MAGNETISM & ELECTROMAGNETISM
What is Magnetism?
The following pages explain the science behind how magnets work. Before you continue reading, watch our short video about magnetism:
When playing with magnets, you probably noticed that a magnet can be used to attract certain materials or objects, but not others. Figure 9, below, shows a magnet picking up metal screws and paper clips, but having no effect on wood, rubber, Styrofoam®, or paper.
Figure 9. A magnet can be used to pick up many metal objects, like screws or paper clips (left), but has no effect on some materials, including plastic, rubber, wood, or even certain metals (right).
If you have ever played with two or more magnets at once, you probably noticed that magnets can either attract or repel each other, depending on how they are positioned. This is because every magnet has a north pole and a south pole. Opposite poles attract each other (north and south) and similar poles repel each other (north-north or south-south). Magnets are often labeled with an N for the north pole and an S for the south pole, as shown in Figure 10.
Figure 10. Every magnet has a north pole and a south pole. Opposite poles pull toward each other, and similar poles push away from each other.
If you watched the video above, you may have noticed that magnetic poles can push and pull on each other without touching each other. Magnets can do this because they are surrounded by a magnetic field. It is the magnetic field that creates the force (a push or a pull) on other magnets or magnetic materials in the field. The magnetic field gets weaker as you get farther and farther away from a magnet; so magnets can be very strong up close, but they do not have much of an effect on objects (like other magnets) that are very far away.
Magnetic fields are invisible; you cannot see them with your eyes. So, how do we know they are there, or what they look like? Scientists represent the invisible magnetic field by drawing magnetic field lines. These are lines that point from the north pole to the south pole outside of the magnet (inside the magnet they point from the south pole to the north pole). The magnetic field is strongest (or the magnet has the strongest pull or push on other magnetic material) where these lines are bunched closely together, and weakest where they are spaced farther apart. A common way to visualize magnetic field lines is to sprinkle many tiny iron filings near a magnet. The iron filings line up with the magnetic field lines, as shown in Figure 11.
Figure 11. On the left, magnetic field lines point from the north pole of a magnet to the south pole outside the magnet (image credit Wikimedia Commons user Geek3, 2010). On the right, you can see these lines using iron filings.
You can also detect a magnetic field by using a compass. A compass—like the one shown in Figure 12—is actually a small bar magnet that is free to rotate on a pivot.
Figure 12. A compass is a device with a rotating magnetic needle that can be used to navigate. The N, S, E and W on the compass stand for north, south, east, and west, respectively. In this image, the N and S are partially hidden behind the needle.
Normally, a compass will align with Earth’s magnetic field, so its needle will align itself roughly with the geographic north-south direction (not perfectly, though; there is actually a slight offset between Earth’s magnetic and geographic poles). This means that a compass can be used to navigate so you can determine which directions are north, south, east, and west. However, if you bring a compass very close to another magnet, that magnet will have a stronger effect on the needle than Earth’s magnetic field. The compass needle will align with the local (or “nearby”) magnetic field (the lines shown in Figure 11).
Earth actually acts like it has a big “upside-down” bar magnet inside of it. The south pole of that bar magnet is actually near (but not perfectly lined up with) Earth’s north pole, and vice versa. So, the part of a compass needle (usually the red end) that points toward the south pole of a magnet (like in Figure 13), will point toward Earth’s geographic north pole. This can be confusing; just look at Figure 13 if you need to remember which end of the compass needle is which!
Figure 13. You can imagine Earth’s magnetic field like there is a giant bar magnet buried inside Earth. The magnet’s south pole is close to Earth’s geographic north pole, and the magnet’s north pole is close to Earth’s geographic south pole. Earth’s magnetic and geographic poles do not line up with each other perfectly, but they are very close.
There are several different types of magnets. Permanent magnets are magnets that permanently retain their magnetic field. This is different from a temporary magnet, which usually only has a magnetic field when it is placed in a bigger, stronger magnetic field, or when electric current flows through it. The bar magnet and paper clips from Figure 9 are examples of permanent and temporary magnets, respectively. The bar magnet is always surrounded by a magnetic field, so it is a permanent magnet. The paper clips do not normally have a magnetic field; in other words, you cannot use one paper clip to pick up another paper clip. However, when you bring the bar magnet near the paper clips, they become magnetized and behave like magnets, so they are temporary magnets. Another type of temporary magnet, called an electromagnet, uses electricity to create a magnet. See the Electromagnetism tab to learn more about electromagnets.
In everyday language, we usually just refer to magnets, and materials that are attracted to magnets, as “magnetic.” Technically, these materials are called ferromagnetic. It is important to note that not all metals are ferromagnetic. You will notice this if you try to pick up a copper penny or a piece of aluminum foil with a magnet. The most common ferromagnetic metals are iron, nickel, and cobalt.
Ferromagnetic material contains many tiny magnetic domains at the microscopic level. Each magnetic domain has its own tiny magnetic field with a north and a south pole. Normally, these domains point randomly in all different directions, so all the tiny magnetic fields cancel each other out, and the overall material is not surrounded by a magnetic field. However, when a material is magnetized (usually by putting it in a strong magnetic field), all of these tiny magnetic fields line up, creating an overall larger magnetic field.
Figure 14. In ferromagnetic material, tiny magnetic fields can be oriented randomly in different directions, canceling each other out. In this case, the material will not show magnetic characteristics (left). When the magnetic fields line up and all point in the same direction, they combine and create a large magnetic field. The material will then show the characteristic of a magnet (right).
How, exactly, the tiny magnetic fields are generated depends on how electrons move inside atoms. This is one example of how magnetism and electricity are connected.
Magnets are fun and useful, but can also be dangerous if they are not handled properly. Small magnets should always be kept away from small children and pets, because they can cause serious injury if they are swallowed. Very strong magnets, like neodymium magnets, can pull together with a very high force, pinching your fingers if they are caught in between. You should always keep magnets away from electronic devices, like computers and cell phones, and away from credit cards (or any other card with a magnetic strip). This is because the data on these devices is often stored using magnetic recording, and can be erased when it comes close to a strong magnetic field. If you are doing a Science Buddies project involving magnets, be sure to read the safety precautions for that specific project before you start.