Ah yes, today we will learn about magnetic fields. Let’s explore how to grow your own magnetic field. And discuss why they are so attractive.

Yeah, right. Magnetic fields enable much of the technology that we take for granted today. Transportation systems such as the mach level train in China, power generation equipment such as wind power, hydro electric systems. In fact, magnetic fields affect almost every aspect of our daily lives and even in today’s technology, and the technologies of tomorrow.

Hi, I’m Dave Wilson, Motion Product Specialist with Freescale Semiconductor and welcome to this tutorial on this very exciting aspect of engineering and science. Today we’re going to take a look at magnetic fields and study some of the history of magnetic fields, some of the terminology and also the operation of magnetic fields in order to lay the framework for some of the topics that we will be covering later in this tutorial series. So sit back, enjoy the presentation. But before we begin, let’s wind the clock back several thousand years to ancient Greece.

Magnetic materials were first documented in Greek literature as far back as 600 B.C. in fact the name magnet actually comes from the name magnesia which is a province in Central Greece. It was discovered that certain magnetite rocks called lodestones had a unique ability to attract metal. But not all magnetite rocks were magnetic. So why were some rocks magnetic and others weren’t? The answer is actually quite shocking. Take an ordinary magnetite rock add a little electricity from Mother Nature and the next thing you know you have a magnetized lodestone. It turns out that the electric loads from the lighting bolt are sufficiently strong enough to permanently magnetize the magnetite. But it was the Chinese who first put these magnetic rocks to good use. The compass is the first documented instrument in history to actually utilize precise scale. The design for this self pointing spoon compass dates back to the Han dynasty nearly 200 years BC. The spoon or ladle is constructed from magnetic lodestone and positioned over a bronze plate. This particular type of compass was often used for demanation as well as orienting buildings and determining burial sites. Amazingly there’s no recorded evidence of the compass actually being used for navigation until over a thousand years later.

Up until the 18th century, the relationship between magnetism and electricity was not that well understood. But this all changed one April evening, in 1820 when a Danish physicist named Hans Christian Oersted was conducting a demonstration for some of his students and his friends at his house. As he was demonstrating the heating effect of current through a platinum wire, he noticed quite accidentally that the needle of a compass on his desk would move each time he switched the current on and off or whenever he moved the wore near the compass. He wasn’t able to explain this phenomenon. So he started studying this particular effect further. Oersted later published his findings proving that a measurable and predict table causality existed between electricity and magnetism. He speculated that the current in the wire must somehow be generating a magnetic field on its own in order to have the effect that it did on the compass. His publication caught the attention of another physicist Andre Ampere who quantified this effect mathematically and submitted his findings in September of the very same year. You know when you think about it, it must have been an exciting time to be alive. The race was now on to understand and quantify this mysterious relationship between electricity and magnetism. Oersted and Ampere had already established that magnetism could be created from electricity. But the real question was could electricity also be created from magnetism?

This was the question that perplexed a London physicist named Michael Faraday. He had tried on 4 different occasions in 1820s to create electricity from magnetism but with no luck. Then on August 29th, 1831, Faraday finally got the break that he needed. He took an iron ring as shown here. On one side of it he wrapped a coil of wire and connected it to battery. On the other side of the ring he wrapped a second coil of wire. He then switched the current on to the first coil and looked for signs of current flowing in the second coil. Unfortunately nothing happened. But he did notice a rather subtle effect which he was not expecting. Every time he connected or disconnected the first coil to the battery he noticed a small blip on his galvanometer which was connected to the second coil. Over the next few months Faraday returned to the strange phenomenon whenever he got a chance to and tried to understand what was going on. He later concluded that current could be induced to flow in the second coil whenever the magnetic field in the iron ring was changed. This later led to the relationship we know today as Faraday’s law and we will explore that more in this tutorial.

Little could Faraday have known the significance of this magnificent discovery? This was in fact the missing piece. Man could now build the mighty electrical machines that later drive the industrial revolution. So the next time you make what you think is an insignificant breakthrough in your project, remember Michel Faraday and his little experiment. Who knows the next big discovery about our universe might be named after you.

In fact it was Faraday that first speculated that magnetic fields exist as lines of force which we today refer to as magnetic flux. Essentially the more flux there is the stronger the magnet will be. But the honour of naming how much flux was bestowed upon a German physicist Wilhelm Weber. We conventionally think of flux leaving the North Pole and re-entering the South Pole of a magnet as shown in this diagram. If we measure how much flux cuts through a surface area which is perpendicular to the flux path, then we have a measure of the flux density at that particular spot in space. One Weber of flux cutting through one square meter of area constitutes a flux density of one Tesla named after the Serbian engineer Nicola Tesla who is also the inventor of the AC induction motor.

Referring back to Oersted’s experiment of the compass, if the magnetic field generated by a conducting wire can exert a force on a magnetic compass then it only stands to reason that a stationary magnet can also exert a force on a conducting wire. Here we have a wire conducting current in the direction shown and it’s suspended in a magnetic field. We can determine in which direction the force will act on the wire by using something called the right hand rule. So try this for yourself. If you point in the direction of the magnetic field with your index finger, and simultaneously point in the direction of current flow in the wire with your thumb then your middle finger will actually be pointing in the direction of the force on the wire. So, go ahead try it. Use the diagram here, take your right hand, which direction will the wire be motivated to move. Is it up, come up down. I don’t know may be left or right. What do you think it is? If you answered down, then you are correct.

So let’s take what we have just learned and see how we can use this effect to actually rotate an object. Here we see the cross-section of a cylindrical body with 2 conductors running down opposite sides of the left of the cylinder. The current in the right conductor is flowing into the page and the current in the left conductor is shown flowing out of the page. If we apply the right hand rule on both conductors separately, we see that the conductor on the left creates a force to the left but the conductor on the right will create a force to the right. The forces on each wire act through a radius R on the cylinder structure to create a clockwise torque. In this case, the torque created is proportional to the sign of the angle Alfa which means it will be greatest when the plane containing the two wires is completely vertical. However, once the cylinder has rotated, so that this plane is horizontal, then the torque will go to zero since the sign of zero is zero. If the cylinder continues to rotate, so that the plane goes half horizontal, the torque will now become negative, or counter clockwise since the sign of a negative angle results in a negative number. So, we can see here that the wires will exert a force on the cylinder to drive it to a steady state angular position where the plane containing the wires is horizontal. If we want further rotation, we must switch the current to a new pair of wires, who are at a different angle than the first two. We will take a look at this effect further when we study DC motors in later tutorial.

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Watch & Read:
Part 2/4: Freescale Magnetic Fields - Introduction
Part 3/4: Magnetic Fields Introduction