- 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.
Take a piece of pure metal, cool it to 0°C using ice, and then gradually heat it from 0°C to 100°C.
If we record its resistance at regular intervals while the temperature rises, we find that the electrical resistance of the metal piece increases gradually with the increase in temperature. If we plot the resistance variation with temperature, that is, a resistance versus 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 cuts 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 to as the inferred zero resistance temperature.
The metal never actually follows the straight line down to zero resistance. Its rate of resistance variation with temperature is not constant throughout the whole range of temperature, and the actual graph bends away from the line, as also shown in the figure below. Only superconductors, cooled below their material-specific critical temperature, exhibit exactly zero resistance.
Let R1 and R2 be the measured resistances at temperatures t1oC and t2oC respectively. Then we can write the equation below,
From the above equation we can calculate the resistance of any material at different temperatures. Suppose we have measured the resistance of a metal at t1oC as R1.
If we know the inferred zero resistance temperature, that is, 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 variations inside electrical machines. For example, during the temperature rise test of transformer, the winding temperature rise is determined by applying the above equation. Accessing the winding inside an electrical power transformer insulation system for direct temperature measurement is impossible, but the resistance variation with temperature relationship provides another way. After measuring the 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 the 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.
