Introduction: (Initial Observation)
Magnets and electromagnets form crucial and important parts of many machinery and devices that we use in our daily lives. In an electric factory, magnets are used in generators to produce electricity. In power stations, multi coil electromagnets (transformers) are used to increase or decrease the voltage. In our homes, electromagnets are used in our door bells, speakers, radios, televisions, computer monitors, computer hard drives and computer floppy drives.
Motors that run hair dryers, vacuum cleaners, washing machines and electric fans all use magnets and electromagnets. Electromagnets are the main component of anti theft pedestals, metal detectors, MRI machines and many other medical and research equipment.
Designers of such machines use a combination of conductivity and magnetic properties of different materials to achieve specific functions. The ability to become a permanent magnet, the ability to become a temporary magnet and the inability to become a magnet are among such properties.
Magnetic permeability is the ability to direct magnetic flux. This ability is minimum in an empty space. Permeability of all other materials are measured in relation to the permeability of empty space.
Information Gathering:
Find information on the magnetic force, magnetic flux and magnetic permeability. Read books, magazines or ask professionals who might know in order to learn about the magnetic permeability of different material.
Keep track of where you got your information from. The following are samples of information that you may find:
Magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. Some well known materials that exhibit easily detectable magnetic properties are iron, some steels, and the mineral lodestone; however, all materials are influenced to one degree or another by the presence of a magnetic field, although in most cases the influence is too small to detect without special equipment.
Magnetic forces are fundamental forces that arise due to the movement of an electrical charge. Maxwell’s equations describe the origin and behavior of the fields that govern these forces (see also Biot-Savart’s Law). Thus, magnetism is seen whenever electrically charged particles are in motion. This can arise either from movement of electrons in an electric current, resulting in ‘electromagnetism’, or from the constant subatomic movement of electrons, resulting in what are known as ‘permanent magnets’.
The tesla (symbol T) is the compound derived unit of magnetic flux density or magnetic inductivity.
1 T = 1 V · s · m -2 = 1 kg · s -2 · A -1 = 1 N · A -1 m -1 = 1 Wb · m -2
A smaller derived unit, the gauss = 10-4 T, was once used.
Magnetic flux, usually denoted by Greek letter Φ, is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. The flux through an element of area perpendicular to the direction of magnetic field is given by the product of the magnetic field density and the area element. More generally, magnetic flux is defined by a scalar product of the magnetic field density and the area element vector. The Maxwell’s equations in the absence of magnetic monopoles requires that the magnetic flux through a closed surface is zero.
Weber (symbol: Wb) is a unit of magnetic flux, equal to the flux linking a circuit of one turn that produces an electromotive force of one volt when reduced uniformly to zero in one second.
The compound derived CGS unit, the maxwell, abbreviated as Mx, is the unit for the magnetic flux. The unit was previously called a line. The unit name honors James Clerk Maxwell, who presented the unified theory of electromagnetism.
1 maxwell = 1 gauss * cm2 = 10–8 weber
In a magnetic field of strength one gauss, one maxwell is the total flux across a surface of one square centimeter perpendicular to the field.
Inductance is a physical characteristic of an inductor, which is an electrical device that produces a voltage proportional to the instantaneous change in current flowing through it. The symbol L is used for inductance in honor of the physicist Heinrich Lenz. The unit of inductance is the Henry (H).
In a typical inductor, whose geometry and physical properties are fixed, the voltage generated is as follows:
v = -L \frac {di} {dt},
where
v is the voltage generated, measured in volts
di/dt is the rate of change of current, measured in ampere/second
L is the inductance of the device, measured in henry.
The henry (symbol H) is the unit of inductance. If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry.
The henry has dimensions V·A-1·s = m²·kg·s-2·A-2 in units.
Source: Wikipedia Encyclopedia
Some materials are more susceptible to magnetic fields than others. We say they are more “permeable” to the magnetic field. .
This diagram illustrates the magnetic field in the air between two poles of a horseshoe magnet.
This diagram is for the same magnet with a piece of soft iron placed between the poles. The soft iron is more permeable to the magnetic field than the air is. Notice how the field seems to focus the magnetic field. This is a result of the iron actually becoming a magnet while placed in the existing field. As a result, when a compass is moved to map the field it is influenced by the iron. Thus showing a more “focused” or stronger field.
The determination of the relative magnetic permeability of weakly magnetic materials is an important requirement in the manufacture of devices and components for space, particle physics and defense applications. The relative magnetic permeability of a material can be measured by the solenoid method or by the comparator method in accordance with BS 5884:1999. A number of types of commercial instruments are now available for determining the relative magnetic permeability of such materials. Source...
High magnetic permeability ferrite is mainly used in pulse transformers and common-mode chokes for information equipment.
Is there any material that can block a magnetic force? Specifically does lead block magnetic fields?
No, magnetic fields (forces are caused by magnetic fields) cannot be blocked. That is, there is no such thing as a magnetic insulator.
