Magnets are found in motors, dynamos, refrigerators, credit cards, debit cards, and electronic instruments such as electric guitar pickups, stereo speakers, and computer hard drives. They can be permanent magnets made of naturally magnetized metal or iron alloys or electromagnets. The latter are made thanks to the magnetic field developed by electricity passing through a copper coil wrapped around an iron core. There are several factors that play a role in the strength of magnetic fields and different ways to calculate it; both are described in this article.
Steps
Method 1 of 3: Determine Factors Affecting Magnetic Field Strength
Step 1. Evaluate the characteristics of the magnet
Its properties are described using these criteria:
- Coercivity (Hc): represents the point at which a magnet can be demagnetized by another magnetic field; the higher the value, the more difficult it is to cancel the magnetization.
- Residual magnetic flux, abbreviated as Br: is the maximum magnetic flux that the magnet can produce.
- Energy density (Bmax): it is related to the magnetic flux; the greater the number, the stronger the magnet.
- Temperature coefficient of the residual magnetic flux (Tcoef of Br): it is expressed as a percentage of degrees Celsius and describes how the magnetic flux decreases as the temperature of the magnet increases. A Tcoef of Br equal to 0.1 means that if the temperature of the magnet increases by 100 ° C, the magnetic flux decreases by 10%.
- Maximum Operating Temperature (Tmax): The maximum temperature at which a magnet operates without losing the field strength. When the temperature falls below the value of Tmax, the magnet recovers all its field intensity; if it is heated above Tmax, it irreversibly loses part of the magnetic field intensity even after the cooling phase. However, if the magnet is brought to the Curie point (Tcurie), it will demagnetize.
Step 2. Pay attention to the magnet material
Permanent magnets typically consist of:
- Alloy of neodymium, iron and boron: it has the highest value of magnetic flux (12,800 gauss), coercivity (12,300 oersted) and energy density (40); it also has the lowest maximum operating temperature and the lowest Curie point (respectively 150 and 310 ° C), a temperature coefficient equal to -0.12.
- Alloy of samarium and cobalt: magnets made from this material have the second strongest coercivity (9,200 oersteds), but have a magnetic flux of 10,500 gauss and an energy density of 26. Their maximum operating temperature is much higher. compared to that of neodymium magnets (300 ° C) and the Curie point is established at 750 ° C with a temperature coefficient equal to 0.04.
- Alnico: is a ferromagnetic alloy of aluminum, nickel and cobalt. It has a magnetic flux of 12,500 gauss - a value very similar to that of neodymium magnets - but a lower coercivity (640 oersted) and, consequently, an energy density of 5.5. Its maximum operating temperature is higher than the samarium and cobalt alloy (540 ° C), as well as the Curie point (860 ° C). The temperature coefficient is 0.02.
- Ferrite: has a much lower magnetic flux and energy density than other materials (respectively 3,900 gauss and 3, 5); however, the coercivity is greater than in the anico and is equal to 3,200 oersteds. The maximum operating temperature is the same as that of samarium and cobalt magnets, but the Curie point is much lower and stands at 460 ° C. The temperature coefficient is -0.2; as a result, these magnets lose their field strength faster than other materials.
Step 3. Count the number of turns of the electromagnetic coil
The greater the ratio of this value to the length of the core, the greater the intensity of the magnetic field. Commercial electromagnets consist of cores of variable length and made with one of the materials described so far, around which large coils are wound; however, a simple electromagnet can be made by wrapping copper wire around a nail and attaching its ends to a 1.5 volt battery.
Step 4. Check the amount of current flowing through the coil
For this you need a multimeter; the stronger the current, the stronger the magnetic field generated.
Ampere per meter is another unit of measurement related to magnetic field strength and describes how it grows as the current strength, the number of turns, or both increases
Method 2 of 3: Test the Magnetic Field Strength Range with Staples
Step 1. Prepare a holder for the magnet
You can make a simple one using a clothespin and a paper or Styrofoam cup. This method is suitable for teaching the concept of magnetic field to elementary school children.
- Secure one of the long ends of the clothespin to the base of the glass using masking tape.
- Place the glass upside down on the table.
- Insert the magnet into the clothespin.
Step 2. Bend the paper clip to shape it like a hook
The easiest way to do this is to spread out the outside of the paper clip; keep in mind that you will need to hang several staples on this hook.
Step 3. Add more paper clips to measure the strength of the magnet
Put the bent paper clip in contact with one of the poles of the magnet so that the hooked portion remains free; attach more staples to the hook until their weight makes it detach from the magnet.
Step 4. Make a note of the number of staples that manage to drop the hook
Once the ballast manages to break the magnetic link between the magnet and the hook, carefully report the quantity.
Step 5. Add masking tape to a magnetic pole
Arrange three small strips and attach the hook again.
Step 6. Connect as many staples until you break the link again
Repeat the previous experiment until you get the same result.
Step 7. Write down the amount of staples you had to use this time to make the hook buckle
Do not neglect the data relating to the number of strips of masking tape.
Step 8. Repeat this process several times, gradually adding more strips of sticky paper
Always note the number of staples and pieces of tape; you should find that increasing the amount of the latter decreases the amount of staples needed to drop the hook.
Method 3 of 3: Testing the Magnetic Field Strength with a Gaussmeter
Step 1. Calculate the original or reference voltage
You can do this with a gaussmeter, also known as a magnetometer or magnetic field detector, which is a device that measures the strength and direction of the magnetic field. It is a widely available tool that is simple to use and is useful for teaching the basics of electromagnetism to middle and high school kids. Here's how to use it:
- Sets the maximum measurable voltage value at 10 volts with direct current.
- Read the data shown on the display by keeping the instrument away from the magnet; this value corresponds to the original or reference value and is indicated by V0.
Step 2. Touch an instrument sensor to one of the magnet poles
On some models this sensor, called Hall sensor, is built into an integrated circuit, so you can actually put it in contact with the magnetic pole.
Step 3. Note the new voltage value
This data is referred to as V.1 and can be less than or greater than V.0, according to which magnetic pole it is tested. If the voltage increases, the sensor is touching the south pole of the magnet; if it decreases, you are testing the north pole of the magnet.
Step 4. Find the difference between the original voltage and the next one
If the sensor is calibrated in millivolts, divide the number by 1000 to convert it to volts.
Step 5. Divide the result by the sensitivity of the instrument
For example, if the sensor has a sensitivity of 5 millivolts per gauss, you should divide the number you got by 5; if the sensitivity is 10 millivolts per gauss, divide by 10. The final value is the strength of the magnetic field expressed in gauss.
Step 6. Repeat the test at various distances from the magnet
Place the sensor at predefined distances from the magnetic pole and note the results.
