Temperature effects on resistance
How is resistance related to temperature? In an ohmic conductor Originally Answered: What is relation between resistance and temperature? Materials vary: . An intuitive approach to temperature dependence leads one to expect a fractional change in resistance which is proportional to the temperature change. I need to create a function to calculate the electrical resistance of graphite given some temperature for modeling a brushed DC motor. Google.
If the temperature were to rise to 35o Celsius, we could easily determine the change of resistance for each piece of wire. Recalculating our circuit values, we see what changes this increase in temperature will bring: As you can see, voltage across the load went down from Though the changes may seem small, they can be significant for power lines stretching miles between power plants and substations, substations and loads.
In fact, power utility companies often have to take line resistance changes resulting from seasonal temperature variations into account when calculating allowable system loading. Most conductive materials change specific resistance with changes in temperature. This is why figures of specific resistance are always specified at a standard temperature usually 20o or 25o Celsius.
The resistance-change factor per degree Celsius of temperature change is called the temperature coefficient of resistance. A positive coefficient for a material means that its resistance increases with an increase in temperature.
Pure metals typically have positive temperature coefficients of resistance. Coefficients approaching zero can be obtained by alloying certain metals.
A negative coefficient for a material means that its resistance decreases with an increase in temperature. Others within each atom are held so tightly to their particular atom that even an electric field will not dislodge them.
The current flowing in the material is therefore due to the movement of "free electrons" and the number of free electrons within any material compared with those tightly bound to their atoms is what governs whether a material is a good conductor many free electrons or a good insulator hardly any free electrons.
Temperature Coefficient of Resistance
The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher the temperature the more violently the atoms vibrate. In a conductor, which already has a large number of free electrons flowing through it, the vibration of the atoms causes many collisions between the free electrons and the captive electrons.
Each collision uses up some energy from the free electron and is the basic cause of resistance.
The more the atoms jostle around in the material, the more collisions are caused and hence the greater the resistance to current flow. In an insulator however, there is a slightly different situation. There are so few free electrons that hardly any current can flow. Almost all the electrons are tightly bound within their particular atom.
Relationship between Resistance and Temperature - Basics of Electrical Engineering
Heating an insulating material vibrates the atoms, and if heated sufficiently, the atoms vibrate violently enough to actually shake some of their captive electrons free, creating free electrons to become carriers of current. Therefore at high temperatures the resistance of an insulator can fall, and in some insulating materials, quite dramatically.
Different materials within either group have different temperature coefficients. Materials chosen for the construction of the resistors used in electronic circuits are carefully selected conductors that have a very low positive temperatur coefficient.