Magnets and electromagnets- What affects the strength of an electromagnet?

Introduction: (Initial Observation)

An electromagnet is a main component of many household and industrial equipments. Electric bell, speaker, television, relay, electric valve, computer hard drive and thousands of other machines could not exist without electromagnets.

Because of the importance of the electromagnet and its massive use in different machinery, it has been a challenge for scientists to make stronger electromagnets without increasing its size and without using more material. For example, an electric valve used in a dish washing machine is using an electromagnet to open and close the valve. If the electromagnet is not strong enough, the valve may fail to open or may fail to close when needed.

The above picture shows the Joseph Henry’s Electromagnet of 1831
Source: Photographic Services: Smithsonian National Museum of American History

In this project you will study the factors affecting the strength of an electromagnet.

The outcome of this project can be used in designing stronger electromagnets.

Information Gathering:

Gather information about electromagnets. Read books, magazines or ask professionals who might know in order to learn about the factors that may affect the strength of an electromagnet. Keep track of where you got your information from.

Following are sample of information that you may find about electromagnet.

An electromagnet is simply a coil of wire. It is usually wound around an iron core. When connected to a DC voltage or a current source, the electromagnet becomes energized, creating a magnetic field just like a permanent magnet.

Electromagnetism II

In your studies about electromagnetism, you have learned about the electromagnetic field formed around a single conductor. A different condition exists when two parallel conductors are carrying equal currents in the same direction. A magnetic field forms in the clockwise direction around each conductor, with the magnetic lines between the conductors opposing each other. The magnetic field between the conductor is canceled out, leaving essentially no field in this area. The two conductors then move toward each other, from a strong field into a weak field.
Two conductors lying alongside each other carrying equal currents in the same direction create a magnetic field equivalent to one conductor carrying twice the current. When several more conductors are placed side by side, the magnetic effect is increased as the lines from each conductor join and surround all the conductors.

A straight current-carrying wire, when formed into a single loop, has the same magnetic field surrounding it as when it was straight. Using the Right Hand Rule, all the lines of force enter the inside of the loop of wire on one side and leave on the other side. The lines of force are concentrated inside the loop. A single loop of wire carrying current is called a basic electromagnet.

When a current-carrying wire is wound into a number of loops to form a coil, the resulting magnetic field is the sum of all the single loop’s magnetic fields added together. This arrangement is similar to several conductors lying side by side carrying current in the same direction. With the lines of force leaving the coil at one end and entering at the other end, a north and south pole are formed at the coil ends, the same as in a bar magnet.

In order to determine the polarity of the coil ends, apply the Right Hand Rule for Coils by grasping the coil with the fingers pointed in the direction of current flow. The thumb will then point toward the N pole of the coil. Of course, if the current direction through the coil is reversed, the polarity of the coil ends will also reverse.
When a coil is wound over a core of magnetic material such as soft iron, the assembly becomes a usable electromagnet. The strength of the magnetic field at the N and S poles is increased greatly by the addition of the soft iron core. The reason for this increase is that air is a very poor conductor of magnetic lines, and iron is a very good conductor. The use of soft iron in a magnetic path will increase the magnetic strength by about 2,500 times over that of air.

The strength of the magnetic poles in an electromagnet is directly proportional to the number of turns of wire and the current in amperes flowing in the coil. An electromagnet having one ampere flowing through 1,000 turns and another electromagnet having 10 amperes flowing through 100 turns will each create 1,000 ampere-turns. This is a measure of the magnetic field strength. The attraction on magnetic materials located in the magnetic field of each of these electromagnets will be the same.

Just as electric current flows through a closed circuit, the lines of force created by a magnet occupy a closed magnetic circuit. Since the same number of lines that come out of the N pole must also enter the S pole, a complete circuit must be present for each magnetic field.

The resistance that a magnetic circuit offers to lines of force, or flux, is called reluctance. The reluctance is similar to resistance in an electrical circuit.

There is an equation for an electromagnetic circuit that is similar to Ohm’s Law for the electric circuit. This equation for the magnetic circuit can be expressed as follows:

Number of Magnetic lines is proportional to:

Ampere-Turns
Reluctance

It is not necessary for us to study this equation in detail at this time. However, there are two important facts that should be observed. The first observation is that the number of magnetic lines, or strength of the field, is directly proportional to the ampere-turns. To an electromagnet, the field strength will be increased if the current in amperes flowing through the coil is increased.

