- Resistance Variation Definition: Resistance variation with temperature refers to how the resistance of materials changes as their temperature changes.
- Metals and Temperature: Metals show an increase in resistance as temperature rises.
- Non-Metals and Temperature: Non-metallic substances generally have decreased resistance with increased temperature.
- Resistance vs Temperature Graph: The resistance vs temperature graph for metals is usually a straight line that can infer zero resistance at a certain temperature.
- Practical Applications: This concept is used to determine temperature variations in electrical machines, like in transformer temperature rise tests.
Metals like silver, copper, and aluminum have many free electrons, making them good conductors with low resistance. However, their resistivity changes with temperature. Generally, metals have higher electrical resistance when the temperature increases, while non-metallic substances usually show decreased resistance with higher temperatures.
If we take a piece of pure metal and cool it to 0°C using ice, then gradually heat it from 0°C to 100°C.
During increasing of temperature if we take its resistance at a regular interval, we will find that electrical resistance of the metal piece is gradually increased with increase in temperature. If we plot the resistance variation with temperature i.e. resistance Vs temperature graph, we will get a straight line as shown in the figure below. If this straight line is extended behind the resistance axis, it will cut the temperature axis at some temperature, – t0oC. From the graph it is clear that, at this temperature the electrical resistance of the metal becomes zero. This temperature is referred as inferred zero resistance temperature.
Although zero resistance of any substance cannot be possible practically. Actually rate of resistance variation with temperature is not constant throughout all range of temperature. Actual graph is also shown in the figure below.
Let’s R1 and R2 are the measured resistances at temperature t1oC and t2oC respectively. Then we can write the equation below,
From the above equation we can calculate resistance of any material at different temperature. Suppose we have measured resistance of a metal at t1oC and this is R1.
If we know the inferred zero resistance temperature i.e. t0 of that particular metal, then we can easily calculate any unknown resistance R2 at any temperature t2oC from the above equation.
The resistance variation with temperature is often used for determining temperature variation of any electrical machine. For example, in temperature rise test of transformer, for determining winding temperature rise, the above equation is applied. This is impossible to access winding inside the an electrical power transformer insulation system for measurement of temperature but we are lucky enough that we have resistance variation with temperature graph in our hand. After measuring electrical resistance of the winding both at the beginning and end of the test run of the transformer, we can easily determine the temperature rise in the transformer winding during test run.
20°C is the standard reference temperature for measuring resistance. So, if we say a substance has a resistance of 20Ω, it means this resistance was measured at 20°C.