A major reason for this has to do with one of Maxwell’s Equations:
However, magnetic fields can be re-routed around objects. This is a form of magnetic shielding. By surrounding an object with a material which can “conduct” magnetic flux better than the materials around it, the magnetic field will tend to flow along this material and avoid the objects inside. This allows the field lines to terminate on the opposite poles, but just gives them a different route to follow.
Why can’t I just use lead or copper or aluminum foil for magnetic shielding?
In the strictest sense, magnetic shielding is not truly shielding at all. Unlike the way a lead shield stops X-rays, magnetic shielding materials create an area of lower magnetic field in their vicinity by attracting the magnetic field lines to themselves. The physical property which allows them to do this is called “permeability“.
Unlike X-rays, sound, light or bullets, magnetic field lines must travel from the North pole of the source and return to the South pole. Under usual circumstances, they will travel through air, which by definition has a permeability of “1”. However, if material with higher permeability is nearby, the magnetic field lines, efficient creatures that they are, will travel the path of least resistance (through the higher permeability material), leaving less magnetic field in the surrounding air.
Here’s how the permeabilities of some common materials compare:
Air ……….. 1
Copper …… 1
Aluminum … 1
Tin …………. 1
Lead ………. 1
Nickel ……………… 100
Commercial Iron … 200
Stainless Steel ……. 200
Permeability
Permeability is described as the ease with which a material can be magnetized. For non-ferrous metals such as copper, brass, aluminum etc., the permeability is the same as that of “free space”, i.e. the relative permeability (mr) is one. For ferrous metals however the value of mr may be several hundred, and this has a significant influence on eddy current response.
Source…
Keywords:
Some common magnetic terms include:
- Magnetism — The physical phenomenon associated with the attraction of certain materials.
- Magnetic Field — A region of physical attraction produced by an electrical current.
- Solenoid — A current-carrying coil of wire that acts like a magnet when a current passes through it.
- Magnetic PERMEABILITY — the ratio of the magnetic induction or flux-density in any medium to the inducing magnetic force.
In electromagnetism, permeability is the degree of magnetisation of a material in response to a magnetic field. Absolute permeability is represented by the symbol μ. In SI units, permeability is measured in henrys per metre.
where
μ is the permeability, measured in henry per metre
B is the magnetic flux density (also called the magnetic induction) in the material, measured in tesla
H is the magnetic field strength, measured in ampere per metre
Absolute permeability
Absolute permeability is represented by the symbol μ0 and is the permeability of the vacuum, where μ0 = 4π × 10−7 N A−2 (exactly).
Together with permittivity, permeability defines the speed of light.
Relative permeability
Relative permeability, sometimes denoted by the symbol μr, is the ratio of the permeability of a specific medium to the permeability of free space μ0:
| Relative permeability for some materials | |
|---|---|
| Medium |  |
| Hydrogen | 0.008 × 10-6 |
| Copper | −6.4 × 10-6 |
| Water | −8.0 × 10-6 |
| Aluminium | 22.2 × 10-6 |
| Platinum | 265 × 10-6 |
Source: WIKIPEDIA ENCYCLOPEDIA
The most important class of magnetic materials are the ferromagnets: iron, nickel, cobalt and manganese, or their compounds.
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 compare the magnetic permeability of different metals and nonmetals. Specifically I want to compare the magnetic permeability of air, glass, copper, zinc, lead, Iron, chromium, wood and graphite.
You may substitute the above material with other material that are available to you.
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.
Independent variable (also known as manipulated variable) is the type of material. Possible values are Aluminum, Zinc, Copper, Iron, Glass, Nickel, Chromium, Wood, glass, water, plastic…
Dependent variable is the relative permeability of each material.
Constants are the size of material and the experiment 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. This is a sample hypothesis:
Iron has the highest permeability followed by nickel and aluminum. Air will have the lowest magnetic permeability.
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: Observing permeability
Introduction: Magnetic flux can be observed by iron powder. In this experiment we use different material to see how they modify the direction and the concentration of magnetic flux.
Procedure:
Place a bar magnet on a wooden or glass table with no nails or metal objects around. Cover the magnet with a cardboard and sprinkle some iron filings on the board. Observe how iron filings rearrange themselves on the direction of magnetic flux.
Repeat this experiment with a piece of iron rod placed close to the rod magnet. (You will need to tape the magnet and the iron rod to the table. Otherwise they will move and attract each other.)
Observe how the iron rod redirects the magnetic flux.
Repeat this experiment with a few other materials. Can you see which one has a high magnetic permeability and which one doesn’t?
Experiment 2: Relative permeability
Introduction: The resistance of a coil changes depending on the permeability of the core material. We will use this property to compare the permeability of different material. (Note: We have received a report that this method does not provide measurable results. There might be some other details missing from this experiment. Please use the experiment 3 instead of this experiment. We will review and update this experiment later. If you have tried this experiment, please share your results by sending an email to info@ScienceProject.com)
Procedure:
Make a coil of about 500 loops of magnet wire on a paper tube or plastic tube with an inner diameter of one inch.
