AP®︎/College Physics 2
What are magnetic fields?
Learn what magnetic fields are and how to calculate them.
What is a magnetic field?
A magnetic field is a picture that we use as a tool to describe how the magnetic force is distributed in the space around and within something magnetic.
Most of us have some familiarity with everyday magnetic objects and recognize that there can be forces between them. We understand that magnets have two poles and that depending on the orientation of two magnets there can be attraction (opposite poles) or repulsion (similar poles). We recognize that there is some region extending around a magnet where this happens. The magnetic field describes this region.
There are two different ways that a magnetic field is typically illustrated:
- The magnetic field is described mathematically as a vector field. This vector field can be plotted directly as a set of many vectors drawn on a grid. Each vector points in the direction that a compass would point and has length dependent on the strength of the magnetic force.Figure 1: Vector field plot for a bar magnet
- An alternative way to represent the information contained within a vector field is with the use of field lines. Here we dispense with the grid pattern and connect the vectors with smooth lines. We can draw as many lines as we want.The field-line description has some useful properties:Figure 2: Field line plot for a bar magnet
- Magnetic field lines never cross.
- Magnetic field lines naturally bunch together in regions where the magnetic field is the strongest. This means that the density of field lines indicates the strength of the field.
- Magnetic field lines don't start or stop anywhere, they always make closed loops and will continue inside a magnetic material (though sometimes they are not drawn this way).
- We require a way to indicate the direction of the field. This is usually done by drawing arrowheads along the lines. Sometimes arrowheads are not drawn and the direction must be indicated in some other way. For historical reasons the convention is to label one region 'north' and another 'south' and draw field lines only from these 'poles'. The field is assumed to follow the lines from north to south. 'N' and 'S' labels are usually placed on the ends of a magnetic field source, although strictly this is arbitrary and there is nothing special about these locations.
- Field lines can be visualized quite easily in the real world. This is commonly done with iron filings dropped on a surface near something magnetic. Each filing behaves like a tiny magnet with a north and south pole. The filings naturally separate from each other because similar poles repel each other. The result is a pattern that resembles field lines. While the general pattern will always be the same, the exact position and density of lines of filings depends on how the filings happened to fall, their size and magnetic properties.Figure 3: Magnetic field lines around a bar magnet visualized using iron filings.
How do we measure magnetic fields?
Because a magnetic field is a vector quantity, there are two aspects we need to measure to describe it; the strength and direction.
The direction is easy to measure. We can use a magnetic compass which lines up with the field. Magnetic compasses have been used for navigation (using the Earth's magnetic field) since the 11ᵗʰ century.
Interestingly, measuring the strength is considerably more difficult. Practical magnetometers only came available in the 19ᵗʰ century. Most of these magnetometers work by exploiting the force an electron feels as it moves through a magnetic field.
Very accurate measurement of small magnetic fields has only been practical since the discovery in 1988 of giant magnetoresistance in specially layered materials. This discovery in fundamental physics was quickly applied to the magnetic hard-disk technology used for storing data in computers. This lead to a thousand-fold increase in data storage capacity in just a few years immediately following the implementation of the technology (0.1 to 100 between 1991 and 2003 ). In 2007 Albert Fert and Peter Grünberg were awarded the Nobel Prize in Physics for this discovery.
In the SI system, the magnetic field is measured in tesla (symbol , named after Nikola Tesla). The Tesla is defined in terms of how much force is applied to a moving charge due to the field. A small refrigerator magnet produces a field of around and the Earth's field is about . An alternative measurement is also often used, the Gauss (symbol ). There is a simple conversion factor, . Gauss is often used because 1 Tesla is a very large field.
In equations the magnitude of the magnetic field is given the symbol . You may also see a quantity called the magnetic field strength which is given the symbol . Both and have the same units, but takes into account the effect of magnetic fields being concentrated by magnetic materials. For simple problems taking place in air you won't need to worry about this distinction.
What is the origin of the magnetic field?
Magnetic fields occur whenever charge is in motion. As more charge is put in more motion, the strength of a magnetic field increases.
Magnetism and magnetic fields are one aspect of the electromagnetic force, one of the four fundamental forces of nature.
There are two basic ways which we can arrange for charge to be in motion and generate a useful magnetic field:
- We make a current flow through a wire, for example by connecting it to a battery. As we increase the current (amount of charge in motion) the field increases proportionally. As we move further away from the wire, the field we see drops off proportionally with the distance. This is described by Ampere's law. Simplified to tell us the magnetic field at a distance from a long straight wire carrying current the equation is
Here is a special constant known as the permeability of free space. . Some materials have the ability to concentrate magnetic fields, this is described by those materials having higher permeability.
Since the magnetic field is a vector, we also need to know the direction. For conventional current flowing through a straight wire this can be found by the right-hand-grip-rule. To use this rule imagine gripping your right hand around the wire with your thumb pointing in the direction of the current. The fingers show the direction of the magnetic field which wraps around the wire.Right-hand-grip rule used to find the direction of the magnetic field (B) based on the direction of a current (I). 
- We can exploit the fact that electrons (which are charged) appear
- Although atoms often have many electrons, they mostly 'pair up' in such a way that the overall magnetic field of a pair cancels out. Two electrons paired in this way are said to have opposite spin. So if we want something to be magnetic we need atoms that have one or more unpaired electrons with the same spin. Iron for example is a 'special' material that has four such electrons and therefore is good for making magnets out of.
