Материал: Андрианова English Thesaurus of Student-Physicist 2013

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

capacitance ,electric potential energy, charged conductor, equipotentials, electric potential

1._____ differs significantly from gravitational potential energy, however, in that there are two kinds of electrical charge - positive and negative - whereas gravity has only positive "gravitational charge" (i.e. mass).

2._____ is characteristic the field only, independent of a test charge that may be placed in that field.

3.All points on the surface of a _____ in electrostatic equilibrium are at the same potential.

4.The _____ of a point charge are a family of spheres centre on the point charge.

5.The equivalent _____ of a parallel combination of capacitors is greater than any of the individual capacitances.

UNIT 17

CURRENT AND RESISTANCE

Read the following texts. Study the thesaurus on fig. 17, answer the questions after the texts

Many practical applications and devices are based on the principles of static electricity, but electricity was destined to become an inseparable part of our daily lives when scientists learned how to produce a continuous flow of charge for relatively long periods of time using batteries. The battery or voltaic cell was invented in 1800 by the Italian physicist Alessandro Volta. Batteries supplied a continuous flow of charge at low potential, in contrast to earlier electrostatic devices that produced a tiny flow of charge at high potential for brief periods. This steady source of electric current allowed scientists to perform experiments to learn how to control the flow of electric charges in circuits. Today, electric currents power our lights, radios, television sets, air conditioners, computers, and refrigerators. They ignite the gasoline in automobile engines, travel through miniature compo-

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nents making up the chips of microcomputers, and provide the power for countless other invaluable tasks.

Electric current

If charges move in a direction perpendicular to a surface, the current is the rate at which charge flows through this surface. One ampere of current is equivalent to one coulomb of charge passing through the crosssectional area in a time interval of 1 s. The direction of conventional current used in this book is the direction positive charges flow. (This historical convention originated about 200 years ago, when the ideas of positive and negative charges were introduced.) In a common conductor such as copper, the current is due to the motion of negatively charged electrons, so the direction of the current is opposite the direction of motion of the electrons. On the other hand, for a beam of positively-charged protons in an accelerator, the current is in the same direction as the motion of the protons. In some cases - gases and electrolytes, for example - the current is the result of the flows of both positive and negative charges. Moving charges, whether positive or negative, are referred to as charge carriers. In a metal, for example, the charge carriers are electrons.

In electrostatics, where charges are stationary, the electric potential is the same everywhere in a conductor. This is no longer true for conductors carrying current: as charges move along a wire, the electric potential is continually decreasing (except in the special case of superconductors). The phrases flow of current and current flow are commonly used, but here the word flow is redundant because current is already defined as a flow (of charge). Avoid this construction!

Current and drift speed

Macroscopic currents can be related to the motion of the microscopic charge carriers making up the current. It turns out that current depends on the average speed of the charge carriers in the direction of the current, the number of charge carriers per unit volume, and the size of the charge carried by each.

If the carriers move with a constant average speed, it called the drift speed. To understand the meaning of drift speed, consider a conductor in which the charge carriers are free electrons. If the conductor is isolated, these electrons undergo random motion similar to the motion of the molecules of a gas. The drift speed is normally much smaller than the free elec-

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trons' average speed between collisions with the fixed atoms of the conductor. When a potential difference is applied between the ends of the conductor (say, with a battery), an electric field is set up in the conductor, creating an electric force on the electrons and hence a current. In reality, the electrons don't simply move in straight lines along the conductor. Instead, they undergo repeated collisions with the atoms of the metal, and the result is a complicated zigzag motion with only a small average drift speed along the wire. The energy transferred from the electrons to the metal atoms during a collision increases the vibrational energy of the atoms and causes a corresponding increase in the temperature of the conductor.

Current and voltage measurements in circuits

To study electric current in circuits, we need to understand how to measure currents and voltages. The circuit consists of only a battery and a light-bulb. The word "circuit" means "a closed loop" of some sort around which current circulates. The battery pumps charge through the bulb and around the loop. No charge would flow without a complete conducting path from the positive terminal of the battery into one side of the bulb, out the other side, and through the copper conducting wires back to the negative terminal of the battery. To measure the current in the bulb, we place an ammeter, the device for measuring current, in line with the bulb so there is no path for the current to bypass the meter; all of the charge passing through the bulb must also pass through the ammeter. The voltmeter measures the potential difference, or voltage, between the two ends of the bulb's filament.

Resistance and Ohm's law

When a voltage (potential difference) is applied across the ends of a metallic conductor the current in the conductor is found to be proportional to the applied voltage. The proportionality constant R is called the resistance of the conductor. Resistance has SI units of volts per ampere, called ohms.

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The concepts of electric current, voltage, and resistance can be compared with the flow of water in a river. As water flows downhill in a river of constant width and depth, the flow rate (water current) depends on the steepness of descent of the river and the effects of rocks, the riverbank, and other obstructions. The voltage difference is analogous to the steepness, and the resistance to the obstructions. Based on this analogy, it seems reasonable that increasing the voltage applied to a circuit should increase the current in the circuit, just as increasing the steepness of descent increases the water current. Also, increasing the obstructions in the river's path will reduce the water current, just as increasing the resistance in a circuit will lower the electric current. Resistance in a circuit arises due to collisions between the electrons carrying the current with fixed atoms inside the conductor. For many materials, including most metals, experiments show that the resistance remains constant over a wide range of applied voltages or currents. This statement is known as Ohm's law.

Ohm's law is an empirical relationship valid only for certain materials. Materials that obey Ohm's law, and hence have a constant resistance over a wide range of voltages, are said to be ohmic. Materials having resistance that changes with voltage or current are nonohmic.

Electrons don't move in straight-line paths through a conductor. Instead, they undergo repeated collisions with the metal atoms. The resistance of an ohmic conductor increases with length, which makes sense because the electrons going through it must undergo more collisions in a longer conductor. A smaller cross-sectional area also increases the resistance of a conductor, just as a smaller pipe slows the fluid moving through it. The resistance is proportional to the conductor's length and inversely proportional to its cross-sectional area, the constant of proportionality is called the resistivity. The resistivity ρ, and hence the resistance, of a conductor depends on a number of factors. One of the most important is the temperature of the metal. For most metals, resistivity increases with increasing temperature.

There is a class of metals and compounds with resistances that fall virtually to zero below a certain temperature called the critical temperature. These materials are known as superconductors.

Electrical energy and power

If a battery is used to establish an electric current in a conductor, chemical energy stored in the battery is continuously transformed into kinetic energy of the charge carriers. This kinetic energy is quickly lost as

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