Three Right Hand Rules of Electromagnetism
Teaching electricity and magnetism is complicated by the challenge that the magnetic forces are perpendicular to the motion of the particles and currents. This requires a three-dimensional perspective which can introduce a variable of a "wrong" direction. To prevent errors, let us be "right" and use the right-hand rule.
Some would claim that there is only one right-hand rule, but I have found the convention of three separate rules for the most common situations to be very convenient. These are for (1) long, straight wires, (2) free moving charges in magnetic fields, and (3) the solenoid rule – which are loops of current. Calling these "rules" is the right name. They are not laws of nature, but conventions of humankind. We use rules to help us solve problems, laws would be the underlying cause as to why the rules work.
Danish Physicist and Chemist Hans Christian Ørsted
Rule #1 – Oersted's Law
Our story begins with Oersted's Demonstration, which was performed for the first time during a lecture in 1821. What Oersted showed for the first time that when a current carrying wire passes over a compass the needle – which is a magnet – the needle deflects. When it is underneath the magnet it deflects the other way. The direction that the magnet points is parallel to the magnetic field around the wire. And you can predict that with your right hand!
Point your right hand's thumb along the flow of current – defined as the flow of positive charge. Now curl your fingers as if they were wrapping around the wire. The direction that your fingers point, is the field. I sometimes like to call this the RIGHT-HAND CURL, or Ampere's Law. Ampere himself described it as the face of a clock: if the current flows into the face of the clock then the magnetic field would wrap clockwise.
A good way to demonstrate this phenomenon is with a set of the Small Clear Compasses. When these are wrapped around a vertical wire, with no current, they will all initially point North. But, if the current is switched on, the compasses will align in a loop around the current. It is important to note that the compasses do affect each other, so finding the right distance between them can help make the demonstration more dramatic.
Rule #2 – The Lorentz Force
This second right-hand rule is usually applied to freely moving charges, called cathode rays, or otherwise to push on electric currents.
Since electric current is made of moving charges we can also push it around with magnets. One way to show this is with an Electric Swing Apparatus. This will highlight that the current, field, and force are all three at right angles.
This 2nd rule is usually called the Lorentz Force named after H. A. Lorentz, a contemporary of Einstein, although its effects were known at the time of Michael Faraday.
Now, some people and some books prefer to use the palm to represent the force, that would be current field force (open hand).
Another way to demonstrate this is with the Electricity and Magnetism Light bulb demo. When there is alternating current, the wire vibrates, but when it is direct current we can apply force in a specific direction. Using your right hand, it is possible to predict the direction the current is flowing.
Rule #3 – The Solenoid Rule
An air core solenoid can act just like a bar magnet. Repelling north and attracting south. In fact – if you trace the magnetic field with a compass, you can see that it truly matches the behavior a bar magnet perfectly
Using a third right-hand rule, we can we predict which side of the coil is north.
Let your curling fingers be the direction the current is flowing. It is looping around. Then your thumb will be NORTH end of the electromagnet.
Left Hand Rule
The right-hand rules assume conventional current, that is… current flows from positive to negative. College-based courses all go with that concept. NOT ALL high school physics courses use that concept. For example, some high schools use the “left-hand” rules because it deals with ELECTRON FLOW, that is… current flow from negative to positive (the direction that electrons flow from a battery for example).
The hand rules work the same but they are based on two different current concepts. In this blog we focused strickly on the right hand rule.
Replicating Oersted’s demo is quite easy to do. I am using about 5 amps.
As the current flows upward, the magnetic field will wrap around.
Normally they just point North, but when I turn the current on we see them all pointing around it, just as we predict with our right hand.
This cathode ray tube computer screen was originally all red. But these magnets have deflected the electrons from landing on their proper pixels.
The magnetic field acts on the current in 3D.
Fingers are directed along the vectors using the right hand.
The filaments of the Edison-style bulb are deflected.
The solenoid will behave exactly like a bar magnet with a clearly defined north and south pole.
The north end of the solenoid repels the north end of this bar magnet.