Chapter 2. Energy, power and voltage ( энергия, мощность и напряжение)

Exercise 1. Read and translate the text, using the words below.

We are going to use the hydraulic analogy again. It takes energy to pump water uphill, and the potential energy of being at a high elevation is released when it descends to a lower elevation. The amount of energy (in joules) is mgh, where m is the mass of liquid (say, in kilograms), g is the strength of gravity (9.8 meters per second squared, or 9.8 newtons per kilogram), and h is the elevation distance. Some elementary physics: The SI unit of energy is the joule, and is one newton-meter, and is the energy required to push something with a force of one newton through a distance of one meter. The unit of force is the newton, and is the force required to increase the speed of a one kilogram object by one meter per second per second. The unit of rate of flow of energy is the watt, and is one joule per second. Note that the watt relates to all of physics, not just electricity.

For a given difference in elevation h, it takes gh joules of energy per kilogram to lift water (or any other liquid) through that height. That same amount of energy is released when the water flows down by that height. It might drop directly—old-fashioned water wheels use this method, or the water might be stored in a reservoir, causing the water to have enormous pressure at the bottom. That pressure gives the water a lot of energy per kilogram, which can be recovered by turbines—modern hydroelectric power stations do this. The amount of pressure at the bottom of a column of water of height h is enough to release gh joules per kilogram.

The electrical analogue of this pressure is voltage. The unit is the volt, named after Alessandro Volta. It is the amount of "pressure" that releases one joule per coulomb of charge. A given region of space (or, more commonly in practice, a given point in an electrical circuit) has a certain voltage potential. If two points differ in potential by V volts, then the passage of Q coulombs of charge from one point to the other releases QV joules of energy. This "potential difference" or "voltage difference" is analogous to the difference in height (and hence difference in pressure) between water at the top of a reservoir and water at the bottom.

In fact, some very old books refer to voltage as "electrical pressure". Another term for voltage that one sees in older books is "electro-motive force" or its abbreviation "emf". It's just voltage.

There is another useful analogy to water. The level of water on a pond is uniform, that is, the surface is level. This is because the water can travel very easily from one point to another. If the surface were higher at one point than at another, the water would simply flow from the higher point to the lower one, until equilibrium is reached. Similarly, in a conductor (e.g. a wire), electrons can flow very easily from one point to another, so there is essentially no voltage difference. If there were, the electrons in the wire would simply flow from the higher voltage to the lower voltage (well, actually, they would flow the other way because their charge is negative) until the voltage is equalized. Hence, all points in an electric circuit that are connected by a wire are at the same voltage.

This only applies to situations in which the water or the charges can move easily. In a river, there is some resistance to the motion, so the water is constantly moving from higher elevations to lower elevations. Not all conductors are perfect, so there is actually a slight voltage drop (and dissipation of power) across a conductor. How great this effect is depends on the resistance, in accordance with Ohm's law, which we will discuss presently.

Words to be remembered.

pump–насос; подавать насосом, качать

to descend–спускаться; сходить

elevation–возвышение, возвышенность; высота

to release–освобождать; высвобождать; избавлять; выпускать; отпускать

uniform–однообразный; однородный

equilibrium–равновесие

to equalize–уравнивать

dissipation–1. диссипация, рассеяние 2. рассеивание

reciprocal–взаимный; обоюдный; эквивалентный

flippant–легкомысленный, ветреный

oscillation–1. колебание 2. вибрация 3. генерация

Exercise 2. Answer the following questions.

1. What is the amount of energy?

2. Does watt relate to all of physics or just to electricity?

3. What method do old-fashioned water-wheels use?

4. What do modern hydroelectric power stations do?

5. After whom was the unit of volt named?

6. What is analogous to the difference in height between water at the top of a reservoir and water at the bottom?

7. What is the term for voltage that one sees in older books?

8. Is there any voltage difference in a conductor?

9. What would happen if there were any voltage difference in a conductor?

10. What depends on the resistance, in accordance with Ohm`s law?

Exercise 3. Translate the following word combinations and use them in the sentences of your own.

High elevation, water wheels, resistance to the motion, voltage drop, dissipation of power, amount of energy, strength of gravity, useful analogy, frequency of an oscillation, flippant term.

Exercise 4. Translate the following sentences from Russian into English.

1. Сила измеряется ньютонами, то есть это сила, требуемая для увеличения скорости объекта весом в один килограмм на один метр в секунду. 2. Это давление даёт воде много энергии на килограмм. 3. Электрическим аналогом этого давления является напряжение. 4. Данная точка в электрической цепи имеет определённый потенциал напряжения. 5.Другой термин для напряжения, который можно увидеть в старых книгах – это электродвижущая сила или ЭДС. 6. Вода может очень легко перетекать из одной точки к другой. 7. Если бы была разница напряжения, то электроны в проводе просто передвигались бы от более высокого напряжения к более низкому. 8. Все точки в электрической цепи, соединённые проводом, имеют одинаковое напряжение. 9. Не все проводники совершенны. 10. То, насколько велик этот эффект зависит от сопротивления, согласно закону Ома.

Exercise 5. Substitute the Russian words for the English ones.

1. Единица of force is the newton, and is the force required to increase the speed of a one kilogram object by one meter per second.

a) the use

b) the one

c) the unit

2. Note that the watt относится to all of physics, not just electricity.

a) relates

b) consumes

c) corresponds

3. The electrical analogue of this давление is voltage.

a) tension

b) pressure

c) intention

4. A given region of пространство has a certain voltage potential.

a) space

b) room

c) region

5. The level of water on a pond is однородный, that is, the surface is level.

a) unique

b) united

c) uniform

6. If the surface were higher at one point than at another, the water would simply flow from the higher point to the lower one, until равновесие is reached.

a) equipoise

b) equilibrium

c) equation

7. In a river, there is some resistance to the движение, so the water is constantly moving from higher elevations to lower elevations.

a) motion

b) mobility

c) direction

8. How great this effect is depends on the сопротивление, in accordance with Ohm`s law.

a) insulation

b) conductivity

c) resistance

Exercise 6. Retell the text, using as many new words and word expressions as possible.