Electrostatics and Coulomb's Law

Even though they didn't fully understand it, ancient people knew about electricity. Thales of Miletus, a Greek philosopher known as one of the legendary Seven Wise Men, may have been the first human to study electricity, circa 600 B.C. By rubbing amber – fossilized tree resin – with fur, it was able to attract dust, feathers and other lightweight objects. These were the first experiments with electrostatics, the study of stationary electric charges or static electricity.

By the later 1700s, the scientific community was beginning to get a clearer picture of how electricity worked. Benjamin Franklin ran his famous kite experiment in 1752, proving that lightning was electrical in nature. He also presented the idea that electricity had positive and negative elements and that the flow was from positive to negative. Approximately 30 years later, a French scientist by the name of Charles Augustin de Coulomb conducted several experiments to determine the variables affecting an electrical force. His work resulted in Coulomb's law, which states that like charges repel and opposite charges attract, with a force proportional to the product of the charges and inversely proportional to the square of the distance between them.

Coulomb's law made it possible to calculate the electrostatic force between any two charged objects, but it didn't reveal the fundamental nature of those charges. What was the source of the positive and negative charges? As we'll see in the next section, scientists were able to answer that question in the 1800s.

Ex. 51. Fill the gaps with the words from the list below. Be ready to interpret the text.

magnetic field, electrons, magnetism, steam engines, nuclear fission, generator, "pressure", current, rotations, amp

The … responsible for lining up all those little bits of metal into a proper Mohawk haircut is due to the movement of…. If you allow electrons to move through a metal wire, a magnetic field will form around the wire.

We can see that there's a definite link between the phenomena of electricity and…. A generator is simply a device that moves a magnet near a wire to create a steady flow of electrons. The action that forces this movement varies greatly, ranging from … to…, but the principle remains the same.

One simple way to think about a … is to imagine it acting like a pump pushing water through a pipe. Only instead of pushing water, a generator uses a magnet to push electrons along. This is a slight oversimplification, but it paints a helpful picture of the properties at work in a generator. A water pump moves a certain number of water molecules and applies a certain amount of pressure to them. In the same way, the magnet in a generator pushes a certain number of electrons along and applies a certain amount of … to the electrons.

In an electrical circuit, the number of electrons in motion is called the amperage or …, and it's measured in amps. The "pressure" pushing the electrons along is called the voltage and is measured in volts. For instance, a generator spinning at 1,000 … per minute might produce 1 amp at 6 volts. The 1 amp is the number of electrons moving (1 … physically means that 6.24 x 1018 electrons move through a wire every second), and the voltage is the amount of pressure behind those electrons.

Ex. 52. Find the definitions for the words from the list below. Be ready to interpret the definitions.

alternating current, circuit breaker, coulomb, cycles-per-second, diode, electrolyte, frequency, Joule’s law, Ohm’s Law, reactive power, three-phase

1. an electrical current which reverses direction repeatedly due to a change in voltage which occurs at the same frequency. Often abbreviated AC or ac.

2. a device which can stop the flow of electricity around a circuit by switching itself off if anything goes wrong.

3. a unit of electric charge, equal to the quantity of electricity conveyed in one second by a current of one ampere.

4. a measure of the frequency in an ac electric system. Abbreviated cps or cycles. Now replaced with the unit Hertz.

5. an electronic semiconductor device that predominantly allows current to flow in only one direction.

6. a nonmetallic conductor of electricity usually consisting of a liquid or paste in which the flow of electricity is by ions.

7. the number of complete alternations or cycles per second of an alternating current. It is measured in Hertz. The standard frequency in the US is 60 Hz. However, in some other countries the standard is 50 Hz.

8. the law which defines the relationship between current in a wire and the thermal energy produced. In 1841an English physicist James P. Joule experimentally showed that W = I2 x R x t where I is the current in the wire in amperes, R is the resistance of the wire in Ohms, t is the length of time that the current flows in seconds, and W is the energy produced in Joules.

9. the law which defines the relationship between voltage, resistance, and current. In 1828 the German physicist George Simon Ohm showed by experiment that the current in a conductor is equal to the difference of potential between any two points divided by the resistance between them. This may be written as I = E / R where E is the potential difference in volts, R is the resistance in Ohms, and I is the current in amperes.

10. the mathematical product of voltage and current consumed by reactive loads. Examples of reactive loads include capacitors and inductors. These types of loads when connected to an ac voltage source will draw current, but since the current is 90o out of phase with the applied voltage they actually consume no real power in the ideal sense.

11. an ac electric system or load consisting of three conductors energized by alternating voltages that are out of phase by one third of a cycle. This type of system has advantages over single-phase including the ability to deliver greater power using the same ampacity conductors and the fact that it provides a constant power throughout each cycle rather than a pulsating power, as in single-phase. Large power installations are three-phase.

Ex. 53. Fill the gaps with the words from the list below. Be ready to interpret the text.

"pressure", wall outlet, wattage, resistance, ohms, Ohm's law, equation, alternating

As mentioned earlier, the number of electrons in motion in a circuit is called the current, and it's measured in amps. The … pushing the electrons along is called the voltage and is measured in volts. If you know the amps and volts involved, you can determine the amount of electricity consumed, which we typically measure in watt-hours or kilowatt-hours. Imagine that you plug a space heater into a wall outlet. You measure the amount of current flowing from the … to the heater, and it comes out to 10 amps. That means that it is a 1,200-watt heater. If you multiply the volts by the amps, you get the …. In this case, 120 volts multiplied by 10 amps equals 1,200 watts. This holds true for any electrical appliance. If you plug in a light and it draws half an amp, it's a 60-watt light bulb.

Now let's add one more factor to current and voltage: …, which is measured in …. We can extend the water analogy to understand resistance, too. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate and the resistance is like the pipe size.

A basic electrical engineering equation called … spells out how the three terms relate. Current is equal to the voltage divided by the resistance. It's written like this:

I = V/R

where I stands for current (measured in amps), V is voltage (measured in volts) and R symbolizes resistance (measured in ohms).

Let's say you have a 120-watt light bulb plugged into a wall socket. The voltage is 120 volts, and a 120-watt bulb has 1 amp flowing through it. You can calculate the resistance of the filament by rearranging the…:

R = V/I

So the resistance is 120 ohms.

Beyond these core electrical concepts, there is a practical distinction between the two varieties of current. Some current is direct, and some current is … – and this is a very important distinction.

Ex. 54. Translate the following text into English.

Закон Ома – це основний закон електротехніки, який застосовується для розрахунку таких величин, як: струм, напруга і опір в електричному ланцюзі.

Електричний струм, тобто потік електронів, виникає в ланцюзі між двома крапками з різними потенціалами. Тоді слід вважати, що чим більше різниця потенціалів, тим більша кількість електронів переміщаэться з крапки з низьким потенціалом (Б) в крапку з високим потенціалом (А). Кількісно струм виражається сумою зарядів, який проходить через задану крапку і збільшення різниці потенціалів, тобто прикладеної напруги до резистора R, приведе до збільшення струму через резистор.

За допомогою закону Ома для ділянки ланцюга можна обчислити прикладену напругу до ділянки ланцюга, або напруга на вхідних затисках ланцюга.