Electricity and magnetism are closely linked. Moving charges produce magnetic fields, and the moving electrons in a circuit are no exception. From circuit breakers to transformers, from motors to generators, from microphones to door bells, the interplay between electricity and magnetism has proven useful in a range of applications.
A current-carrying wire generates its own magnetic field. The strength of the field depends on the amount of current flowing in the wires. When the wires are wrapped around a ferrous material and a current is passed through the wires, the resulting magnetic field magnetizes the iron core, producing an electromagnet. This induced magnetism is the basic principle behind many of the aforementioned electrical devices.
The magnetic field formed by a current-carrying wire can be made stronger by winding the wire into a coil. If the coil is wound onto a ferrous (iron) core, the magnetic field around the coil becomes even stronger, since magnetic lines of force travel more easily through iron than through air. When current first flows through a coil, the magnetic field builds relatively slowly. This is because the expanding magnetic field generates a voltage in the coil that opposes the original current flow, known as counter-electromotive force, or counter-emf. When the current is cut off, the magnetic field then collapses, and this collapsing magnetic field generates a voltage in the coil that keeps the current flowing. This resistance to the change in current flow in a circuit is known as self-induction and is a property exhibited by electrical components known as inductors. Inductors resist change in current flow. If current is increasing, the inductor opposes the increase by generating a voltage that moves against the applied current. If current decreases, the inductor uses the magnetic energy in the coil to oppose the decrease and to keep the current flowing.
Inductor Symbol
Induction is measured in a unit known as henries, and the symbol used to represent induction is L. Inductors work exactly opposite to capacitors, in the sense that they allow DC to pass easily, but resist the flow of AC. This resistance to current flow is known as inductive reactance and is measured in ohms. It increases with increasing frequency of the AC signal.
A transformer is used to increase or decrease the voltage in a circuit. A transformer uses the properties of an inductor to accomplish this. Alternating current flowing in wires wrapped around an iron core magnetizes the core, and in turn produces a changing magnetic field in the core. This changing magnetic field generates a voltage in a neighboring coil of wire. Depending on the number of turns of the wire in the primary versus the secondary coil, and the proximity of the coils, a smaller or larger voltage can be induced in the secondary coil. The primary coil is the one that is connected to the source. The secondary coil is the one in which an electric current is induced. A larger number of secondary coils means a larger voltage. The closer the secondary coils are to the primary coils, the more efficient the transformer is in producing a voltage in the secondary coils.
Transformers are especially useful to transmit electricity from power plants to residences and businesses. It is more energy efficient to transmit low current, high voltage electricity. However, by the time it arrives at homes, it must be reduced or “stepped down” to the standard 120–240 volts that most appliances in our homes use.
A current-carrying wire positioned in a permanent magnetic field will experience a force (push or pull). This force can be harnessed to produce useful mechanical energy. This is the principle by which a simple motor works. The mechanical energy can be used to move the tires on a car, spin the blades of a fan, or lift heavy objects, just to name a few examples. The generated force on the current-carrying wire is directly proportional to the amount of current flowing, the length of the wire, and the strength of the magnetic field. Increasing any of these will increase the force on the wire and thus the capacity to do work. Motors today are more complex than this, and many use electromagnets in place of the current-carrying wire alone, but the underlying principle remains the same. A generator, on the other hand, is simply a motor in reverse. Moving a wire in a permanent magnetic field induces a current in the wire if it is connected in a complete circuit. Thus, a generator takes mechanical energy and converts it into electrical energy.
Here’s how an expert test taker would approach a question about the structure of electrical systems.
Question | Analysis |
Given the following circuit, with a switch that has been closed for a long time, what will happen to the lamp immediately after the switch is opened? | Step 1: The question asks what will happen to the lamp in the circuit when the switch is opened. Will it stay on or turn off? Burn less brightly or more brightly? |
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Step 2: Normally, in a simple series circuit, when the switch is opened, the lamp will turn off immediately. However, the presence of an inductor in the circuit must be accounted for. An inductor allows DC to pass easily with little resistance, and so most of the current will pass through the inductor and less through the lamp. Thus initially, the lamp will not be glowing very brightly. Once the switch is opened, the current generated by the voltage source will stop, but since the inductor resists this change, it will briefly produce a voltage that keeps current flowing in the circuit. |
Step 3: Therefore, immediately after the switch is opened, the lamp will still stay lit briefly and will burn more brightly, since all the voltage generated by the inductor will be dropped across it. | |
(A) The lamp will stay lit for a long time. (B) The lamp will burn more dimly. (C) The lamp will immediately turn off. (D) The lamp will briefly burn more brightly and then turn off. |
Step 4: Choose option (D). |
Now try a question on your own.
Answer choice (C) is correct. A transformer can take low current, high voltage electricity from a power plant and reduce the voltage, or step it down, before it enters a home. In general, a transformer can increase or decrease the source voltage depending on the application.