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Physics and Current Help

By — McGraw-Hill Professional
Updated on Sep 5, 2011

Current

Whenever there is movement of charge carriers in a substance, there is an electric current . Current is measured in terms of the number of charge carriers , or particles containing a unit electric charge, passing a single point in 1 second.

Charge carriers come in two main forms: electrons, which have a unit negative charge, and holes, which are electron absences within atoms and which carry a unit positive charge. Ions can act as charge carriers, and in some cases, atomic nuclei can too. These types of particles carry whole-number multiples of a unit electric charge. Ions can be positive or negative in polarity, but atomic nuclei are always positive.

Usually, a great many charge carriers go past any given point in 1 second, even if the current is small. In a household electric circuit, a 100-W light bulb draws a current of about 6 quintillion (6 × 10 18 ) charge carriers per second. Even the smallest minibulb carries a huge number of charge carriers every second. It is ridiculous to speak of a current in terms of charge carriers per second, so usually it is measured in coulombs per second instead. A coulomb (symbolized C) is equal to approximately 6.24 × 10 18 electrons or holes. A current of 1 coulomb per second (1 C/s) is called an ampere (symbolized A), and this is the standard unit of electric current. A 60-W bulb in a common table lamp draws about 0.5 A of current.

When a current flows through a resistance—and this is always the case, because even the best conductors have resistance—heat is generated. Sometimes visible light and other forms of energy are emitted as well. A light bulb is deliberately designed so that the resistance causes visible light to be generated. However, even the best incandescent lamp is inefficient, creating more heat than light energy. Fluorescent lamps are better; they produce more light for a given amount of current. To put this another way, they need less current to give off a certain amount of light.

In physics, electric current is theoretically considered to flow from the positive to the negative pole. This is known as conventional current . If you connect a light bulb to a battery, therefore, the conventional current flows out of the positive terminal and into the negative terminal. However, the electrons, which are the primary type of charge carrier in the wire and the bulb, flow in the opposite direction, from negative to positive. This is the way engineers usually think about current.

Static Electricity

Charge carriers, particularly electrons, can build up or become deficient on objects without flowing anywhere. You’ve experienced this when walking on a carpeted floor during the winter or in a place where the humidity is low. An excess or shortage of electrons is created on and in your body. You acquire a charge of static electricity . It’s called static because it doesn’t go anywhere. You don’t feel this until you touch some metallic object that is connected to an electrical ground or to some large fixture, but then there is a discharge, accompanied by a spark and a small electric shock. It is the current, during this discharge, that causes the sensation.

If you were to become much more charged, your hair would stand on end because every hair would repel every other one. Objects that carry the same electric charge, caused by either an excess or a deficiency of electrons, repel each other. If you were massively charged, the spark might jump several centimeters. Such a charge is dangerous. Static electric (also called electrostatic) charge buildup of this magnitude does not happen with ordinary carpet and shoes, fortunately. However, a device called a Van de Graaff generator , found in some high-school physics labs, can cause a spark this large. You have to be careful when using this device for physics experiments.

On the grand scale of the Earth’s atmosphere, lightning occurs between clouds and between clouds and the surface. This spark is a greatly magnified version of the little spark you get after shuffling around on a carpet. Until the spark occurs, there is an electrostatic charge in the clouds, between different clouds, or between parts of a cloud and the ground. In Fig. 12-2, four types of lightning are shown. The discharge can occur within a single cloud (intracloud lightning , part a) , between two different clouds (intercloud lightning , part b) , or from a cloud to the surface (cloud-to-ground lightning , part c ), or from the surface to a cloud (ground-to-cloud lightning , part d) . The direction of the current flow in these cases is considered to be the same as the direction in which the electrons move. In cloud-to-ground or ground-to-cloud lightning, the charge on the Earth’s surface follows along beneath the thunderstorm cloud like a shadow as the storm is blown along by the prevailing winds.

Direct Current What Does Electricity Do? Electromotive Force

 

Fig. 12-2 . (a) Lightning can occur within a single cloud (intracloud), (b) between clouds (intercloud), or between a cloud and the surface (c) cloud to ground or (d) ground to cloud.

The current in a lightning stroke can approach 1 million A. However, it takes place only for a fraction of a second. Still, many coulombs of charge are displaced in a single bolt of lightning.

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