The second observation is that the number of lines or field strength is inversely proportional to the reluctance; that is, if the reluctance increases then the field strength decreases. Most magnetic circuits consist of iron and short air gaps. The reluctance of such a series circuit is equal to the iron reluctance added to the air gap reluctance.

The effect of an air gap on the total reluctance of a circuit is very pronounced. This is because air has a much higher reluctance than iron. To illustrate this fact, consider a magnetic circuit with a short air gap that has a field strength of 10,000 lines of force. If the length of the air gap is doubled, the reluctance will almost double, and the field strength will be reduced to approximately 5,000 lines of force. Although the air gap represents only a very short segment of the total magnetic path, increasing the air gap from .1 inch to .2 inch may cut the field strength almost in half.

The subject of electromagnetism can be summarized in the following statements.

• Electricity and magnetism are related, because a magnetic field is established around a conductor that is carrying a current.

• An electromagnet has a N pole at one end and a S pole at the other end of the iron core, much like a bar magnet.

• Every magnetic field has a complete circuit that is occupied by the lines of force.

• The amount of flux created by an electromagnet is directly proportional to the ampere-turns, and inversely proportional to the reluctance.

More about Electromagnetism

Any moving charge creates a magnetic field around it. Both a simple beam of charges (such as in a TV set) or current moving in a wire generate a magnetic field. The direction of the magnetic field can be found using the right-hand grip rule. The wire is gripped in the right hand so that the thumb lines up with the direction of current flow. The direction of the magnetic field is given by the curl of the fingers.

Electromagnetism is a temporary effect caused by the flow of electric current and disappears when the current is turned off. The total field can be increased by using a conductor shaped into a loop, or series of coils (solenoid). The shape of the magnetic field can be given by another right-hand rule. Grip the coil with the right-hand with the fingers wrapped in the direction of current flow, and the thumb will point to the North pole or in the direction of the magnetic field through the center of the coil. This is often called the right-hand solenoid rule.

The field along the central axis of a solenoid is quite strong.

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Question/ Purpose:

The purpose of this investigation is to establish a connection between possible factors and the strength of an Electromagnet. Each possible factor is one independent variable and its effect must be testable by an experiment. The following are individual questions that can be studied in order to determine the effect of different variables on the strength of an electromagnet.

1. How does the number of turns of wire in a coil affect the strength of an electromagnet?
2. How does the current affect the strength of an electromagnet?

Additional questions that can be studied are:

1. How does the type of core material affect the strength of an electromagnet?
2. How does the diameter of core affect the strength of an electromagnet?

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.

For question number 1, variables may be defined as follows:

• Independent variable (also known as manipulated variable) is the number of loops of wire.
• Dependent variable (also known as responding variable) is the strength of the electromagnet.
• Constants are the type and diameter of wire, type and size of the core, electric current, experiment and measurement method.

For question number 2, variables may be defined as follows:

• Independent variable (also known as manipulated variable) is the electric current used to establish and maintain the magnetism of the electromagnet. We change the current by changing the voltage or by including a variable resistor in the circuit.
• Dependent variable (also known as responding variable) is the strength of the electromagnet.
• Constants are the type and diameter of wire, number of loops of wire, type and size of the core, experiment and measurement method.

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. For every question or every variable, you need to propose one hypothesis. Following are two sample hypothesis.

1. As the number of coils increases the strength of the magnetic field will also increase (therefore it should hold on more amount of iron filings)
2. As the current increases the magnetic field becomes stronger.

You may come up with your own hypothesis.

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 1:

How does the number of turns of wire in a coil affect the strength of an electromagnet?

Introduction:
An electromagnet is often a piece of metal in a coil of wire. When the wire has no current flowing through, the metal is just a regular piece of metal that possesses no magnetic ability. However when the current is passed through the wire, the metal becomes magnetized. This is because the flow of electrons in wire creates a magnetic field; thus the metal adjacent to it also becomes magnetized.

