Introduction: (Initial Observation)
Electric current is the most important element in each electric circuit or electronic circuits. In designing a circuit and selecting material such as wires, fuses and switches and even electronic components, current is the main factor that should be considered. When a circuit is open, there is no current but when it is closed, current is determined by what is in the circuit. In this project we will investigate the factors that affect the electric current in a simple circuit.
Find out about electric current. Read books, magazines or ask professionals who might know in order to learn how does the type of conductor, temperature and filament affect the electric current. Keep track of where you got your information from.
Initial studies on the basics of electricity shows when the concentration of electrons in one spot is more than the concentration of electrons in another spot, connecting these two spots using a conductor will create a flow of electrons from the concentrated side to the other side. This flow continues until the concentration of electrons on both sides of the conductor are the same. The amount of electrons that pass trough the conductor per second is electric current and has a unit called Ampere. But how much electrons will pass per second (current) depends on how easy they can pass.
Think about a narrow hall way. The number of people who can pass trough this hall way is limited by the width of the hall way and other objects in the hall ways that may act as a resistance and slow down the passing traffic.
The same is true for conductors. some conductors are like a clean hall way and are also wide, so they create no resistance to slow down the passing traffic. Some other conductors are like a narrow and long hall way that is also filled with too many furniture that you have to jump over them. So they have a higher level of resistance toward passing electrons.
In other words less resistance results a higher electric current and more resistance results a lower electric current. So current and resistance have an opposite relation. This is helpful to us because we can focus on the resistance of the conductor. Testing the resistance is much easier than testing the current. Specially current is depended on voltage too, but resistance is not.
The property of a conductor of electricity that limits or restricts the flow of electric current is called its resistance. Electrical pressure is required to overcome this resistance, which is the attractive force holding the electrons in their orbits. The materials from which electrical conductors are manufactured, usually in the form of extruded wire, are materials that offer very little resistance to current flow.
While wire of any size or resistance value may be used, the word “conductor” usually refers to materials which offer low resistance to current flow, and the word “insulator” describes materials that offer high resistance to current. There is no distinct dividing line between conductors and insulators; under the proper conditions, all types of material conduct some current. Materials offering a resistance to current flow midway between the best conductors and the poorest conductors (insulators) are sometimes referred to as “semiconductors,” and find their greatest application in the field of transistors.
The best conductors are materials, chiefly metals, which possess a large number of free electrons; conversely, insulators are materials having few free electrons. The best conductors are silver, copper, gold, and aluminum, but some nonmetals, such as carbon and water, can be used as conductors. Materials such as rubber, glass, ceramics, and plastics are such poor conductors that they are usually used as insulators. The current flow in some of these materials is so low that it is usually considered zero. The unit used to measure resistance is called the ohm. The symbol for the ohm is the Greek letter omega (W). In mathematical formulas, the capital letter “R” refers to resistance. The resistance of a conductor and the voltage applied to it determine the number of amperes of current flowing through the conductor. Thus, 1 ohm of resistance will limit the current flow to 1 ampere in a conductor to which a voltage of 1 volt is applied.
Factors Affecting Resistance
Among the four major factors affecting the resistance of a conductor, one of the most important is the type of conductor material. It has been pointed out that certain metals are commonly used as conductors because of the large number of free electrons in their outer orbits. Copper is usually considered the best available conductor material, since a copper wire of a particular diameter offers a lower resistance to current flow than an aluminum wire of the same diameter. However, aluminum is much lighter than copper, and for this reason as well as cost considerations, aluminum is often used when the weight factor is important.
A second resistance factor is the length of the conductor. The longer the length of a given size of wire, the greater the resistance. Figure 8-14 pictures two wire conductors of different lengths. If 1 volt of electrical pressure is applied across the two ends of the conductor that is 1 foot in length and the resistance to the movement of free electrons is assumed to be 1 ohm, the current flow is limited to 1 ampere. If the same size conductor is doubled in length, the same electrons set in motion by the 1 volt applied now find twice the resistance; consequently, the current flow will be reduced by one-half.
A third factor affecting the resistance of a conductor is cross-sectional area, or the end surface of a conductor. This area may be triangular or even square, but is usually circular. If the cross-sectional area of a conductor is doubled, the resistance to current flow will be reduced in half. This is true because of the increased area in which an electron can move without collision or capture by an atom. Thus, the resistance varies inversely with the cross-sectional area of a conductor.
To compare the resistance of one conductor with that of another having greater cross-section area, a standard, or unit, size of conductor be established. The most convenient unit of measurement of wire diameter is the mil (0.001 of an inch). The most convenient unit of wire length is the foot. Using these standards, the unit of size will be the mil-foot. Thus, a wire will have unit size if it has a diameter of 1 mil and the length of 1 foot.
The resistance specified in ohms of a unit conductor of a certain material is called the specific resistance, or specific resistance of the substance.
The square mil is a convenient unit of cross-sectional area for square or rectangular conductors. A square mil is the area of a square, each side of which measures 1 mil.
To compute the cross-sectional area of a conductor in square mils, the length in mils of one side is squared. In the case of a rectangular conductor, the length of one side is multiplied by the length of the other. For example, a common rectangular bus bar (large, special conductor) is 3/8 inch thick and 4 inches wide. The 3/8 inch thickness may be expressed as 0.375 inch. Since 1,000 mils equals 1 inch, the width in inches can be converted to 4,000 mils. The cross-sectional area of the rectangular conductor is 0.375 x 4,000 or 1,500 square mils.
More common than the square or rectangular shape is the circular conductor. Because the diameters of round conductors may be only a fraction of an inch, it is convenient to express these diameters in mils to avoid the use of decimals. The circular mil is the standard unit of wire cross-sectional area used in American and English wire tables. Thus, the diameter of a wire that is 0.025 inch may be more conveniently expressed as 25 mils.
Figure 8-15 illustrates a circle having a diameter of 1 mil. The area in circular mils is obtained by squaring the diameter measured in mils. Thus, a wire with a diameter of 25 mils has an area of 25 squared, or 25 x 25, or 625 circular mils.
In comparing square and round conductors, it should be noted that the circular mil is a smaller unit of area than the square mil. To determine the circular mil area when the square mil area is known, the area in square mil is divided by 0.7854. Conversely, to find the square mil area when the circular mil area is known, the area in circular mils is multiplied by 0.7854.
Wires are manufactured in sizes numbered according to a table known as the American wire gauge (AWG). Wire diameters become smaller as the gauge numbers become larger.
The last major factor influencing the resistance of a conductor is temperature. Although some substances, such as carbon, show a decrease in resistance as the ambient (surrounding) temperature increases, most materials used as conductors increase in resistance as temperature increases. The resistance of a few alloys, such as constantan and manganin, change very little as the temperature changes. The amount of increase in the resistance of a 1 ohm sample of a conductor per degree rise in temperature above 0° Centigrade (C), the assumed standard, is called the temperature coefficient of resistance. For each metal this is a different value; for example, for copper the value is approximately 0.00427 ohm. Thus, a copper wire having a resistance of 50 ohms at a temperature of 0° C will have an increase in resistance of 50 x 0.00427, or 0.214 ohm, for each degree rise in temperature above 0° C. The temperature coefficient of resistance must be considered where there is an appreciable change in temperature of a conductor during operation. Charts listing the temperature coefficient of resistance for different materials are available.
The purpose of this project is to learn how is current affected by type of conductor, temperature, filament, etc.
Variables that may affect the electric current in a conductor are:
- The voltage supplied to the conductor
- The type of conductor (material type/ resistance of material)
- The thickness or diameter of conductor
- The length of the conductor
- The temperature of conductor
Based on your gathered information, make an educated guess about what types of things affect the electric current. Identifying variables is necessary before you can make a hypothesis. Since we have suggested 7 different possible variable, I suggest one hypothesis for each variable.
- The voltage supplied has a positive affect on the electric current. In other words by increasing the voltage, current will increase too.
- The type of conductor (material type/ resistance of material) has an effect opposite the resistance of the conductor. Higher resistance results lower current.
- The thickness or diameter of conductor actually has an affect on the resistance, as a result it will affect the current too. More thickness on the conductor results a higher electric current.
- The length of the conductor also affects the resistance. Longer conductors have a higher resistance. So length has a negative affect on the electric current.
- The temperature of conductor will increase the resistance of conductor, so it reduce the electric current in the conductor.
You need to make some preparation prior to start your experiments. First make a simple electric circuit consisting of a switch, a 6 volts battery, a meter to show the electric current and finally your conductor. You will need to test many different conductors, so make this simple to exchange. If you decide to test resistance and use that to calculate current, you will not need all this. Details is described further down.
You may mount two nails or two alligator clips to be the base for your conductor. This makes it easier to change the conductor in a short period of time.
For a meter, you can use an ammeter (Ampere meter) or a general purpose multi-meter. If you want to use a multi-meter, make sure you set it up to measure current before connecting it to the circuit. Failure to do so can damage your multi-meter.
To test each conductor, connect it to the circuit, close the switch, read the current (How many amps or milliamps) open the switch and record the result. Do not keep the circuit closed for a long time because doing that will discharge your battery and your experiment results will not be accurate. If you are using an analogue meter and meter goes to the max, stop the experiment, switch your meter to a higher range and try again. Lower the range if the meter just makes a small move. If the meter moves backward, that means that it has a wrong polarity. open it and reinstall it in the opposite direction.
If you really understand the relation between resistance of a conductor (such as a wire) and the electric current created in that conductor in a closed circuit, it is much easier to just test the resistance of your conductor. By knowing the resistance, you can simply calculate what the current will be for any possible voltage. To do this use the formula V = I * R or I = V / R
In the above formula, V is voltage, I is current and R is resistance. For example if you test the resistance of a conductor and it is 5 ohms, you can connect the conductor to a 6 volts battery and current will be 6 / 5 or 1.2 Amps.
In this experiment we test the effect of voltage on the current. You can use any conductor such as thin copper wire, thin Iron wire, salt water, a strip of aluminum foil or pencil lead. First connect your conductor and test the current while you have one regular 6 volts battery in the circuit.
Record the results and place a second battery in series with the first one to produce 12 volts. Test the current again and record the results.
Remove the second battery after this experiment. All other experiments will be done by one battery only.
In this experiment we test the type of conductor. conductors that you can test are graphite (lead of a pencil), copper wire, Iron or steel wire, aluminum wire and tungsten wire. All the wires that you test must be the same length and the same diameter as your graphite sample. If you can not find any of the material for this experiment, simply skip this experiment and go to the next. The purpose of this experiment is to show that each material has it’s own specific resistance regardless of size and shape. So if we test same size, same shape conductors made of aluminum, graphite, copper and tungsten, you will get 4 different currents.
In this experiment we want to see how does the diameter of a conductor affect the current. You can use graphite as a conductor. Use a very thin graphite rod and test the current. Then use a thick graphite rod and do the test again. Record the current in both tests. You should test the same length of both samples.
If you do not have graphite rods with different diameters, use your pencil and a ruler to draw 3 lines on a piece of paper. One line must be narrow. Second one a little wider (about double the first). and third one must be the widest (double the second one). Now use a multi-meter and measure the resistance of 1 centimeter of each line.
You will see that wider line has less resistance, so can create a higher electric current.
In this experiment we test the effect of conductor length on the current. Use a long graphite rod as conductor and test the current. Then cut it in half and test the current again.
Again you can use one of the lines that you draw in the previous experiment to do the same, but measure resistance instead of current. First measure the resistance of 1 centimeter of your thick line. Then measure the resistance of 2 centimeter of the same line.
You will see that by doubling the length, resistance also doubles, but current reduces to half.
In this experiment we test the effect of temperature on the electric current. For this experiment use a very thin iron wire and measure it’s resistance. While it is still connected to the meter, bring the flame of a candle below the wire and see if you notice any change in the resistance.
If the heat slightly increases the resistance, based on the relation of current and resistance we can conclude that heat will slightly reduce current.
Materials and Equipment:
Material that you may use for this experiment are:
- Graphite or pencil lead
- Ammeter or multi-meter
- Copper wires in different diameters (no insulation)
- two 6 volt batteries (known as lantern battery)
- Aluminum wire with the same diameter as graphite
- Iron wire with the same diameter as graphite
- Other optional material such as tungsten filament (can be carefully removed from a broken lamp)
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.
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.
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.
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.
Using light bulb instead of multi-meter
Make a basic electric circuit including a light bulb, a battery and a switch. If the switch is off, the light will stay off because air is not conductive. And when the switch is on, light will turn on because switch is conductive and passes an electric current, creating light in the bulb. Less current creates less light and more current creates more light. Now eliminate switch and replace it with different types of conductors. With changes of the light you may determine the changes of the electric current.You can also make an instrument to measure electric current. Such an instrument is a short coiled wire that will become magnetized with electric current and in turn will spin an indicator.