Electromagnetic Induction: Conductor to Conductor & Transformers

Mutual Inductance

Using a shared magnetic field, one coil transfers power to another.

Have you ever seen a power transformer on a utility pole? Or how about an AC adapter? Maybe you've used a transponder car key, or even an electric toothbrush? Either way, I'll bet you answered 'yes' to at least one of these questions, which makes an important point. We use a phenomenon known as mutual inductance on a daily basis without even knowing it. We're going to explore mutual inductance in greater detail so that we can understand how it is used in all sorts of everyday devices.
The ability of one current-carrying conductor to induce a voltage in another conductor through a mutual magnetic field is known as mutual inductance. When an alternating electric current flows through a wire, it creates an alternating magnetic field around the wire. If we wrap this wire into the shape of a coil, we can concentrate the magnetic field into the coil's center and make it much stronger. We'll call this the primary coil. Now, if we were to bring a secondary coil of wire with the same number of turns into close proximity, the alternating magnetic field would induce a voltage approximately equal to that in the primary coil.

Transformers

At this point, you might be wondering why we would go through so much effort just to get the same voltage in both coils. One reason is that the coils don't have to be physically connected to transfer energy. This has some useful applications that we'll look at a bit later. Another reason is that we can achieve a different voltage in the secondary coil simply by changing the number of turns.
For example, if the secondary coil has twice as many turns as the primary coil, then the induced voltage will be twice as large. On the other hand, if the secondary coil has half the number of turns as the primary coil, the induced voltage will be half. As we can see, the voltage ratio between the two coils is the same as the turns ratio. The device that we've described here is called a transformer because it transforms one voltage into another voltage.

Current and voltage are inversely related.

Getting a higher voltage out of a transformer than what we put in may seem like we're getting something for nothing, but we need to look at what happens to the current and power as well. When it comes to transformers, voltage and current are inversely related. In other words, if the secondary voltage is twice as much as the primary voltage, then the secondary current will be half as much as the primary current.
Because of this relationship, power, which is the product of current and voltage, is the same going into and out of the transformer. In a way, a transformer is similar to a lever used to move a really heavy object. While your small force at one end gets turned into a big force at the other end, your large movement also gets reduced to a small movement by the same amount. In the natural world, there is always a trade-off that keeps us from getting something for nothing.
In order for mutual inductance to take place, the magnetic field must always be changing. At the beginning of our discussion, we specified that the primary coil of our transformer was connected to an alternating current source, which produced an alternating magnetic field. If instead we connected the primary coil to a direct current source, such as a battery, the magnetic field would be constant and unchanging. Since an unchanging field cannot induce a voltage in the secondary coil, transformers do not work with direct current. Alternating current is much better suited for use with transformers, which is a big reason why it is used for long-distance power transmission, as we'll talk about next.

Applications of Mutual Inductance

Between a power plant and your home, the voltage must be adjusted inside of transformers.

Transformers are one of the most important applications of mutual inductance and can be found both outside and inside your home. Let's look at how they're used to send electricity from the power plant to your home. To minimize power loss in the long transmission lines, it is necessary to transmit the electricity at very high voltages, often in excess of 700,000 volts! The generators in the power plant generate much lower voltages, so the electricity has to be sent through a step-up transformer before it can be sent out to your house.