Emiliy Khristianovich Lenz (1804 - 1865) - Russian famous physicist. He is one of the founders of electromechanics. His name is associated with the discovery of a law that determines the direction of the induction current, and a law that determines the electric field in a conductor with current.
In addition, Emilius Lenz and the English physicist Joule, studying the thermal effects of current experimentally , independently discovered the law according to which the amount of heat released in the conductor will be directly proportional to the square of the electric current that passes through the conductor, its resistance and the time during which the electric current is kept constant in the conductor.
This law is called the law of the Joule-Lenz, its formula expresses as follows:
Q = kl²Rt, (1)
where Q is the amount of heat released, l is the current, R is the conductor resistance, t is time; k is called the heat equivalent of work. The numerical value of this quantity depends on the choice of units in which the measurements of the remaining quantities included in the formula are made.
If the amount of heat is measured in calories, current in amperes, resistance in Ohms, and time in seconds, then k is numerically equal to 0.24. This means that the current in 1A releases in the conductor, which has a resistance of 1 ohm, in one second the number of heat, which is 0.24 kcal. Based on this, the amount of heat in calories released in the conductor can be calculated by the formula:
Q = 0.24l²Rt.
In the SI system of units, energy, amount of heat and work are measured in units - joules. Therefore, the proportionality coefficient in the Joule-Lenz law is equal to unity. In this system, the Joule-Lenz formula has the form:
Q = l²Rt. (2)
The Joule-Lenz law can be tested experimentally. A current is passed through a wire spiral immersed in a liquid poured into a calorimeter. Then, the amount of heat released in the calorimeter is calculated. The resistance of the spiral is known in advance, the current is measured by an ammeter and time by a stopwatch. By changing the current in the circuit and using various spirals, you can check the Joule-Lenz law.
Based on Ohm's Law
I = U / R,
Substituting the current value into formula (2), we obtain a new expression for the formula for the Joule – Lenz law:
Q = (U² / R) t.
It is convenient to use the formula Q = l²Rt when calculating the amount of heat generated during a series connection, because in this case the electric current is the same in all conductors. Therefore, when several conductors are connected in series, each of them will produce a quantity of heat that is proportional to the resistance of the conductor. If you connect, for example, three wires of the same size in series - copper, iron and nickel, then the greatest amount of heat will be released from nickel, since its resistivity is the largest, it is stronger and heats up.
If the conductors are connected in parallel, then the electric current in them will be different, and the voltage at the ends of such conductors is the same. The calculation of the amount of heat that will be released during such a connection is best done using the formula Q = (U² / R) t.
This formula shows that with a parallel connection, each conductor will emit such an amount of heat that will be inversely proportional to its conductivity.
If you connect three equal thicknesses of wire - copper, iron and nickel - in parallel with each other and pass a current through them, then the greatest amount of heat will be released in the copper wire, it will heat more strongly than the others.
Based on the Joule-Lenz law, various electric lighting installations, heating and heating appliances are calculated. The conversion of electricity to heat energy is also widely used.