We know that materials present degrees of difficulty for the passage of electric current. This degree of difficulty is called **electrical resistance**. Even metals, which are generally good conductors, have resistance.** Resistance unit of measure is ohm **( ).

Devices that are used in an electrical circuit are called **resistors****. Resistors are used in a circuit to increase or decrease the intensity of the electrical current that flows through it.**

We can compare the electrical resistance to those barriers we encounter on the running tracks for the obstacle course. The more obstacles the slower is the average speed of the runners. In a circuit it happens the same way: the more electrical resistance, the less current that flows through the conductor wire.

The most common application of resistors is to convert electrical energy to thermal energy. This is because the electrons moving in the resistor collide with the crystalline lattice that forms it, generating heat. This phenomenon is called the joule effect in our daily lives: in electric showers, irons, electric stoves, etc. Note that all of these appliances "provide heat."

The incandescent lamp itself converts more electrical energy into thermal energy than light energy, the latter being its great purpose: **85% of the energy it consumes is transformed into heat.** In contrast, fluorescent lamps, considered “cold light bulbs”, have a much smaller share of electrical energy converted to heat and are therefore economical.

## Ohm's First Law

It was observed experimentally in some resistors, which **The current set in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit devices and the wires that connected them.** That is: the higher the generator voltage, the higher the current and the higher the resistance, the lower the current. This relationship is expressed mathematically by:

on what: **U** it's the tension

**R** is the resistance

** i **is the current

Let's look at an example:

A small lamp has a voltage of 12 V. Knowing that its resistance is determine the current through the lamp.

We know that .

How ,

We have to: