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Introduction to Current Electricity

Good conductors have large numbers of free charges in them. Figure 20.6 shows how free electrons move through an ordinary conductor. The distance that an individual electron can move between collisions with atoms or other electrons is quite small. The electron paths thus appear current electricity meaning nearly random, like the motion of atoms in a gas. But there is an electric field in the conductor that causes the electrons to drift in the direction shown (opposite to the field, since they are negative). The drift velocity vdvd is the average velocity of the free charges.

The unit of current is the ampere, but this word is often shortened to ‘amp’. Probably the most common place to see something rated in amps is in the circuit breaker box in your house. The numbers on the switches tell you how many amps of current can flow through the breaker before it shuts off to protect the wires. Current is measured by the amount of electrical charge that moves past a given point, such as the circuit breaker, in the time period of one second. Since electric charge is measured in coulombs and time in seconds, the true unit for current is coulombs per second.

  1. The current flows through the copper wires because copper is a good conductor.
  2. You see, the atoms in a conducting material have lots of free electrons that float around from atom to atom and everywhere in between.
  3. Cold electrodes can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (called cathode spots or anode spots) are formed.
  4. In the case of alternating current, where the current changes direction 50 or 60 times per second, most of the electrons never make it out of the wire.
  5. The caveat is that, in this case, the pipe is always full of water.

Drift velocity is quite small, since there are so many free charges. If we have an estimate of the density of free electrons in a conductor, we can calculate the drift velocity for a given current. The larger the density, the lower the velocity required for a given current.

Electric Shocks

The electric field does work in moving the electrons through a distance, but that work does not increase the kinetic energy (nor speed, therefore) of the electrons. The work is transferred to the conductor’s atoms, possibly increasing temperature. Thus a continuous power input is required to keep a current flowing. An exception, of course, is found in superconductors, for reasons we shall explore in a later chapter.

Defining Current and the Ampere

More commonly, though, when we speak of electric current, we mean the more controlled form of electricity from generators, batteries, solar cells or fuel cells. The picture of charge flow being developed here is a picture in which charge carriers are like soldiers marching along together, everywhere at the same rate. Their marching begins immediately in response to the establishment of an electric potential across the two ends of the circuit. There is no place in the electrical circuit where charge carriers become consumed or used up. And there is no place in the circuit where charge carriers begin to pile up or accumulate.

The Nature of Charge Flow

The SI unit of electrical current is the ampere, defined as 1 coulomb/second. Current is a quantity, meaning it is the same number regardless of the direction of the flow, without a positive or negative number. However, in circuit analysis, the direction of current is relevant. When you hear the word ‘current,’ what does it make you think of?

Making Connections: Take-Home Investigation—Electric Current Illustration

In gases, electric current arises due to ions and free electrons obtained after ionization of the gas. In semiconductors, electric current is due to holes in p-type semiconductors, and electrons in n-type semiconductors. Direct current is the type of current that has a fixed magnitude and direction. The net flow of electrons in direct current occurs in a fixed direction, from the negative to the positive terminal of the applied voltage. On the other hand, alternating current has time-varying magnitude and direction.

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