Svenja Lohner, PhD, Science Buddies
OverviewHow was magnetism responsible for the destruction of dozens of ships during World War II? In this lesson, your students will explore the concepts of magnetic fields and forces using the example of World War II magnetic mines. With the help of a magnetometer, a smartphone, and a sensor app, students will investigate what factors affect the strength of a magnetic field. Then they will use their knowledge to try to discover the location of hidden "mines" and investigate how they can cloak a magnetic field to become undetectable by a magnetic trigger mechanism.
- Describe what factors affect the strength of magnetic forces.
- Provide evidence that magnetic fields exist between objects exerting forces on each other even though the objects are not in contact.
- Conduct investigations to measure the strength of a magnetic field.
NGSS AlignmentThis lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
- MS-PS2-3. Ask questions about data to determine the factors that affect the strength of electric and magnetic forces.
- MS-PS2-5. Conduct an investigation and evaluate the experimental design to provide evidence that fields exist between objects exerting forces on each other even though the objects are not in contact.
|Science & Engineering Practices||Disciplinary Core Ideas||Crosscutting Concepts|
|Science & Engineering Practices||Planning and Carrying Out Investigations.
Collect data to produce data to serve as the basis for evidence to answer scientific questions or test design solutions under a
range of conditions.
Conduct an investigation and evaluate the experimental design to produce data to serve as the basis for evidence that can meet the goals of the investigation.
Analyzing and Interpreting Data. Analyze and interpret data to provide evidence for phenomena.
Constructing Explanations and Designing Solutions. Undertake a design project, engaging in the design cycle, to construct and/or implement a solution that meets specific design criteria and constraints.
Engaging in Argument from Evidence. Construct and present oral and written arguments supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon or a solution to a problem.
|Disciplinary Core Ideas||PS2.B: Types of Interactions.
Electric and magnetic (electromagnetic) forces can be attractive or repulsive, and their sizes depend on the magnitudes of
the charges, currents, or magnetic strengths involved and on the distances between the interacting objects.
Forces that act at a distance (electric, magnetic, and gravitational) can be explained by fields that extend through space and can be mapped by their effect on a test object (a charged object, a magnet, or a ball, respectively).
|Crosscutting Concepts||Cause and Effect.
Cause and effect relationships may be used to predict phenomena in natural or designed systems.
Systems and System Models.
Models can be used to represent systems and their interactions—such as inputs, processes and outputs—and energy and matter flows within systems.
Influence of Science, Engineering, and Technology on Society and the Natural World.
The uses of technologies and any limitations on their use are driven by individual or societal needs, desires, and values; by the findings of scientific research; and by differences in such factors as climate, natural resources, and economic conditions.
Materials per group of 2–3 students:
- Ceramic disc magnets, 18mm diameter (5), available from Amazon.com
- Printout of grid paper template (5-6). Note: You might need to adjust the size of the template based on the boxes you use.
- Construction paper (darker color but not black)
- Duct tape
- Pencil or pen
- Transparent shoe or storage box, available from Amazon.com
- Aluminum foil
- Materials for cloaking activity: your students can get creative here. Some possibilities are aluminum foil, additional magnets, copper wire (non-magnetic), iron wire (magnetic), foam sheets, paper, etc.
- Smartphone with a sensor app such as phyphox, available for free on Google Play for Android devices (version 4.0 or newer) or from the App Store for iOS devices (iOS 9.0 or newer).
Materials for teachers:
- Two bar magnets
Disclaimer: Science Buddies participates in affiliate programs with Home Science Tools, Amazon.com, Carolina Biological, and Jameco Electronics. Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity, and keep our resources free for everyone. Our top priority is student learning. If you have any comments (positive or negative) related to purchases you've made for science projects from recommendations on our site, please let us know. Write to us at firstname.lastname@example.org.
Background Information for TeachersThis section contains a quick review for teachers of the science and concepts covered in this lesson.
A material that you can turn into a magnet is called a ferromagnetic material. Some examples are iron, steel, nickel, and cobalt. Once a ferromagnetic material becomes magnetic, it is able to attract other magnetic materials. A magnet always has two opposite poles, which are referred to as north and south. Similar or "like" poles will repel each other (push each other away). Opposite or "unlike" poles will attract each other (pull towards each other) as shown in Figure 1.
Figure 1. Diagram showing how magnets can attract and repel each other depending on the orientation of their poles. The north and south poles are labeled "N" and "S" respectively.