Material:

1. Iron filings
2. Small analog or digital scale
3. Battery or power supply
4. Insulated solid copper wire (AWG 23 up to AWG 28)

Procedure:

1. Wrap 10 loops of insulated solid copper wire around a large nail. Leave about 15 cm of free wire on each end. Remove the insulation from the ends of the wire where the wire will touch the battery.
2. Connect the ends of the coil wire to a power supply or battery.
3. Hold the nail over a container of iron filings or small nails (or other metal pieces). Let the nail attract and hold as much iron filings as possible.
4. Move the nail away from the iron filing container and hold it over a piece of paper. At this time turn the power off (disconnect the battery) so the iron filings will drop. Weigh all of the iron filings that drop off the nail.

5. Repeat this experiment (steps 1 to 4) 3 times with 10, 20, 30, 40, 50 turns of wire. (You may optionally go up to 100 turns of wire). For each number of loops calculate the average weight of iron filings.

Record your results in a table like this:

6. Use your results table to draw a line graph. Use the X axis for the number of loops of wire and Y axis for the weight of iron filings lifted.

Safety Notes: Only use batteries or low voltage DC power suppliers for this experiment. Adult supervision is required.

Experiment 2:

How does the current affect the strength of an electromagnet?

1. Coil 2.5 meter of insulated solid copper wire around a large nail. Leave about 15 cm of free wire on each end. Remove the insulation from the ends of the wire where the wire will touch the battery.
2. Connect the ends of the coil wire to a power supply or battery.
3. Hold the nail over a container of iron filings or small nails (or other metal pieces). Let the nail attract and hold as much iron filings as possible.
4. Move the nail away from the iron filing container and hold it over a piece of paper. At this time turn the power off (disconnect the battery) so the iron filings will drop. Weigh all of the iron filings that drop off the nail.

5. Repeat this experiment (steps 1 to 4) 3 times with 5 different currents. For each current calculate the average weight of iron filings.

Record your results in a table like this:

6. Use your results table to draw a line graph. Use the X axis for the number of loops of wire and Y axis for the weight of iron filings lifted.

Note 1: In this experiment only change the current. All other factors will be kept constant. Measure the amount of iron filings at 5 different currents. Repeat each experiment three times for accuracy.

Note 2: If you are using a DC power supply, it most likely has a built-in ampere meter, otherwise you need to connect an ampere meter in series with your coil and your electric source.

How do I change the current?

There are three different ways that you can change the current for this experiment:

1. Increase the voltage so the current will increase as well. If you are using batteries, you can connect your batteries in series to increase the voltage, so that they can produce a higher voltage. For example one AA battery has a 1.5 volt output. 2 AA batteries in series will give 3 volts. 3 AA batteries in series will give 4.5 volts.
2. You may use 6-volt lantern batteries for this experiment. If you use lantern batteries, you may use a variable resistor in series with your ammeter to control the current.
3. You may use a DC power supply that can control the current. With such a power supply you can control the output voltage and current.

Make sure

1. To use the same amount of iron filings in each test
2. Ampere meter reading is as accurate as possible
3. Try to make an exact number of Coil turns (e.g. no bent wires etc)
4. Same amount of wire because the resistance of it can vary the results from each other.

Safety Notes: Only use batteries or low voltage DC power suppliers for this experiment. Adult supervision is required.

Materials and Equipment:

1. Insulated wire, stripped at both ends
2. Connector wires (optional)
3. Ampere meter (or multi-meter)
4. AC power supply (Or batteries)
5. Iron nail (or soft iron rod)
6. Variable resistor (6 volt)
7. Crocodile clips (optional)
8. Iron filings
9. Scale

Most or all of the above material are available at MiniScience.com.

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.

Following are some sample results. DO NOT trust this sample results table. Do your own experiments.

This is a table, containing sample experiment results for experiment 1, but you need to do your own tests.

Mass of paper (0.62) is now taken away from the results

Calculations:

Each test is repeated 3 times, so we needed to calculate the average of 3 results in each test.

Summary of Results:

Analyzing Evidence

Clear Patterns: Method-Experiment 1

Here there is a very clear curve. It starts very closely and then increases rapidly. This shows that one coil does not increase the magnetic power one unit (m).

Conclusions-Experiment 1

The number of coils around an electromagnet is not proportional to the electromagnetic strength.

Scientific Explanation-Experiment 1

When more coils are in contact with the core, which is what the experiment shows, more area of interaction is available, allowing more domains to be magnetized quicker than if the area is smaller. So if there is more area, the magnetic area will become stronger.

