12 February 1804, Dorpat, Governorate of Livonia, Russian Empire
In the previous lecture we learned the remarkable fact that, the Electricity and the Magnetism both have the ability to generate each other. This remarkable fact will lead to some remarkable corollaries we will learn shortly.
Going with the vision of this university to simply things, we will try to explore this intuitively and logically with out getting into hardcore mathematics.
Taking a good look at the illustrative diagram above, we can imagine that its an illustration of our recently learned concept of Electromagnetic Induction. As we move the magnet towards or away from the coil, we can generate Electricity.
Other way, if we hold the magnet still and move the coil towards or away from it then also we will have the same effect.
The question is what will be the direction of flow of electricity?
This is the precise nature of the generate electricity which is the subject of this lecture and the gentleman on the top gave us the answer.
For a moment forget the direction and just take the fact that the movement of the magnet has caused the current/electricity. Now we also know that the electricity in a conductor will cause magnetic field. Also, we all know that a magnet has a north Pole and a south pole and opposite poles attract and similar poles repel each other. This logically infers and intuitively appears that if a magnetic field is created by current then by reversing the current direction the magnetic field direction should also get reversed. This actually gives us the clue to address the subject.
If the current generated by the moving magnet in above diagram created the opposite pole than the pole facing the coil then both would attract each other and you will not need o actually push the magnet towards the coil, it will automatically pull and hence it will automatically generate electricity on its own. This appears contradictory to the Law of Conservation of Energy. If Electricity which is a form of energy is generated on its own, like in our hypothetical example then and infinite electrical energy will get generated with any work or expense. this quickly resolves to the fact that, there must be the generation of electricity with a current direction in such a way that the magnetism created by the generated current should oppose the movement of the external magnet.
This is precisely the Lenz’s Law, which states, “the direction of the current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field.”
Or as informally, yet concisely summarised by D.J. Griffiths: Nature abhors a change in flux.
Lets us complete our learning by some quick rules.
What is the direction of Magnetic Field in a magnet?
If we look at the image above, we see that the Magnetic field comes out from the North Pole and gets in at the South Pole. This must be remembered! Since we are talking so much about the magnetic field, has anyone ever wondered if earth’s magnetic poles can flip? Well, those who are really interested in learning about this can check out web pages like http://www.hillwilsonstarmap.net/2020/08/22/earths-magnetic-poles-can-flip-but-that-probably-will-not-happen-soon/.
Understanding the Awesome Right-Hand Thumb Rule:
This rule states that if you naturally curl your fingers forming a fist and take out the thumb then if the direction of curl of your fingers align with the current or magnetic field then the thumb direction will be the direction of generated current or magnetic field.
This law also works well with Screws… the direction of turn when aligned with the curl, the thumb gives the direction of movement of screw, inside or outside.
A coil creating a magnetic field in a direction.
A magnetic field creating a current in a conductor.