PHY_10_27_V2_DM2_Magnetic field energy

Magnets and Potential Energy

1                 Potential energy (PE) is sometimes called stored energy. Potential energy is the energy stored within a physical system. Energy is the ability to do work or cause movement. You can think of potential energy as work waiting to happen. All objects at rest have some type of PE. The PE of an object changes with its position. What does this mean? The PE that an object has is measured in relation to a specific starting point.

2                 Let’s look at some examples. Sometimes we talk about chemical potential energy. This type of PE is

in the chemical bonds of all matter. The amount of chemical PE an object has depends on the

types of chemical bonds found in that matter. Sometimes we talk about elastic PE. The amount of elastic PE in that system depends on how far the object is stretched or compressed.

Sometimes we talk about gravitational PE. The amount of gravitational PE an object has depends on how far it is from the ground.

3                 Gravity gives us the most common examples of PE. A book on the table has more gravitational potential energy than a book on the floor. A roller coaster has more potential energy at the top of the track than at the bottom of the track. An apple high in a tree has farther to fall than an apple low in a tree. The force of gravity will do more work to bring it to the ground. Therefore, the higher apple will have greater potential energy than the lower apple. In all cases, the farther the object is from the ground, the more PE it has.

4                 It is important to note that PE applies to all forces acting at a distance. Acting at a distance means that contact between objects is not required. The force has an effect even when the objects do not touch. Forces acting at a distance include gravity, electricity, and magnetism. Magnetism has some interesting uses related to potential energy.

5                 How do magnets work? Remember that each magnet produces its own magnetic field. The orientation of the field gives each magnet two poles. These are the north pole and the south pole. When two magnets come close to each other, their fields interact. This makes a force that can act over a distance. The way the fields interact changes, based on which poles of the two magnets come together. If two north poles come close, the two magnets will repel each other, or push each other away. If a north pole and a south pole come close, the two magnets will attract each other.

6  There is another thing to remember about potential energy. Physical systems act in ways that reduce the stored potential energy. In the case of gravitational PE, an unsupported object will fall and lower its potential energy. A ball will drop when it is let go. The same thing happens with magnetic PE. A magnet that can move will align itself with another, stronger magnetic field. That is how it reaches the state of lowest PE. You can see this happen. The needle of a compass is a magnet. If you move the compass, the needle will twist to line up with Earth’s magnetic field.

7 What does this really tell us about potential energy and magnets? Gravitational PE only depends on distance. Magnetic PE depends on both distance and orientation in the magnetic field. A south pole of one magnet can approach the north pole of another. The magnetic fields are aligned. This makes an attractive force between the two magnets. The farther apart they are, the higher their potential energy. If allowed to move freely, they will snap together to minimize PE. This is just like the ball falling to the ground to lower gravitational PE. Two north poles can approach each other. The magnets’ fields are in exact opposite alignment. The field produces a repulsive force pushing them apart. They repel each other. In this case, the closer they are, the higher their potential energy will be. If allowed to move, they may move as far apart as possible or one will swing around to change its orientation. Either way, the PE will be lowered.

8     Magnets come in two varieties. They can be permanent or temporary. The small magnets on refrigerator doors are examples of permanent magnets. Temporary magnets use electricity to create a magnet. The magnet can be turned on or off. The north and south poles of a temporary magnet can switch sides. Larger electric currents make stronger magnets.

9     Some trains use magnets and PE to make them run along a track. They are called Maglev trains. The word “Maglev” is short for magnetic levitation. These trains have no wheels. Instead, the track has magnetic coils pointing towards the railcar. The railcar also has magnets pointing towards the track. The magnets on the track and train are temporary magnets. When turned on, they repel each other. The railcar lifts off the track between one and ten centimeters. The current from the coils in the track keeps changing. This changes the orientation of the magnetic field of the temporary magnets along the track. The coils in front of the train use magnetic force to pull the train forward. The coils behind the train use magnetic force to push the train forward. This type of train system uses magnetic PE to move the train along the track with very little friction.

10 There is another thing to remember about potential energy. Physical systems act in ways that reduce the stored potential energy. In the case of gravitational PE, an unsupported object will fall and lower its potential energy. A ball will drop when it is let go. The same thing happens with magnetic PE. A magnet that can move will align itself with another, stronger magnetic field. That is how it reaches the state of lowest PE. You can see this happen. The needle of a compass is a magnet. If you move the compass, the needle will twist to line up with Earth’s magnetic field.

11  What does this really tell us about potential energy and magnets? Gravitational PE only depends on distance. Magnetic PE depends on both distance and orientation in the magnetic field. A south pole of one magnet can approach the north pole of another. The magnetic fields are aligned. This makes an attractive force between the two magnets. The farther apart they are, the higher their potential energy. If allowed to move freely, they will snap together to minimize PE. This is just like the ball falling to the ground to lower gravitational PE. Two north poles can approach each other. The magnets’ fields are in exact opposite alignment. The field produces a repulsive force pushing them apart. They repel each other. In this case, the closer they are, the higher their potential energy will be. If allowed to move, they may move as far apart as possible or one will swing around to change its orientation. Either way, the PE will be lowered.

12  Magnets come in two varieties. They can be permanent or temporary. The small magnets on refrigerator doors are examples of permanent magnets. Temporary magnets use electricity to create a magnet. The magnet can be turned on or off. The north and south poles of a temporary magnet can switch sides. Larger electric currents make stronger magnets.

13  Some trains use magnets and PE to make them run along a track. They are called Maglev trains. The word “Maglev” is short for magnetic levitation. These trains have no wheels. Instead, the track has magnetic coils pointing towards the railcar. The railcar also has magnets pointing towards the track. The magnets on the track and train are temporary magnets. When turned on, they repel each other. The railcar lifts off the track between one and ten centimeters. The current from the coils in the track keeps changing. This changes the orientation of the magnetic field of the temporary magnets along the track. The coils in front of the train use magnetic force to pull the train forward. The coils behind the train use magnetic force to push the train forward. This type of train system uses magnetic PE to move the train along the track with very little friction.