Introduction
Exploring the interaction between temperature and resistance is important in electrical engineering, as temperature acts as a conductor in the movement of electrons within conductive materials. This relationship has significant implications for a wide range of applications, from basic household devices to intricate electronic system. In this blog, we will understand the relationship between temperature and resistance, uncovering the mechanisms at play and their real-world consequences.
In general, the resistance of a material changes with the change in temperature. The effect of temperature upon resistance varies according to the type of material as discussed below :
Figure 1
- The resistance of pure metals (e.g. copper, aluminium) increases with the increase of temperature. The change in resistance is fairly regular for normal range of temperatures so that temperature/ resistance graph is a straight line as shown in Fig 1 (for copper). Since the resistance of metals increases with the rise in temperature, they have positive temperature co-efficient of resistance.
- The resistance of electrolytes, insulators (e.g. glass, mica, rubber etc.) and semiconductors (e.g. germanium, silicon etc.) decreases with the increase in temperature. Hence these materials have negative temperature co-efficient of resistance.
- The resistance of alloys increases with the rise in temperature but this increase is very small and irregular. For some high resistance alloys (e.g. Eureka, manganin, constantan etc.), the change in
resistance is practically negligible over a wide range of temperatures.
Figure 1 shows temperature/resistance graph for copper which is a straight line. If this line is extended backward, it would cut the temperature axis at −234.5°C. It means that theoretically, the resistance of copper wire is zero at −234.5°C. However, in actual practice, the curve departs (point A) from the straight line path at very low temperatures.
Key points regarding Temperature Coefficient of Resistance
1. Ohm's Law and the Basics of Resistance
Before delving into the temperature-resistance relationship, permit's revisit Ohm's Law. According to Ohm's Law, the modern (I) flowing thru a conductor is at once proportional to the voltage (V) throughout it and inversely proportional to its resistance (R). This foundational precept sets the stage for knowledge how modifications in temperature have an effect on resistance.
2. The Role of Temperature in Resistance
In most conductive substances, the atoms vibrate with thermal strength. As temperature increases, these vibrations intensify, leading to more collisions between electrons and vibrating atoms. This extended collision frequency impedes the flow of electrons, resulting in a upward thrust in resistance. Conversely, as temperature decreases, the vibrational strength decreases, permitting electrons to transport greater freely and decreasing resistance.
3. Coefficient of Temperature
To quantify the temperature-resistance relationship, materials are characterized by using their temperature coefficient of resistance (TCR). This coefficient represents the proportion exchange in resistance per degree Celsius exchange in temperature. Different materials exhibit various TCR values, presenting engineers with crucial statistics for designing circuits which can perform inside specific temperature degrees.
4. Positive and Negative Temperature Coefficients
Materials will have wonderful or bad temperature coefficients. A fantastic TCR means that resistance increases with temperature, as visible in maximum metals. In evaluation, substances with a bad TCR revel in a lower in resistance as temperature rises. This phenomenon is normally observed in semiconductor materials.
5. Practical Applications and Challenges
The impact of temperature on resistance is not only a theoretical attention; it has realistic implications. Electronic gadgets, from household home equipment to industrial machinery, often enjoy temperature versions during operation. Engineers need to account for these modifications to make sure the reliability and overall performance of digital structures.
6. Compensating for Temperature Effects
In sure applications, compensatory measures are employed to counteract the outcomes of temperature on resistance. One not unusual approach is using temperature-touchy resistors, referred to as thermistors, which show off a rather nonlinear reaction to temperature adjustments. These devices find programs in temperature compensation circuits and temperature-touchy structures.
Temperature Co-efficient of Resistance
Consider a conductor having resistance `R_0` at 0°C and `R_t` at t °C. It has been found that in the normal range of temperatures, the increase in resistance (i.e. `R_t` - `R_0` )
- is directly proportional to the initial resistance i.e.
`R_t` - `R_0` ∝ `R_0`
2.is directly proportional to the rise in temperature i.e.
`R_t` - `R_0` ∝ t
3. depends upon the nature of material. Combining the first two, we get,
`R_t` - `R_0` ∝ `R_0`t
or `R_t` - `R_0`=`alpha_0``R_0`t
where `alpha_0` is a constant and is called temperature co-efficient of resistance at 0°C.Its value depends upon the nature
of material and temperature.
After Rearranging above equation we get,
`R_t`=`R_0`(1+`alpha_0`t)
Definition of `alpha_0`. From above equation we get,
`alpha_0`= (`R_t` - `R_0` / `R_0`t)
=Increase in resistance/ohm original resistance/°C rise in
temperature
Hence temperature co-efficient of resistance of a conductor is the increase in resistance per ohm original resistance per °C rise in temperature.
A little reflection shows that unit of α will be ohm/ohm°C i.e./°C. Thus, copper has a temperature co-efficient of resistance of 0.00426/°C. It means that if a copper wire has a resistance of 1 Ω at 0°C, then it will increase by 0.00426 Ω for 1°C rise in temperature i.e. it will become 1.00426 Ω at 1°C.Similarly, if temperature is raised to 10°C, then resistance will become 1 + 10 × 0.00426 = 1.0426 ohms.
The following points may be noted carefully :
- Those substances (e.g. pure metals) whose resistance increases with rise in temperature are said to have positive temperature co-efficient of resistance. On the other hand, those substances whose resistance decreases with increase in temperature are said to have negative temperature coefficient of resistance.
- If a conductor has a resistance `R_0`,`R_1` and `R_2` at 0°C, `t_1` and `t_2` respectively, then,
`R_1`= `R_0`(1+`alpha_0``t_1`)
This relation is often utilised in determining the rise of temperature of the winding of an electrical machine. The resistance of the winding is measured both before and after the test run. Let `R_1` and `t_1` be the resistance and temperature before the commencement of the test. After the operation of the machine for a given period, let these values be `R_2` and `t_2`.Since `R_1` and `R_2` can be measured and `t_1` (ambient temperature) and `alpha_0` are known, the value of `t_2` can be calculated from above equation.The average rise in temperature of the winding will be (`t_2`-`t_1`)°C.
Note:
The life expectancy of electrical apparatus is limited by the temperature of its insulation; the higher the temperature, the shorter the life. The useful life of electrical apparatus reduces approximately by half every time the temperature increases by 10°C.This means that if a motor has a normal life expectancy of eight years at a temperature of 100°C, it will have a life expectancy of only four years at a temperature of 110°C, of two years at a temperature of 120°C and of only one year at 130°C.
Conclusion
The effect of temperature on resistance is a fundamental concept in the world of electricity and electronics. Understanding how different materials respond to temperature changes is essential for designing efficient and reliable electrical systems. Whether you're an electrical engineer, a hobbyist, or simply someone who uses electronic devices in everyday life, knowing how temperature affects resistance can help you make more informed choices and ensure the reliable operation of electrical systems in a wide range of conditions.
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