Faradays lag och transformatorn

By the end of this episode, you'll be able to answer three questions. Why does a magnet fall slowly through a copper pipe? How does a transformer step up voltage? And why do transformers only work with alternating current? Welcome to PodThis and Study Session. Let's start with Faraday's Law. Imagine you're holding a magnet near a coil of wire. If they're both still, nothing happens.

Faraday’s Law of Induction gives us the framework for understanding how magnetism creates electricity. Back in eighteen thirty-one, Michael Faraday performed a series of experiments that revealed this relationship. He used two separate coils of wire that weren't physically touching each other. One coil was connected to a battery to create a magnetic field. We often call this the primary coil. The other coil was connected to a galvanometer. That's a sensitive instrument used to detect the presence of an electric current. Faraday recorded that the needle on the galvanometer stayed still most of the time. It only moved at the exact moment he connected the battery to the primary coil. The needle also moved again at the specific moment he disconnected the battery. While the battery was fully connected and the current was steady, the needle didn't budge. Faraday concluded that a static magnetic field does not induce electricity in a nearby conductor. The electricity only appeared when the magnetic field was in the process of growing or collapsing. This discovery shifted the scientific understanding of how energy could be transferred between systems. It proved that having magnetism present isn't enough to generate power. Instead, the interaction requires a state of change to occur within the magnetic field. These observations are the basis of what we now call Faraday’s Law. The law states that a changing magnetic field is the necessary condition to induce an electromotive force. This force is commonly referred to as E-M-F. You can think of E-M-F as the electrical pressure or voltage that causes electrons to flow through a circuit. The specific measurement used to describe this process is called magnetic flux. Magnetic flux represents the total magnetic field passing through a given area, like the center of a wire loop. If you picture a hoop, the flux is the amount of magnetic field lines passing through that hoop. According to Faraday’s Law, an E-M-F is induced in a conductor whenever there is a change in this magnetic flux. If the amount of magnetism passing through the loop stays the same, no electricity is generated. The change in flux can happen in a few different ways. You might move a permanent magnet toward or away from a coil of wire. You could also change the size or shape of the wire loop while it sits in a magnetic field. Another method involves changing the strength of the magnetic field itself, just like Faraday did with his battery. The magnitude of the induced E-M-F depends on how quickly the magnetic flux changes. A faster change in the magnetic field results in a higher voltage being produced. This relationship explains why spinning a turbine quickly generates more power than spinning it slowly. The core requirement stays the same regardless of the method you use. A conductor must experience a fluctuating magnetic environment for induction to take place. This fundamental principle allows us to convert mechanical motion into the electrical energy we use every day. The law focuses entirely on the magnitude of the induced voltage based on the rate of change.

Lenz's Law determines the direction of the induced current, which builds on the calculation of its magnitude. This principle explains exactly how electricity and magnetism interact to maintain the balance of energy in a system. When a magnetic field changes near a conductor, it induces an electromotive force, or EMF. This EMF is the electrical pressure that can drive a current. Lenz's Law tells us that this induced current will always flow in a direction that creates its own magnetic field to oppose the original change in magnetic flux. Magnetic flux is a measurement of the total magnetic field passing through a given area, such as the space inside a loop of wire. You can think of this law as a form of physical resistance to change. If you try to increase the magnetic flux through a wire loop, the loop will produce a current that generates a field pointing in the opposite direction. This opposing field tries to cancel out the increase. On the other hand, if you try to decrease the flux by pulling a magnet away, the loop will create a current that generates a field to pull the magnet back. It effectively tries to keep the flux exactly where it was. According to the physics documented by Heinrich Lenz, this opposition is a fundamental requirement for the conservation of energy. If the induced field supported the change instead of opposing it, we would encounter a significant problem with the laws of nature. A magnet moving toward a coil would be pulled in faster by the induced field, which would then create an even stronger current. This would cause the magnet to accelerate indefinitely without any external energy being added. Since we know energy cannot be created from nothing, the induced field must push back against the motion that created it. This push ensures that you have to do work to move the magnet, and that work is what actually transforms into electrical energy. A very clear example of this opposition is the behavior of a neodymium magnet when it is dropped through a copper pipe. Neodymium is a material used to make exceptionally strong permanent magnets. Copper is a metal that conducts electricity well, but it is not naturally magnetic, so the magnet will not stick to the walls of the pipe. If you drop a piece of plastic through the pipe, it falls at the normal speed of gravity. However, when you drop the neodymium magnet, it appears to float slowly downward. As the magnet falls, the magnetic flux through the copper pipe changes rapidly. This change induces circular paths of electricity within the copper itself. These are known as eddy currents. Because these eddy currents are moving charges, they produce their own magnetic field inside the pipe. Following Lenz's Law, this new magnetic field must oppose the downward motion of the magnet. The field from the copper pushes upward, acting as a magnetic brake that fights against gravity. The magnet falls at only a small fraction of its normal speed because of this constant push. The energy that would normally make the magnet fall faster is instead converted into the electrical energy of the eddy currents. Question. What specific condition is required to induce a voltage in a coil according to Faraday's Law? Answer. A changing magnetic field is required to induce a voltage. A static or stationary magnetic field will not produce an induction effect.