Clear Patterns: Method-Experiment 2

Here there is a very clear curve at the beginning, but at the final three amps there is a similar per amp rate of increase suggesting the opposite of experiment 2. This shows that one amp does increase the magnetic power one unit. So basically, the more expended, the same return on the investment.

The Scientific Explanation-Experiment 2

As the current is passed on to the wire, it becomes a magnet itself, in accordance with the right hand rule. It magnetizes the “substance” at the core. Inside the “substance” there are domains. When they are magnetized they aligned themselves onto the same direction, making it magnetic. This needs energy to perform and maintain the process. It is not cumulative so the more energy put into it the more energy that will be produced.

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.

Following is a sample conclusion.

The strength of the electromagnet does increase by the number of coils of wire around the nail, however such increase is not proportional.

A similar conclusion can be used for the effect of a current on the strength of electromagnet.

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:

Visit your local library and find books about Electricity and Magnetism. Many physics books contain a chapter about magnetism.

Following are some web resources:

A guide to Magnetism throughout the ages

Magnetism on Encyclopedia.com

Introduction to magnetism

Strong Electromagnets:

In the winter of 1820, it occurred to Professor Oersted, of Copenhagen, to try a new way to find the answer. On a table before him lay a compass and beside it was one of Volta’s batteries. He connected the wires to complete the circuit of the battery, and brought one wire close to the side of the compass parallel to the needle. The needle swung around, just as if he had a magnet in his hand. When the current was sent through the wire toward the north, the needle moved to the left. When the current was sent through the wire toward the south, the needle swung to the right. Oersted saw he had made a discovery. Passing an electric current through a wire makes a magnet of the wire. “Magnetism,” he said, “is but electricity in motion.” Oersted’s discovery was of importance, for it led to the invention of the electromagnet, one of the most useful electrical inventions.
The first electromagnet was made by Sturgeon, an Englishman. He took a round bar of very soft iron and bent it in the shape of a horseshoe. Around this he wrapped a wire, and through the wire he passed an electric current. He varnished the core, as the iron bar is called, to keep the electricity from flowing off or away from it.

Sturgeon was surprised at the way the electromagnet worked. It was very much stronger than a natural or permanent magnet of the same size. But the most surprising element was that the instant the current was turned on, the iron core became a magnet, and when the current was turned off the core practically ceased to be a magnet. It might be thought that this peculiar action of the electromagnet would make it a useless plaything, but it is this very action which makes it so useful. If a needle or other object is picked up with a permanent magnet, the only way to get it off the magnet is to scrape or pull it off; but to get it off an electromagnet, it is necessary only to break the electric current. The electromagnet is thus under our control. To put it to work, we turn on the electric current; to make it stop working, we turn off the current. You do this every time you push the button of an electric doorbell.
We can control also the power of the electromagnet, that is, the size of the load it will lift. The man who taught us how to do this was Joseph Henry, an American. Instead of varnishing the iron core as Sturgeon had done, to keep the electricity from flowing off, or to insulate it, Henry insulated the copper wire by covering it with a wrapping of silk. Instead of putting a single turn of wire round the iron core, he put many turns. On his first electromagnet he put thirty-five feet of wire, making about four hundred turns. These additional turns increased the strength of the magnet very much.

Henry found that the magnet was stronger when wound with a number of separate coils of wire, the ends of each coil being connected with the battery. With a small battery, one of Henry’s electromagnets lifted eighty-five pounds, and in 1831 he exhibited a magnet which lifted thirty-six hundred pounds. Thus by using a small or large battery, small or large iron cores, a few or many coils of wire, electromagnets of different strength can be made.
Henry was also the first to make the electromagnet do work at a distance, and to show us how it could be made useful. In telling of this he says: “I arranged around one of the upper rooms in the Albany Academy a wire more than a mile in length, through which I was enabled to make signals by sounding a bell.” This first electric bell was made up of a permanent magnet about ten inches long, supported on a pivot, and placed with one end between the two poles of an electric magnet. When the current was passed through the electromagnet, this caused the bar magnet to swing and strike the bell.

Small electromagnets by the millions are now in use. In connection with the electric battery, they ring our doorbells, sound alarms, move signals, and the like.