Remove the insulation from the ends of coil wire and connect them in series to a switch, an ammeter and an AC power adapter. In this way if you close the switch, electricity passes through the coil and you can read the current.
Connect a voltmeter to the ends of the coil.
Note: The effective resistance of a coil is a combination of wire resistance and the resistance caused by Eddy current. That is why we cannot simply use an Ohm meter to measure the effective resistance across the coil. The voltmeter and ammeter together will help us to calculate the effective resistance of the coil using the formula V = I . R or R = V / I.
Close the switch and record the voltage and current with an air core. (When nothing is in the coil, that is the air core). Open the switch immediately to prevent battery discharge and the overheat of wires.
Insert a steel rod in the coil. Close the switch and record the voltage and current with the steel core. Open the switch again and then remove the steel core.
Repeat this last step with different core material that you want to test. Record your results in a table like this:
| Core material | Coil Voltage | Current | Effective resistance |
| Air | |||
| Aluminum | |||
| Iron | |||
| Graphite | |||
| ………….. | |||
| …………. |
Fill up the above data table with the voltage and current from your observation. Calculate the effective resistance for each core material and write them in the last column.
Explanation:
Higher permeability in the core material results in higher concentration of magnetic flux inside the core.
Higher magnetic flux induces a higher eddy current into the core wire.
Higher eddy current results a higher resistance because its direction is opposite of the direction of the main current.
In other words we can use higher effective resistance as an indication of higher permeability of the core material.
Get more reliable results:
There is always a chance that you got wrong or inaccurate results. To make your results more trustable, repeat your experiments 2 more times. You may change the order in which you test different core materials. You will calculate 3 different effective resistance for each core material. Calculate the average resistance and enter that in your final results table.
Experiment 3: Relative permeability
Introduction: The more permeable an object is, the easier it can magnetize in a magnetic field. If the magnetic field is generated by an electromagnet connected to AC (Alternative Current) source of electricity, then we will have a changing magnetic field that can induce electrical waves on other coils near itself. The voltage of such electrical wave has a direct relation with the permeability of the object. In this experiment we use two coils to determine the relative permeability of different objects.
Procedure:
Make a transmitter coil with 50 to 500 loops of wire around a plastic or cardboard tube.
Make a receiver coil with 50 to 500 or more loops of wire around a plastic or cardboard tube.
You may choose to make both coils on the same tube.
Remove the insulation from the ends of the transmitter coil wire and connect them to a 6-volt AC power source.
Optional: It is a good idea if you also connect a switch between one of the wires and the transformer. In this way if you close the switch, electricity passes through the coil.
Remove the insulation from the ends of the receiver coil wire and connect them to a multimeter. Set the meter to read the AC voltage.
Close the switch and record the voltage with an air core. (When nothing is in the coil, that is the air core). Open the switch immediately to prevent overheat of the coil wires.
Insert a steel rod in the coil. Close the switch and record the voltage with the steel core. Open the switch again and then remove the steel core.
Repeat this last step with different core materials that you want to test. Record your results in a table like this:
| Core material | Coil Voltage |
| Air | |
| Aluminum | |
| Iron | |
| Graphite | |
| ………….. | |
| …………. |
Fill up the above data table with the induced voltage for each core material you test.
Explanation:
Higher permeability in the core material results in higher concentration of magnetic flux inside the core. Since the power source creates Alternative Current, a changing magnetic field will form in the core. The changing magnetic field of the core induce a voltage on the receiving coil.
Get more reliable results:
There is always a chance that you got wrong or inaccurate results. To make your results more trustable, repeat your experiments 2 more times. You may change the order in which you test different core materials. You will calculate 3 different effective resistance for each core material. Calculate the average resistance and enter that in your final results table.
Important note about the coils:
The experiment is suggesting to use 50 loops to 500 loops of wire on each coil. So I need to explain how does the number of wire loops affect the results. A simple answer is that transmitter coil and receiving coil work like the primary and secondary loops in a transformer.
More wire on the transmitter coil results a stronger magnetic flux on the core. More wires on the receiver result a higher voltage on the receiver coil. If your multimeter is not able to measure millivolts, then you need to have more wire on the receiver coil.
Experiment Pictures:
The transformer is a 6-Volt AC transformer from a computer external speaker.
The connector of the transformer is removed and the wires are prepared for direct contact. Insulation is removed from the ends of the wire.
Two coils are about 1 cm apart. The cardboard tube used here is too long. About 2 inches of that would be enough for this experiment.
Materials and Equipment:
Sample list of material:
- 150 feet magnet wire 28 AWG for making a wire coil.
- Digital or analog voltmeter.
- Digital or analog ammeter with ability to show milliamps.
- 12-Volts AC power adaptor
- One simple switch (any switch that helps you close and open the circuit).
You may use two multimeters, one set to measure the DC voltage and the other set to measure the DC current. You will get better results if you use digital multi-meters that can measure current to milliamps.
AWG stands for American Wire Gauge.
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:
Write your calculations in this part of your report.
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.
