Материал: Андрианова English Thesaurus of Student-Physicist 2013

3. Fill in the gaps with the missing words from the list:

Gauss's law, coulomb constant, electric force, gaussian surface, charge

1.In modern terms, the _____ is said to be quantized, meaning that charge occurs in discrete chunks that can't be further subdivided.

2.The value of the _____ depends on the choice of units.

3.When a positive test charge is used, the electric field always has the same direction as the _____ on the test charge.

4.Though it's not obvious,_____ describes how charges create electric

fields.

5.A _____ is an imaginary surface, created solely to facilitate a mathematical calculation.

UNIT 16

ELECTRICAL ENERGY AND CAPACITANCE

Read the following texts. Study the thesaurus on fig. 16, answer the questions after the texts

By using the principle of conservation of energy, we are often able to avoid working directly with forces when solving problems. The potential energy concept is also useful in the study of electricity. Because the Coulomb force is conservative, we can define an electric potential energy corresponding to that force. In addition, we define an electric potential - the potential energy per unit charge - corresponding to the electric field.

With the concept of electric potential in hand, we can begin to understand electric circuits, starting with an investigation of a common circuit element called a capacitor. These simple devices store electrical energy and have found uses virtually everywhere, from etched circuits on a microchip to the creation of enormous bursts of power in fusion experiments.

Potential difference and electric potential

Electric potential energy and electric potential are closely related concepts. The electric potential turns out to be just the electric potential energy per unit charge. This is similar to the relationship between electric force and the electric field, which is the electric force per unit charge.

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The electric force is conservative, so the electric work depends only on the endpoints of the path. Because the electric field is conservative, the change in potential energy doesn't depend on the path. Because electric potential energy is a scalar quantity, electric potential is also a scalar quantity.

The electric potential difference is a measure of the change in electric potential energy per unit charge. Alternately, the electric potential difference is the work per unit charge that would have to be done by some force to move a charge from point A to point В in the electric field.

Released from rest, positive charges accelerate spontaneously from regions of high potential to low potential. If a positive charge is given some initial velocity in the direction of high potential, it can move in that direction, but will slow аnd finally turn around, just like a ball tossed upwards in a gravity field. Negative charges do exactly the opposite: Released from rest, they accelerate from regions of low potential toward regions of high potential. Work must be done on negative charges to make them go in the direction of lower electric potential.

Electric potential and potential energy due to point charges

In electric circuits, a point of zero electric potential is often defined by grounding (connecting to Earth) some point in the circuit. For example, if the negative terminal of a 12-V battery were connected to ground, it would be considered have a potential of zero, while the positive terminal would have a potential of + 12 V. The potential difference created by the battery, however, is only locally defined. In this Unit we describe the electric potential of a point charge which is defined throughout space.

The electric field of a point charge extends throughout space, so its electric potential does, also. The zero point of electric potential could be taken anywhere but is usually taken to be an infinite distance from the charge, far from its influence and the influence of any other charges.

The electric potential of two or more charges is obtained by applying the superposition principle: the total electric potential at some point due to several point charges is the algebraic sum of the electric potentials due to the individual charges.

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Equipotential surfaces

A surface on which all points are at the same potential is called an equipotential surface. The potential difference between any two points on an equipotential surface is zero. Hence, no work is required to move a charge at constant speed on an equipotential surface.

Equipotential surfaces have a simple relationship to the electric field: The electric field at every point of an equipotential surface is perpendicular to the surface. If the electric field E had a component parallel to the surface, that component would produce an electric force on a charge placed on the surface. This force would do work on the charge as it moved from one point to another, in contradiction to the definition of an equipotential surface.

Capacitance

A capacitor is a device used in a variety of electric circuits - for example, to tune the frequency of radio receivers, eliminate sparking in automobile ignition systems, or store short-term energy for rapid release in electronic flash units. A capacitor consists of two parallel metal plates separated by a distance d. Used in an electric circuit, the plates are connected to the positive and negative terminals of a battery or some other voltage source. When this connection is made, electrons are pulled off one of the plates, leaving it with a charge of + Q, and are transferred through the battery to the other plate, leaving it with a charge of – Q. The transfer of charge stops when the potential difference across the plates equals the potential difference of the battery. A charged capacitor is a device that stores energy that can be reclaimed when needed for a specific application.

The capacitance of a capacitor is the ratio of the magnitude of the charge on either conductor (plate) to the magnitude of the potential difference between the conductors (plates). SI Unit: farad (F) = coulomb per volt (C/V).

Energy stored in a charged capacitor

Almost everyone who works with electronic equipment has at some time verified that a capacitor can store energy. If the plates of a charged capacitor are connected by a conductor such as a wire, charge transfers from one plate to the other until the two are uncharged. The discharge can often be observed as a visible spark. If you accidentally touched the opposite plates of a charged capacitor, your fingers would act as a pathway by which the capacitor could discharge, inflicting an electric shock. The de-

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gree of shock would depend on the capacitance and voltage applied to the capacitor. Where high voltages and large quantities of charge are present, as in the power supply of a television set, such a shock can be fatal.

Capacitors with dielectrics

A dielectric is an insulating material, such as rubber, plastic, or waxed paper. When a dielectric is inserted between the plates of a capacitor, the capacitance increases. If the dielectric completely fills the space between the plates, the capacitance is multiplied by the factor к, called the dielectric constant.

1.What’s the difference between potential difference and electric potential?

2.The electric field of a point charge extends throughout space. What about its electric potential?

3.Formulate the superposition principle.

4.What’s the potential difference between any two points on equipotential surface?

5.Where do we use a capacitor?

6.Why dielectric is used in capacitor?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

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