- Even a tiny piece of material contains billions of atoms. If they are all randomly orientated the overall field will cancel out, regardless of how many unpaired electrons the material has. The material has to be stable enough at room temperature to allow an overall preferred orientation to be established. If established permanently then we have a permanent magnet, also known as a ferromagnet.
- Some materials can only become sufficiently well ordered to be magnetic when in the presence of an external magnetic field. The external field serves to line all the electron spins up, but this alignment disappears once the external field is removed. These kinds of materials are known as paramagnetic.The metal of a refrigerator door is an example of a paramagnet. The refrigerator door itself is not magnetic, but behaves like a magnet when a refrigerator magnet is placed on it. Both then attract each other strongly enough to easily keep in place a shopping list, sandwiched between the two.
Canceling the field of the Earth
Figure 5 shows a setup in which a compass is placed near a vertical wire. When no current is flowing in the wire the compass points north as shown due to the Earth's field (assume the field of the Earth is ).
Figure 5: Compass and wire experiment (viewed from above, no current flowing).
What current (magnitude and direction) would be required to cancel out the field of the Earth and 'confuse' the compass?
Suppose our power supply is limited to a total of . Can you suggest an alternative configuration of the experiment which produces the same effect on the compass?
 Newton Henry Black, Harvey N. Davis (1913) Practical Physics, The MacMillan Co., USA, p. 242, fig. 200 (public domain)
 UK Success Stories in Industrial Mathematics. Philip J. Aston, Anthony J. Mulholland, Katherine M.M. Tant. Springer, Feb 4, 2016
 This is a file from the Wikimedia Commons. This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International, 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.
Want to join the conversation?
- why is the direction of magnetic field is from north to south why not south to north?(13 votes)
- It is not a fact but is an agreement that was once made and is now the standpoint. They are just labels, the north seeking point of an magnet (northpole) turns to the North (magnetic southpole of the earth). The agreement is that all northseeking magnet sides are called northpoles and vice versa.(25 votes)
- In exercise 1a what would the compass point to when distracted by the magnetic field generated by the current flowing through the wire? And also why does the direction of the current matter, I understand it determines the direction of the field with respect to the wire but how does that affect, the effect the field has on the compass? it still generates a magnetic field regardless of the direction the current flows in and would essentially 'confuse' the compass anyway. Right?... HELP. SOMEONE. ANYONE??(5 votes)
- Hello Alex,
Let's consider the compass without the added complexity of the wire. Here the magnetic field of the compass needle is influenced by the magnetic field of the earth.
When current flows in the wire the compass needle moves. It does so because the magnetic field of the wire is considerably stronger than the magnetic field of the earth.
To better understand please research "right hand rule." Also, you may be interested in the Tangent galvanometer.
Please leave a comment if you still have questions.
- Hello, Consider this situation, we have a magnetic field going into the page/screen through which a charged particle is moving with a velocity v perpendicular to the field. Then using Fleming Left Hand Rule (FLH) we determine the path as a circular path. But my question
1. is the motion of circular path is clockwise or anti clockwise ?
2. How will the motion depend on the charge of the particle, that is, in case it is a electron / proton / neutron ?(1 vote)
- Ok, so we say 1 pole is north other is south. But if like charges repel and unlike charges attract, how in the world we know if one is north or south. How can you even tell the difference. Is there any way? Two north repels. Two south repels. South and North attract and North and south attract. There's no way to tell!! :((1 vote)
- The way they are defined is geographic. The north pole points towards the north and vice versa.(4 votes)
- How do you think the field lines would change if you used a much larger battery, which would provide a much larger current?(2 votes)
- How do l find mu permiability values for different maferials? [1-18-19:2301](2 votes)
- You normally look it up on a table, you can find them on the internet or in some engineering references.(1 vote)
- If all magnetic fields are caused due to moving charges, how do magnets work?(0 votes)
- go to youtube and search for "veritasium how do magnets work" and watch both videos(5 votes)
- is there something like magnetic compass and geographic compass? or all compasses are the same? and what's the difference between earth's magnetic and geographic poles?(1 vote)
- The geographic pole is where the imaginary line representing the earth's axis of spin exits the earth. The magnetic pole is where a compass points.(3 votes)
- I have a doubt that is bothering me very much. Why don't two field lines intersect? I have searched for the answer, and the only answer that I get is: "Because if they intersect, then the magnetic compass will have to point in two different directions which is not possible."
But, can't the needle of the compass point straight in the middle? One field line exerts force towards one direction, the other one towards the other. Then because of the resultant, the needle need not deflect at all. Right?
Please clarify with clear explanation.(1 vote)
- To find the direction of magnetic field on paper, we draw tangent to the magnetic field lines which determines the direction of the field. If two field lines intersect, at the point of intersection, we’ll be drawing two tangents which is not possible. Magnetic field is a vector quantity. It has specific magnitude and direction. Therefore, it can’t have two directions at a single point. This is the reason why magnetic field never intersects.
Magnetic forces between two different sources can intersect but magnetic field lines of any source can’t intersect.
I hope I’ve clarified your doubt.(2 votes)
- Which way does current flow in a standard bar magnet?(1 vote)
- I do not think electric current flows through bar magnets.
(1)If, however, we are talking about magnetic fields, outside the magnet, they flow from north to south.
(2) Magnetic field lines are closed curves
(3) Therefore, it makes sense that they flow from south to north inside bar magnets.(2 votes)