisions with flowing electrons and impeding their flow.
• An object’s resistance can be calculated using the formula R=V/I where R is the resistance, V is the potential difference (voltage) and I is the current.

## Ohm’s Law and Non-Ohmic Conductors

• Ohm’s law states that current is directly proportional to voltage and inversely proportional to resistance. This means that if you increase the voltage, current increases. But, if you increase resistance, current decreases.
• Some materials obey Ohm’s law and are called ohmic conductors. Their resistance stays constant as the current changes. These materials have an I-V characteristic that is a straight line.
• Other materials do not obey Ohm’s law and are called non-ohmic conductors. These materials change their resistance with the current. They have a curved I-V characteristic.

## Resistors in Series and Parallel:

• For resistors in series, the total resistance (Rt) is equal to the sum of all the individual resistances (R1 + R2 + R3…).
• For resistors in parallel, you can calculate the total resistance using the formula 1/Rt = 1/R1 + 1/R2 + 1/R3…
• Generally, adding a resistor in series will increase total resistance, but adding a resistor in parallel will decrease total resistance.

## Effects of Resistance

• Resistance causes electrical energy to be converted into heat energy. This is why electrical appliances get warm or hot when they’re being used.
• The heating effect of resistance is utilised in some appliances such as electric heaters and toasters.
• Wires and components with a high resistance can become dangerously hot and cause a fire if too much current is passed through them. For this reason, materials with low resistances (like copper) are often used for wiring.