Heinrich Lenz (1804 - 1865) - Russian famous physicist.He is one of the founders of Electromechanics.With his name is associated with the discovery of a law defining the direction of the induced current, and the law defining the electric field in the conductor.
In addition, Emil Lenz and English physicist Joel, studying the experience of the Joule effect, independently of one another opened a law under which the amount of heat that is released in a conductor is directly proportional to the square of the electric current that passes through a wire,its resistivity and the time during which the electric current is kept constant in the conductor.
This law is called the Joule - Lenz, the formula expresses it as follows:
Q = kl²Rt, (1)
where Q - the number of released heat, l - current, R - resistance of the conductor, t - time;the value of k is the thermal equivalent of work.The numerical value of this quantity depends on the choice of units in which the measurements of other variables in the formula.
If the amount of heat measured in calories, the current in amperes, the resistance in ohms, and the time in seconds, then k is numerically equal to 0.24.This means that the current in the conductor 1a selects which has a resistance of 1 ohm per second, the number of heat which is equal to 0.24 kcal.On this basis, the amount of heat in calories liberated in the conductor can be calculated using the formula:
Q = 0,24l²Rt.
in SI units are the energy, the amount of heat and work are measured units - joules.Therefore, the coefficient of proportionality in the law Joule - Lenz is one.In this system, the formula Joule - Lenz has the form:
Q = l²Rt.(2)
Joule - Lenz can be verified.By wire spiral, immersed in the liquid poured into the calorimeter, while the current is passed.Then calculated the amount of heat released in the calorimeter.Resistance spiral is known in advance, the current measured by the ammeter and time of a stopwatch.By varying the current in the circuit and using different spiral, you can check the Joule - Lenz.
Based on Ohm's law
I = U / R,
substituting the current value in the formula (2), we obtain a new expression for the formula law Joule - Lenz:
Q = (U² / R) t.
Formula Q = l²Rt convenient to use when calculating the amount of heat released in the series connection, because in this case, the electric current in the conductors of the same.Therefore, when there is a series connection of multiple conductors, each of them will be allocated that amount of heat which is proportional to the resistance of the conductor.If you connect, for example, three consecutive wires of the same size - copper, iron and nikelinovuyu, the largest amount of heat is released from nikelinovoy as the resistivity of its largest, it is stronger and more heated.If
conductors connected in parallel, the electric current in them will vary, and the voltage at the ends of such conductors is the same.The calculation of the amount of heat that will stand out in this connection, it is better to conduct, using the formula Q = (U² / R) t.
This formula shows that the parallel connection of each conductor will allocate an amount of heat which is inversely proportional to its conductivity.
If we connect three of the same thickness of wire - copper, iron and nikelinovuyu - in parallel with each other and pass through them current, the largest amount of heat released in a copper wire, it heats up and stronger than the others.
Taking as a basis for the Joule - Lenz, calculates various electric light installations, heating and heating appliances.Also widely used electric energy conversion into thermal energy.
