8.1 Electromagnetism

I've finally made it to Module 8 of the Navy Basic Electricity and Electronics series from the 1970s. The previous modules have been:

• Module 1: Current
• Module 2: Voltage
• Module 3: Resistance
• Module 4: Measuring Current and Voltage in a Series Circuit
• Module 5: Relationships of Current, Voltage, and Resistance
• Module 6: Parallel Circuits
• Module 7: Combination Circuits and Voltage Dividers

Module 8 is about Induction.

1. As we already know, whenever current runs through a conductor, it creates a magnetic field around the conductor. Magnetic lines of flux form circular patterns around the conductor. The "left-hand rule for conductors" says that if you wrap your left hand around a conductor with your thumb pointing in the direction of the current, the direction of your other fingers tells whether the lines of flux are going in a clockwise or counterclockwise direction.

Electricity flows from negative to positive in a conductor and a magnetic field loops from north to south around a magnet.

2. If you wind a conductor around something--say a cylinder of metal--you create a stronger magnetic field (and a definite north and south pole). Using the left hand rule, if you wrap your fingers around the cylinder in the way they are wound, your thumb will point toward the north pole of this magnet created by current.

By these observations, we are building toward understanding a new component we have not encountered yet--the inductor or "choke." It is a coil that is usually wrapped around an iron core.

3. A number of applications are mentioned in this section. The first is a "relay." A relay is used to control a high voltage circuit without being physically connected to it. A current in a low voltage circuit, when closed, causes a magnetic field to arise around a coil, which attracts a conductor on a higher voltage circuit in such a way that the other circuit closes and then current flows in it.

There are several reasons to want to energize a high voltage circuit without simply flipping a switch on it directly. One is the ability to do it by remote control. Rather than run high voltage lines from afar, you can run low voltage lines. This also increases the safety of the situation.

4. A second application is the electric bell. Coils are used such that when the circuit is closed, a magnetic field is created and an armature hammer is pulled to hit a bell. But when it is so pulled, it breaks the circuit and the magnetic field is broken. But when the field is broken, the iron connected to the armature hammer reconnects the circuit, causing the magnetic field to return and the armature hammer to hit the bell again.

This process will occur repeatedly and whatever rate you set up, calling the hammer to hit the bell repeatedly until the overall circuit is opened.

5. The two applications above used a fixed core. A solenoid relay uses a movable core. When the current is running through the coil, a magnetic field is generated which pulls the core into the coil (because the north-south pole created pulls the core into the south pole). If this core is connected to a spring, the spring can close a circuit.

This sort of solenoid relay is used to start a car. The ignition closes a low voltage circuit with the car battery, which pulls an iron core into a coil. As it moves, a spring closes a higher voltage circuit with the starter, which then starts the car.

6. Two symbols are often used in diagrams to show current flowing in or out of a conductor. A dot suggests that the current is flowing out of the conductor, like the tip of an arrow. An x suggests that current is flowing into the conductor, like the back part of an arrow.