The effect of temperature on resistance

Introduction: (Initial Observation)

Resistors are among the most important electronic components that regulate the flow of electricity inside an electric circuit such as a radio and a television. While an electronic device is working, it will get hot and heat often disables the device. I have seen old radios, old televisions and modern computers that fail to function properly, when they get hot.

I am wondering if some of these problems are caused by change of resistance when a resistor gets hot.

Resistance will be measures by an Ohm meter or multi-meter. The above image shows the display of a multi-meter.

If temperature really affect the resistance of conductors, it may also affect the price of electricity in different weather temperatures. There are thousands of miles of wires carrying electricity from power plants to our homes. The resistance of wires converts part of such energy to heat and the portion that gets to the destination is usually lower than the actual production of electric power. If the resistance of these wires is changing by the change in the weather temperature, then the amount of energy wasted during transmission will also change. Such change may eventually affect the production and cost of energy as well.

I have decided to see how does the resistance change by changes in temperature. Actually I have another good idea too. If the change in temperature can cause change in resistance, I can then used such effect to build an electronic thermometer. How else can you benefit from changing resistance by heat?

Information Gathering:

Find out about resistance and resistors. Read books, magazines or ask professionals who might know in order to learn about the factors that may affect the resistance of a conductor. Keep track of where you got your information from.

Following are sample of information that you may find in electronic books.

Common Resistor: Resistors are color coded for easy reading.

To determine the value of a given resistor look for the gold or silver tolerance band and rotate the resistor as in the photo above. (Tolerance band to the right).

Look at the 1st color band and determine its color. This maybe difficult on small or oddly colored resistors. Now look at the chart and match the 1st color to the “Digit it represents”. Write this digit down. Now look at the 2nd color band and match that color to the same chart. Write this digit next to the 1st Digit.

(By now you should have a two digit number such as 20)

The Last color band is the number you will multiply the result by. Match the 3rd color band with the chart under multiplier. This is the number you will multiple the previous 2 digit numbers by. Write it next to the other 2 numbers with a multiplication sign before it. Example : 1 0 x 100.

To pull it all together now, simply multiply the first 2 digit numbers by the Multiplier.

Example:

• First color is red which is 2
• Second color is black which is 0
• third color is yellow which is 10,000
• Tolerance is silver which is 10%
• Therefore the equation is:

2 0 x 10,000 = 200,000 Ohms

Resistor Color Code Chart

 

1st. & 2nd Color Band	Digit it Represents	—–Multiplier—–
BLACK	0	X1
BROWN	1	X10
RED	2	X100
ORANGE	3	X1,000 or 1K
YELLOW	4	X10,000 or 10K
GREEN	5	X100,000 or 100K
BLUE	6	X1,000,000 or 1M
VIOLET	7	Silver is divide by 100
GRAY	8	Gold is divide by 10
WHITE	9	Tolerances Gold= 5% Silver=10% None=20%

 

Tolerance Explanation

Resistors are never the exact value that the color codes indicate. Therefore manufacturers place a tolerance color band on the resistor to tell you just how accurate this resistor is made. It is simply a measurement of the imperfections. Gold means the resistor is within 5% of being dead-on accurate. Silver being within 10% and no color band being within 20%. To determine the exact range that the resistor may be, take the value of the resistor and multiply it by 5,10, 0r 20%. That is the number that the resistor may go either way.

Example: A 1,000 Ohm resistor with a gold band maybe any value between 950 to 1050 Ohms.

Example: A 22,000 Ohm resistor with a silver band maybe any value between 19,800 and 24,200 Ohms.

Like to know more?

If you like to do some additional study (college level), search the Internet for the Temperature Coefficient of Resistance

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to see the effect of temperature on the resistance of a particular conductor.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

The independent variable (also known as manipulated variable) is the temperature.

The dependent variable (also known as responding variable) is the resistance.

