ABSTRACT
What happens when we attach a copper wire from the positive terminal of a battery to one metal contact of a light bulb and another copper wire from the other metal contact of the light bulb to the negative terminal of the battery? The light begins to glow. Why? The difference in electrical charge between the positive and negative terminals of the battery produces a voltage (V) that causes free electrons in the copper wire to flow from the negative terminal to the positive terminal. The flow of electrons is referred to as current (I). The electrons flowing through the copper wire interact with each other and with the copper atoms. This interaction interferes with the flow of the electrons and is referred to as electrical resistance (R). The voltage V, the current I, and the resistance R are related by the equation
V IR (10.1)
where V volts I amperes
R ohms
It is apparent from Equation (10.1) that I and R compete with each other. If R is low, then the electrons flow easily through the material and I is high. This is the case for the copper wire. If R is high, the electrons have difficulty flowing through the material and I is low. This is the case for the filament in the light bulb. The filament is made of a very thin cross section of a tungsten alloy that has much higher electrical resistance than the copper. When the electrons reach the thin tungsten filament, they must exert considerable effort to get through. This results in work that is dissipated in the form of heat. So much heat is produced that the tungsten filament becomes white hot and emits bright light.