Each magnet produces a magnetic field, which is the area around the magnet where a magnetic force exists that can act on other magnetic materials. You can make a magnetic field visible by sprinkling iron filings on and around a magnet. The iron filings react to the magnetic forces and line themselves up along the magnetic field lines as shown in Figure 2. Magnetic fields are usually represented by such magnetic field lines. These lines describe the direction of the magnetic force within the magnetic field. You can measure the direction of a magnetic field with a compass. A compass contains a needle shaped magnet that can freely move on a balanced pivot point. When the magnetic needle is exposed to a magnetic field, it will line up with the magnetic field with its south pole pointing toward the north pole of the magnet and its north pole pointing toward the south pole of the magnet.
Figure 2. Picture showing the magnetic field lines of a bar magnet's magnetic field visualized with iron filings.
Magnetism is often used for navigational purposes. This is possible because the Earth itself creates a magnetic field which is caused by the flow of molten iron inside the Earth's outer core. Earth's magnetic field looks similar to that of a bar magnet with its magnetic north pole near the geographic south pole and the magnetic south pole near the Earth's geographic north pole as shown in Figure 3. The Earth's magnetic field acts on all metal objects on the Earth's surface. Thus, a compass will align itself in the Earth's magnetic field and show the way North or South.
Figure 3. Illustration visualizing Earth's magnetic field which resembles the magnetic field of a bar magnet.
However, Earth's magnetism has not been applied for navigational purposes only. In World War II, magnetism was used to create special undersea explosives called magnetic mines. These mines were first developed by the British and German navy after World War I. When the Germans started to employ these magnetic mines in 1939, they turned out to be devastating for the British navy. The key to these mines was that they had a special magnetic trigger mechanism. But why magnetic mines to destroy ships? A battle ship is usually made of lots of steel, which is a ferromagnetic material. This means that such a ship is like a huge floating magnet with a large magnetic field surrounding it. Part of this magnetic field is caused by the ship components that consist of hard iron. During ship construction, these parts become magnetized by the Earth's magnetic field, creating some amount of permanent magnetism in the ship hull. At the same time, all the ship components made of soft iron will become temporarily magnetized by the Earth's magnetic field, which results in an additional induced magnetism of the ship. As the ship moves through the water, its magnetic field will move with it and interact with any other magnetic field close by.
The magnetism of these battle ships was what triggered the magnetic mines. Their ignition mechanism was designed in a way that it only reacted to a magnetic field. One example is the so-called dip needle mechanism. Such a trigger works very much like a compass turned on its side. A magnetized needle is carefully balanced inside the firing circuit. It is aligned horizontally to the Earth's magnetic field and leveled so that it does not make contact with the rest of the circuit. However, as soon as a ships vertical magnetic field acts on this needle, it will dip up or down and make contact to close the loop of the firing circuit which triggers the mine, as shown in Figure 4.
Figure 4. Simplified schematic of the dip needle firing circuit of a magnetic mine.
This magnetic firing mechanism was very effective and responsible for a lot of destructed ships during World War II. This was until each side managed to develop technologies that could either mask the magnetic field of a ship or that allowed to trigger the magnetic mines from a distance. One of these technologies is called "degaussing". Degaussing a ship means reducing or eliminating the ship's magnetic signature by producing an opposite magnetic field. The opposite magnetic field was induced by letting a controlled amount of direct current flow through degaussing coils that were wrapped around the ship. The objective of these coils was to generate a magnetic field of equal magnitude and opposite direction that would cancel the ship's magnetic signature. With no magnetic signature the ships became undetectable by magnetic mines.
In this lesson plan, your students will simulate the search of magnetic mines represented by small ceramic magnets. With the magnets hidden from student's sight, they will use a mobile device with a built-in magnetometer to detect the hidden magnets' positions. A magnetometer is an instrument used for measuring magnetic forces and the strength of a magnetic field. Although such magnetic sensors were initially developed and used for navigation and tracking purposes, nowadays they are built into many consumer electronics such as mobile phones, tablets, or computers. With a built-in magnetometer, these devices can function as electronic compasses and can measure magnetic fields. Some magnetometers use the Hall effect discovered by Dr. Edwin Hall in 1879, to detect and measure magnetic fields. Hall discovered that magnetic fields have the tendency to deflect a moving charge as it flows through a conductor. This creates a voltage that can be measured by the sensor. The magnitude of the Hall voltage is proportional to magnetic field strength. Other magnetometers use so-called magnetoresistors that consist of special metal alloys that change their resistance in response to a magnetic field.
The magnetic field strength is measured in teslas (T). The stronger a magnet, the stronger is its magnetic field. Earth's magnetic field strength ranges from about 25 to 65 microteslas depending on the location on Earth, whereas ceramic magnets typically have a magnetic field strength of 0.5 to 1 tesla. Your students will use a mobile device and a sensor app with a magnetometer to map the location of hidden "mines" with the help of the device's built-in magnetometer. Based on their measurements, students will be able to derive the hidden magnet's magnetic field strength and in a second part of the lesson can explore ways of how to "cloak" their magnetized ships to make them less detectable.