Controlled variables are the measuring instruments and experiment procedures.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. Following is a sample hypothesis:

Heat can increase the resistance of a resistor. My hypothesis is based on my information about the effect of heat on material. Heat energy can potentially transform a solid to liquid and a liquid to gas. As this happens, the molecules or atoms of the substance distance each other; so it will be more difficult for electrons to jump from one atom to the other.

Another sample hypothesis:

Heat can increase the resistance of a resistor. My hypothesis is based on my gathered information about super conductors. A superconductor is a material that conducts electricity without resistance. Mercury that is a liquid metal will become a super conductor at extremely low temperatures. So all conductors may be a better conductor in lower temperatures.

Your hypothesis may also be opposite to the above hypothesis and be proven wrong. This is an example:

Heat can decrease the resistance. My hypothesis is based on my gathered information that heat can increase the motion in electrons. Since electricity is a movement of electrons, heat must be able to reduce the resistance for such movements.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment:

Introduction:

In this experiment you will test the resistance of a tungsten wire at different temperatures. You use tungsten because it is a commonly used resistor. Tungsten is used as the filament of incandescent light bulbs (regular light bulbs), as well as the heating element of electrical heaters. You will use an Ohm meter to measure the resistance. Almost all multi-meters are also an ohm meter. Multi-meters need a battery or other power source to work as an Ohm-meter.

Materials:

1. Tungsten, carbon or nichrome filaments (nichron is a Nickel and Chromium alloy). Heat element of most heaters is made of nichrome. Filament of most light bulbs is made of tungsten. Can also be purchased from some hardware stores.
2. Ohm meter (Multi-meter, set it to Ohms)
3. Thermometer (Food thermometer works well)
4. Glass beaker or any other glass or ceramic pot that can be used to heat up some oil
5. Heating device such as a hotplate

Caution:

1. Make sure that your resistor is not touching any other conductive material while doing your experiment. That is why you use a glass or ceramic container to hold the hot oil.
2. Make sure your heating condition does not cause chemical change on the resistor. That is why you use hot oil.
3. Make sure the connections are secure. Loose connections can cause additional resistance.

Procedure (Methods):

1. Fill 1/4th of a 400 ml glass beaker with motor oil, mineral oil or liquid paraffin wax.
2. Wind about one foot of the tungsten wire around a rod to make a coil. Remove the rod and connect the ends of the coil to solid, unshielded copper wires (shown in red in the image). You use unshielded copper wire because heat can belt the plastic insulations of the wire and disrupt your experiment.

3. Insert the tungsten wire with attached copper wires into the oil in the beaker.

4. Place the beaker in refrigerator to make it as cold as possible.

5. Remove the beaker and measure the resistance of the resistor. To do that, attach the ohm meter probes to the copper wires hanged outside the beaker. Since the resistance of copper is very low, you can practically ignore it in this experiment. Remember that ohm meters need to be set to zero (0) every time that you want to use them. To do that connect the probes to each other and then use the nub on the meter to make the adjustment.

6. Record the temperature of the oil and the resistance of the wire (tungsten wire).

7. Start to warm up the beaker slowly on a hotplate or electric heater. Make sure the temperature will increase slowly.

8. Record the resistance and the temperature as temperature increases. You can make your recordings after every 20ºC temperature increase or every one minute.

9. Record your results in a table like this:

10. Use the above table to draw a graph.

Some images about this experiment:

Another method of experiment:

You may do this experiment using an ammeter instead of an Ohmmeter. Since lower resistance will result more current, you can see the same results. Multi-meters can also be used as an ammeter.

You may also use the formula of V=I x R to calculate the resistance.

For example if your power supply is producing 6 volts and the current is 0.7 amps, then resistance is R = V : I = 6 : 0.7 = 8.5 Ohms

Materials and Equipment:

List of material can be extracted from the experiment section.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Calculations:

You may need to use the current to calculate the resistance as described in another method of experiment.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

References:

List of References

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html

http://spiff.rit.edu/classes/phys273/manual/temp_coeff.html