# Physics With Magnets for Science & School

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## Introduction: Physics With Magnets for Science & School

Magnets play an important role in physics. Almost every physics book has a chapter on magnetism. In this instructables I would like to show some experiments with magnets that are particularly suitable for physics lessons.

• Measuring weak magnetic fields including a DIY compass
• Measuring medium strong magnetic fields
• Measuring strong magnetic fields
• Levitation of a magnet
• Determination of the specific charge e / m of an electron using a ring magnet
• Gauss rifle
• Measuring the earth magnetic field with a coil and a compass
• a very simple & impressive magnetic field lines indicator (NEW)

The magnetic flux density B (unit Tesla) indicates the strength of a magnetic field. The earth's magnetic field has a strength of around 45 µT, whereas strong neodymium magnets can have flux densities of around 1 Tesla.

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## Step 1: Measuring Weak Magnetic Fields Including a DIY Compass

Fortunately, the earth has a magnetic field. This protects us, for example, from high-energy, charged, cosmic particles. These are deflected in the magnetic field due to the Lorentz force and describe circular orbits around the magnetic field lines.

In Austria, the earth's magnetic field has a strength of around 45 µT, i.e. only 0.000045 T. Such weak magnetic fields can be easily detected and measured with the HMC5883L sensor and the Arduino. In order to obtain useful results with the HMC5883L sensor, the offset in the x and y directions must first be determined. To do this, turn the sensor around the z-axis in 10 ° steps and note the magnetic field in the x and y directions. If you have these 36 pairs of values, you form the x and y mean of pairs 1 and 19, 2 and 20, 3 and 21 to 18 and 36. Finally, you only have to average the 16 x mean values and determine 16 y averages. These two numbers form the x and y offset of the sensor, which must be entered in the Arduino program.

The strength of the magnetic field in the x and y directions can be used to build a simple compass. The relationship exists between the position angle phi and the magnetic field in the x or y direction: tan (phi) = -Bx / By (see figure).

## Step 2: Measuring Medium Strong Magnetic Fields

Medium-strength magnetic fields in the range of [-0.08 T, + 0.08 T] can easily be determined with the Hall sensor SS495A.
For this, a voltage in the range [0.2.5V] only has to be read in with a multimeter or using an Arduino. The following relationship exists between the flux density B (in Tesla) and the voltage value U (in volts): B = (U - 2.5) * 0.04 For example, the SS495A sensor can be used to examine the strength of a magnet as a function of the distance to the magnet.

## Step 3: Measuring Strong Magentic Fields

A different Hall sensor is required to measure even stronger magnetic fields up to 3 Tesla. I am using the CYSJ362A. With a supply of + 5V, this supplies a voltage of 1.5V per Tesla. To determine the flux density B, it is sufficient to measure the voltage U with the Arduino. The following then applies: B = U / 1.5.

Attention: For the CYSJ362A you need a second, independent power supply! I use 9V batteries, one for the Arduino and the display and a second one to power the Hall sensor!

## Step 4: Gauss Rifle

For the Gauss rifle you need a long metal rail in an L-shape, several strong cube magnets and steel balls. The structure is simple. At a distance of e.g. A magnet is placed 15 cm into the metal rail. Then you place 2 steel balls on the right of the magnets. Now you place a single steel ball on the left start of the metal rail and give it a push in the direction of the first magnet.

What will happen?

The steel ball is attracted to the first magnet and thereby accelerated. If the ball collides with the magnet, the momentum conservation law applies and the second metal ball on the right side will whiz away at the final speed of the ball arriving on the left. So that this does not lose speed again due to the magnet, there is just another metal ball between the bullet and the magnet. This ensures that the magnetic field is virtually shielded and that the ball that rushes to the right is no longer felt and braked.

## Step 5: Determination of the Specific Charge E/m of an Electron

Another important parameter in physics, namely the specific charge e / m of the electron, can be determined using a ring magnet and high voltage.

In addition to the ring magnet, high voltage is also required. This is supplied by a DC flyback transformer (a so called diode split transformer or DST) from an old tube television. The NE555 creates a chopper circuit with adjustable frequency. If you connect an output of the high voltage (ground) on the outside to the two ring magnets and now the positive pole of the high voltage is near the center of the ring magnets, a spark will flash over. Depending on the orientation of the magnets (north pole up or down), the spark will be curved to the right or left.

The specific charge e / m can be determined from the radius of curvature r and the voltage U (this must be estimated from the spark length in air, where 1 cm = 10 kV applies) (see figure).

My ring magnets (you will need two of them) have the dimensions: 40 mm (outer diameter) x 20 mm (inner diameter) x 10 mm (height). I've bought them on amazon (amazon)

## Step 6: Levitation

Finally, we make an experiment on levitation. For this we need some electronic parts (e.g. the Hall sensor SS495A, the operational amplifier UA741, the Mosfet IRF4905, the voltage regulator 7805 and some resistors and capacitors) and an electromagnet. I simply made it out of a roll with enamelled copper wire. The Hall sensor must be in the middle of the lower coil opening.

If a spherical magnet approaches the Hall sensor, it detects a stronger field and the OPA switches off the current via the IRF4905. As a result, the ball magnet will fall down. However, this also reduces the magnetic field at the location of the Hall sensor and below a certain strength the OPA switches the current through the electromagnet back on via the IRF4905. As a result, the magnetic ball is pulled up again and does not fall to the ground. This constant switching on and off takes place so quickly that you cannot see it. The magnetic ball will float in the air.

## Step 7: Measuring the Earth's Magnetic Field

With a coil and a tiny compass you can easily determine the earth's magnetic field. The magnetic flux density B within a coil with the length L and N windings and the current I is: B = µ0 * N * I / L

µ0 is the magnetic field constant, which is 1.2566 * 10^-6 N/A²

First you have to orientate the coil in east to west direction. With no current the compass needle shows exactly to the north. Then you slowly increase the current, until the needle shows to northwest or northeast (depending on the direction of the current). The angle alpha between the north direction and the needle has to be 45°. Write down the current I for this situation.

In this case, you don't need any trigonometric functions because the magnetic field of the coil is equal to the earth's magnetic field.

Therefore the earth's magnetic field can be calculated as: B_earth = 1.2566 * N * I / L [in µT]

The result should be in the range of 20-50 µT, depending on the inclination of the magnetic field at your local position, because you can only measure the horizontal part of B_earth.

In earlier days physicians determined with this method (the apparatus is called tangent boussole) the electrical current! Today we simply use an amperemeter...

## Step 8: A Very Simple & Impressive Magnetic Field Lines Indicator

For a very simple and impressive magnetic field indicator you just need magnets, a paper clip, a string and clear sticky tape.

Tie the paper clip to the string. Then stick both sides of the paper clip with transparent adhesive tape for stabilization.
Now hold the string and move the paper clip closer to the magnet. This will line up along the field lines. If you move the thread carefully, you can discover how the field around the magnet is aligned! For example, with a bar magnet, the field lines are bent from pole to pole.

Runner Up in the
Magnets Challenge

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