Transformers use the principles of induction to change voltage levels. This moves us from the theory of induced currents into the world of practical energy control. A transformer is a device that increases or decreases the voltage of an alternating current. It has a simple design with no moving parts. It consists of two separate coils of wire. These are the primary coil and the secondary coil. Both coils are wrapped around a shared iron core. The iron core is a critical part of the design. It acts as a bridge for the magnetic field. When you send a current through the primary coil, it creates a magnetic field. Since the current is alternating, the magnetic field is constantly changing. This magnetic flux travels through the iron core to reach the secondary coil. Flux is just the term for the total magnetic field passing through a specific area. As that changing field moves through the secondary coil, it induces a voltage there. One important detail is that the two coils do not actually touch. They are electrically insulated from each other. The energy moves from the primary side to the secondary side entirely through the magnetic field. This process allows us to change the voltage while keeping the frequency of the electricity the same. The amount of voltage produced in the secondary coil depends on the turns ratio. This ratio is the comparison between the number of loops in the primary coil and the number in the secondary coil. The relationship between the turns and the voltage is linear. If you have more loops on the output side, you get more voltage. If you have fewer loops, the voltage drops. We use specific names for these two setups. A step-up transformer has more turns on the secondary coil than the primary. This increases the voltage. A step-down transformer has fewer turns on the secondary coil. This decreases the voltage. You can calculate the exact change by looking at the number of wire loops. Imagine you have a primary coil with one hundred turns of wire. You wrap a secondary coil around the same core with two thousand turns. Since the secondary coil has twenty times more loops, the output voltage will be exactly twenty times higher than the input. If you started with ten volts, you would end up with two hundred volts. This linear relationship is what makes our modern power grid possible. Power plants generate electricity, but they need to send it over long distances. To do this efficiently, they use massive step-up transformers. These devices can step the electricity up to three hundred forty-five thousand volts. High voltage is necessary for long-distance transport. It reduces the amount of energy lost as heat in the wires. Without the ability to precisely change voltage through the turns ratio, we could not power cities located far away from energy sources. Question. According to Lenz's Law, what is the relationship between an induced current and the motion that caused it? Answer. The induced current creates a magnetic field that directly opposes the motion that caused it.

The requirement for alternating current ensures that a transformer maintains the constant change in magnetism needed for power to move between its coils. To understand why this is necessary, we have to look at what happens when we use direct current, often called DC. Direct current is a type of electricity that flows in a single, steady direction, like the power from a standard AA battery. If you connect a DC source to the primary coil of a transformer, the current creates a magnetic field inside the iron core. This magnetic field is stationary and unchanging because the current itself is not moving or fluctuating. We use the term magnetic flux to describe the total amount of magnetic field passing through a specific area, such as the loops of a coil. According to Faraday’s law, a voltage is only induced in a wire when the magnetic flux through it is changing over time. Because the DC magnetic field is steady, the magnetic flux through the secondary coil does not change at all. Without that change, the secondary coil produces zero output voltage. The transformer fails to transfer any energy to the second circuit because the magnetic push is missing. The transformer becomes a simple loop of wire that does nothing useful with the energy it receives. There is also a physical danger when using DC in this way. The primary coil usually has very low electrical resistance, which is the measure of how much a material opposes the flow of current. In a properly functioning AC system, the changing magnetic field creates a small opposing voltage that limits the current. In a DC system, that opposing voltage never happens. A massive amount of current flows through the primary wire without any opposition. This creates intense heat that can quickly melt the wire insulation and lead to a fire. Alternating current, or AC, solves this problem by constantly reversing its direction of flow. The rate of this reversal is called frequency, and it is measured in units called hertz. Most power grids operate at a frequency of either 50 hertz or 60 hertz. This means the current changes direction many times every single second. As the current oscillates back and forth, the magnetic field it generates also oscillates. This creates a magnetic flux that is constantly growing, shrinking, and reversing within the core. Because the flux is always in motion, it continuously cuts across the wires of the secondary coil. This constant motion satisfies the requirement for induction at every moment the transformer is active. The iron core acts as a bridge, guiding this shifting flux from the first coil to the second one. This is why you will find transformers in AC power grids but never in a purely DC circuit. It is the movement of the electricity itself that allows the energy to jump across the gap between the coils. Without that oscillation, the magnetic field would be as still as a frozen lake. A still magnetic field cannot generate the electrical pressure needed to power your devices. Every time the current flips, it resets the process of induction and keeps the energy moving. Question. If a transformer has 50 turns on the primary coil and 150 turns on the secondary coil, what happens to the voltage? Answer. The output voltage will be exactly three times higher than the input voltage.

