Pretty hard to imagine modern life without electricity. It has become one of the most valuable attributes of a modern existence. In fact, anyone who has ever worked with electricity, knows that the passage of wires for the current they have a tendency to heat up. Why does it depend?

What is current

Current is an ordered motion of charged particles, called electrons. And if current flows through a conductor, it is starting to happen, different physical processes, namely, the electrons collide with the molecules.

The molecules are neutral or those who have lost their negatively charged particle. As a result of collisions or electrons can be a neutral molecule, or in this case knocked out of another same molecule electron, formed positively charged ion. During these clashes consumed the kinetic energy of charged particles. It is this energy and becomes warm.

Thermal heating conductor can also affect the resistance. For example, you can take a body and drag it along the ground. The land in this case is the resistance. What do you do with him? Right, between body and surface will the force of friction, which in turn heats the body. The current in this case behaves the same way.The law of Joule: the history of


And, listening to all of the above, scientists were able to determine the relationship between current, resistance and quantity of heat. This relationship is called the law of Joule, the formula of which is known to all physicists. In the years 1832-1833 Russian physicist Emilia, by Lenz, it was discovered that the thermal effects on metallic conductors, their conductivity major change. It actually complicated the work of the scientist and his ability to count electric circuit.

Meanwhile, a young scientist came up with the idea that perhaps there is some correlation between the current strength and temperature of the conductor. But how can that be? At that time no precise electric devices which can measure current, resistance, didn't even have a stable source of EMF. Lenz did not stop, he decided to spend the experience.

The experiments of the Russian physicist

The essence of this experience was so simple, like all brilliant, that he can repeat even a schoolboy. The scientist has constructed a special device, which was used for measuring the amount of heat generated by the conductor. This device was an ordinary vessel, inside of which Lenz was filled with a solution of diluted alcohol and set conductor is a platinum wire, which is fed electrical current.

After the device was created, the scientist began to conduct experiments. He measured the exact amount of time necessary for the alcohol in the vessel was heated to about 10 C. there have been a lot of not only months, but years. In 1843, 10 years later, was published the law, which essence consisted in that the heating conductor current proportional to the square of employee for the heating current.

Joule and Lenz

But there it was! It turns out that a few years ago, the English physicist James Prescott Joule conducted similar experiments, and have already published their observations. How to be? Lenz gave up and carefully studied the work of Joule and came to the conclusion that, even if they conducted the same experiments Lentz was much more accurate. In this connection, the scientific community has added to the work of Joule and Lenz amended this act became known as the law of Joule. Mathematical formulation of the law is as follows:

  • I – current strength, And
  • U – voltage, V
  • t – the time that the current spends on the passage of the conductor C.

The law is: the amount of thermal energy released in a conductor through which electric current flows, equal to the product of current, voltage and time of current flow through a conductor.

However, whether this is always true statement? You can try to bring him out, using Ohm's law. According to him U = I*R, where R is the resistance in Ohms.

Given Ohm's law, we can substitute the value in the formula Q = I*U*t = I 2 *R*t. From this we can conclude that the amount of heat depends on the resistance of the conductor. Also for law of Joule will be true this statement is: I = Q = I*U*t.

All three formulas are true, but Q = I 2 *R*t will be true for all situations. The other two are also correct, however, in certain circumstances.


Now about the guides. Initially, in his experiments, Joule, and Lenz used a platinum wire as mentioned above. In all similar experiments, the scientists of that time used mainly metal conductors, since they were fairly inexpensive and stable. Not surprising, because still metal conductors is the primary type of conductors, in connection with what was initially believed that the law of Joule is applicable only to them. However, later it was discovered that this law is applicable not only to metallic conductors. It holds true for all of them. The conductors themselves according to classification can be divided into:

  • Metallic (copper, iron, silver, etc.). The main role in them is played by negatively charged particles (electrons) that flow through the conductor.
  • Liquid. They also for the movement of charges answer ions are atoms, in which either too many or too few electrons.
  • Gaseous. Unlike their colleagues in such conductors the current is determined by the movement of ions and electrons.

And despite the differences, in any case, if you increase the amperage or the resistance will increase and the amount of heat.

Application of the law by other physicists

The discovery of the law of Joule promised great prospects. In fact, this law helped to create a kind of different electric heaters and elements. For example, a little later, after the discovery of the law scientists noticed that by heating certain elements they begin to glow. They wanted to experiment with them using different conductors, and in 1874 the Russian engineer Alexander Lodygin invented the modern incandescent lamp, the filament which was made of tungsten.

Apply the law of Joule and electrical engineering – for example, when you create a fuse. The fuse is a part of the circuit, the design of which is made so that during the flow it current higher than the permissible value (for example, if a short circuit) it overheats, melts and opens the power circuit. Even an ordinary electric kettle or microwave, which is, in fact, everyone operates under this law.


Quite difficult to determine the contribution of these scientists in modern electronics and electrical engineering, but one thing is certain – the emergence of the law of Joule turned people's ideas about electricity and gave more specific knowledge about what is electric field in the conductor.

Without a doubt, outdoor these great scientists-physicists of the act was the decisive step in all of science, it is through this opening was subsequently committed other more or less tremendous achievements of other scientists. All science is a close intertwining of discoveries, some solved and unsolved problems. Discussed in this article act in a certain way has influenced many studies and left quite an indelible and distinctive mark in science.