1. As it is impossible to see electric current, its existence ___ its effects.
2. The current can ___ observing their heating, chemical or magnetic effects.
3. When the e.m.f. is applied to the ends of the wire, the free electrons move in ___.
4. The greater the number of participating electrons, ___ is the flow of current.
5. In electric circuits this charge is often carried by moving electrons _____.
6. The unit of resistance is _____.
7. Electric current can be measured using ___.
ELECTRIC FIELD
The concept of the electric field was introduced by Michael Faraday. An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The electric field acts between two charges in a similar manner to the way that the gravitational field acts between two masses, and like it, extends towards infinity and shows an inverse square relationship with distance. However, there is an important difference.
Gravity always acts in attraction, drawing two masses together, while the electric field can result in either attraction or repulsion. Since large bodies such as planets generally carry no net charge, the electric field at a distance is usually zero. Thus gravity is the dominant force at distance in the universe, despite being much weaker.
An electric field generally varies in space, and its strength at any one point is defined as the force (per unit charge) that would be felt by a stationary, negligible charge if placed at that point. The conceptual charge, termed a 'test charge', must be vanishingly small to prevent its own electric field disturbing the main field and must also be stationary to prevent the effect of magnetic fields. As the electric field is defined in terms of force, and force is a vector, so it follows that an electric field is also a vector, having both magnitude and direction. Specifically, it is a vector field.
The study of electric fields created by stationary charges is called electrostatics. The field may be visualized by a set of imaginary lines whose direction at any point is the same as that of the field. This concept was introduced by Faraday, whose term 'lines of force' still sometimes sees use.
The principles of electrostatics are important when designing items of high-voltage equipment. There is a finite limit to the electric field strength that may be withstood by any medium. Beyond this point, electrical breakdown occurs and an electric arc causes flashover between the charged parts. Air, for example, tends to arc across small gaps at electric field strengths which exceed 30 kV per centimeter. Over larger gaps, its breakdown strength is weaker, perhaps 1 kV per centimeter. The most visible natural occurrence of this is lightning, caused when charge becomes separated in the clouds by rising columns of air, and raises the electric field in the air to greater than it can withstand. The voltage of a large lightning cloud may be as high as 100 MV and have discharge energies as great as 250 kWh.
The field strength is greatly affected by nearby conducting objects, and it is particularly intense when it is forced to curve around sharply pointed objects. This principle is exploited in the lightning conductor, the sharp spike of which acts to encourage the lightning stroke to develop there, rather than to the building it serves to protect.
ELECTRIC POTENTIAL
The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires work. The electric potential at any point is defined as the energy required bringing a unit test charge from an infinite distance slowly to that point. It is usually measured in volts, and one volt is the potential for which one joule of work must be expended to bring a charge of one coulomb from infinity.
This definition of potential, while formal, has little practical application, and a more useful concept is that of electric potential difference, and is the energy required to move a unit charge between two specified points.
An electric field has the special property that it is conservative, which means that the path taken by the test charge is irrelevant: all paths between two specified points expend the same energy, and thus a unique value for potential difference may be stated. The volt is so strongly identified as the unit of choice for measurement and description of electric potential difference that the term voltage sees greater everyday usage.
A pair of AA cells. The + sign indicates the polarity of the potential difference between the battery terminals.
For practical purposes, it is useful to define a common reference point to which potentials may be expressed and compared.
Electric potential is a scalar quantity, that is, it has only magnitude and not direction. It may be viewed as analogous to height: just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will 'fall' across the voltage caused by an electric field. As relief maps show contour lines marking points of equal height, a set of lines marking points of equal potential (known as equipotentials) may be drawn around an electro statically charged object. The equipotentials cross all lines of force at right angles. They must also lie parallel to a conductor's surface; otherwise this would produce a force that will move the charge carriers to even the potential of the surface.
The electric field was formally defined as the force exerted per unit charge, but the concept of potential allows for a more useful and equivalent definition: the electric field is the local gradient of the electric potential. Usually expressed in volts per meter, the vector direction of the field is the line of greatest slope of potential, and where the equipotentials lie closest together.
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.
Electric power, like mechanical power, is the rate of doing work, measured in watts, and represented by the letter P. The term wattage is used colloquially to mean "electric power in watts." The electric power in watts produced by an electric current I consisting of a charge of Q coulombs every t seconds passing through an electric potential (voltage) difference of V is
where
Q is electric charge in coulombs
t is time in seconds
I is electric current in amperes
V is electric potential or voltage in volts
I. Answer the following questions:
1. Who introduced the concept of an electric field?
2. How does an electric field act?
3. What is an electrostatics?
4. When do you use the principles of electrostatics?
5. How the electric potential is defined?
6. What property has an electric field?
7. What is an electric potential?
II. Arrange the following synonyms into pairs:
Use, much, conversion, in place of, to generate, usual, to be made up of, readily, poor, quantity, power plant, energy, to be converted, for instance, chief, actually, generally, to connect, to raise, at present, a lot of, bad, application, usually, power, common, instead of, to consist, of, to produce, to increase, now, in fact, main, number, and so on, easily, power station, to be transformed, to join, for example, transformation.
III. Arrange the following antonyms into pairs:
Many, much, direct, new, small, common, negatively, charge, outer, conductor, poor, thin, uneconomic, cool, to raise, little, inner, old, uncommon, non-conductor, indirect, few, to lower, good, economic, warm, thick, positively, large, discharge.
IV. Fill in the gaps with the words given below:
