Electronics/Voltage and Current

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Voltage[edit]

This is a rough draft.

This section talks about the nature of voltage and current. Voltage and current sources and series and parallel voltage and current belong in the DC section. I'm not sure where voltage drops should go.

Voltage is a potential in an electric field between two charges. A voltage drop is a change in potential. Voltages add or subtract in series. (Relate this to voltage symbol)

(Moved to talk page)

Two electrically charged particles separated by a distance will have a potential energy associated with them,

Potential energy between two charges, where k is a constant, q and Q are the value of the two charges and r is the distance. Note that potential falls off as 1/r.

PE = {k q_{1} q_{2} \over r}

The potential divided by the amount of charge is the voltage:

V = {PE \over Q}

Volt = {Joule \over Coulomb}

Ideal voltage sources[edit]

An ideal voltage source is a fundamental electronics component that creates a constant voltage between two points regardless of whatever else is connected to it. Since it is ideal, some circuit configurations are not allowed.

Real voltage sources, such as batteries, power supplies, piezoelectric disks, generators, etc. have an internal source impedance (in series with the source), which is very important to understand. hfdh

Current[edit]

It is sometimes taught that current in electric circuits is composed of electrons, which flow from the negative terminal of the power source to the positive at the speed of light. This is not (completely) true.

  1. Electricity is carried by charged particles.
    This can mean any small particles that carry charge and are free to move. In metals, electrons are free to move and the metal nuclei are not. In salt water, however, electrons, negative ions, and positive ions are free to move, and do, when a voltage is applied (batteries and electrolytic capacitors are examples of electrical components that carry charge as ions). In your own nerve cells, electricity is carried by moving ions, such as potassium and sodium. In semiconductors, electricity is carried by electrons, but is often much more easily understood as movement of "holes"; the absence of an electron. In some static electricity experiments, electricity can be carried by charged dust or small pieces of paper.
  2. Electrons drift through conductors.
    • When you flip the light switch, the light comes on almost instantly. This does not mean that the electrons themselves move that quickly. In fact, they usually move much much slower. A typical speed for electron drift in a DC electronic circuit is slower than molasses. The electrons themselves are moving very quickly, but not in one direction. They are constantly moving randomly from atom to atom, and only have a very gradual drift, or shift in average position over time. The speed electrons drift actually depends on voltage, resistance of the conductor, shape of the conductor, material the conductor is made of, temperature, and other factors.
    • What is actually traveling quickly is electromagnetic waves; the pushing of electrons by their neighbors. This is similar to the way a wave in water works. When you drop a stone in a pond, a wave spreads out from where the stone hit the surface. But does the water itself move? Not really. The water molecules at the surface are just moving back and forth, and their cumulative effect is the wave that you see, which travels in one direction. This is similar to the travel of an AC wave down a transmission line. (we could make a better analogy to waves of car traffic or waves of people in line for a ride at the fair) An interesting analogy would be moving a hand through air. The hand is the wave and the air is the random electron movement.
  3. Electromagnetic waves only travel at the speed of light in a vacuum.
    • What is usually meant when someone says "the speed of light" is actually "the speed of light in a vacuum", as light itself slows down while traveling through materials. A typical speed for a signal traveling down common coaxial cable is 2/3 the speed of light (in a vacuum). (This is about 200,000,000 m/s.) The wave traveling down the cable is actually the same thing as light, just at a different frequency. The waves traveling through your nerves as you read this are traveling at about 120 m/s.

(Section relating voltage to current.) As you increase voltage you apply an electric field to electrons and they travel from the negative to the positive potential. This is why increasing voltage directly increases current. Reversing the voltage reverses the current. Without resistance this is effectively a short meaning the electrons flow unhindered.

(section where you have voltage but no current.) Sometimes you have voltage but no current. Describe its effects on circuits.

So, negative particles drift from negative to positive voltage, and positive particles drift in the opposite direction from positive to negative voltage. The particles drift at different speeds in different materials. speed of "holes" based on bandgap. Given the presence of holes we tend to ignore the particles and focus on the current flow. Current is measured by the amount of charge flow per unit time and represents the speed of the electromagnetics waves. In talking about current we will mainly talk about electrons flowing, as they are the predominant charge carriers in metal and many circuit components.

Voltage = potential between two charges. Defined as the derivative of the flux linkage:

V(t)={d(Ni(t)) \over dt}

Current = flow of electrons Defined as the rate of change of the charge:

I(t)={dQ(t) \over dt}

Ideal current sources[edit]

An ideal voltage source is a fundamental electronics component that creates a constant current through a section of circuit, regardless of whatever else is connected to it. Since it is ideal, some circuit configurations are not allowed.

Real current sources, such as batteries, power supplies, piezoelectric disks, generators, etc. have an internal source impedance (in parallel with the source), which is very important to understand.