Transformer efficiency is the ratio of power output compared to power input. It all comes down to how energy moves through magnetic fields. Large-scale power transformers are actually some of the most efficient machines ever built. They often reach an efficiency of ninety-nine point seven percent. That means for every one thousand watts you put in, nine hundred and ninety-seven watts are delivered at the other end. The remaining three watts don't just disappear. They're converted into heat. That heat comes from two physical processes known as hysteresis and eddy currents. Hysteresis is the energy it takes to keep realigning the magnetic particles inside the iron core. Since the alternating current flips direction many times every second, those particles have to flip their magnetic poles constantly. That movement creates friction at the atomic level, and that's what generates the heat. Eddy currents are the other big cause of energy loss. These are small, circular loops of electricity that get induced right inside the solid metal of the core. Because the iron core is a conductor sitting inside a changing magnetic field, it follows the same laws of induction as the copper wires. Those internal currents hit electrical resistance as they move through the iron. That resistance turns the electricity into heat, which can make a transformer get quite warm while it's running. To keep these losses as low as possible, manufacturers don't use a single solid block of iron for the core. They use a method called lamination instead. The core is built from hundreds of thin sheets made of silicon steel. These sheets are usually between zero point three and zero point five millimeters thick. Every single sheet is coated in a layer of insulating varnish before they're stacked together. That varnish acts as a barrier to stop electrons from flowing between the different layers. By breaking the core into many thin slices, the path for those eddy currents is physically blocked. The currents can only form tiny loops within each thin sheet instead of large loops across the whole core. Smaller loops mean less current flow and much less heat. While eddy currents are usually a problem in power distribution, they're actually the main way some appliances work. An induction stovetop is a great example. The stovetop has a copper coil that creates a magnetic field vibrating at a high frequency. That frequency is usually between twenty and fifty kilohertz. When you put a pan with an iron base on the surface, that high-frequency field creates intense eddy currents right in the metal of the pan. The electrical resistance in the iron base turns those currents into heat. This lets the pan get hot very fast and it transfers that heat directly to your food. Since the heat starts inside the pan itself, the stovetop surface doesn't have to get nearly as hot as a traditional electric or gas burner. Question. Why will a transformer fail to produce an output voltage if connected to a DC battery? Answer. Direct current produces a stationary magnetic field, which means there is no change in magnetic flux to induce a voltage in the secondary coil.

It's time for the recap quiz. Try to answer each question in your head before I give you the answer. Testing yourself like this helps move the information into your long-term memory. It builds a deeper understanding. We're going to go through five questions. They cover the core concepts of induction and transformers. Question. What did Michael Faraday's galvanometer experiment prove is necessary to induce an electric current? Answer. It proved that a changing magnetic field is required to generate a current. A static field will not work. Question. Why does a neodymium magnet fall slowly through a copper pipe? Answer. According to Lenz's Law, the induced current creates its own magnetic field. This field opposes the motion of the falling magnet. Question. If a transformer has 200 turns in the primary coil and 100 turns in the secondary coil, what happens to the output voltage? Answer. The voltage is stepped down. This means the output voltage becomes exactly half of the input voltage. Question. Why is alternating current, or AC, essential for a transformer to function? Answer. Alternating current provides a constantly oscillating magnetic field. This creates the continuous change in magnetic flux needed for induction to occur. Question. What is the purpose of using laminated silicon steel sheets in a transformer core? Answer. These insulated sheets break up circular current paths. This prevents energy loss caused by eddy currents.

How many of five? Replay the chapter for each one you missed.