# Advice 1: The law of Joule: definition, practical significance

The law of Joule opened in 1841 and 1842, two scientists James Prescott Joule and Emilia, by Lenz. Lenz published the results of his work in 1842, a year later than the Joule, but his experiments were more accurate and the conclusion from experiments he brought earlier.

## The Law Of Joule

The law of Joule determines the amount of heat released in the conductor, has a resistance for time t, during the passage of electric current through it.

Q = a*I*2R*t, where
Q - number of the group is allocated to heat (in Joules)
a - coefficient of proportionality
I - current ( Amperes)
R - conductor Resistance (in ohms)
t - the transit Time (in seconds)

The law of Joule-Lenz explains that the electric current is electric charge that moves under the influence of an electric field. The field does work, and the current has power and energy. When this energy passes through a stationary metallic conductor, it becomes heat, towards the heating conductor.

In differential form the law of Joule expressed as a volumetric heat capacity density current in the conductor is equal to the product of specific electrical conductivity to the square of the electric field.

## The application of the law of Joule

Incandescent lamp was invented in 1873 by a Russian engineer Lodygin. In incandescent lamps, as in electric heating devices, applies the law of Joule. They use a heating element which is a conductor with high resistance. Due to this element it is possible to achieve localized heat in the area. The heat will appear when the increase of resistance, increase the length of the conductor, the choice of a particular alloy.

One of the areas of application of the law of Joule is the reduction of energy losses.
Heating effect of current leads to energy loss. When transmission output power is linearly dependent on voltage and amperage, and the power of heat depends on current squared, so if you raise the voltage, thus lowering the strength of the current before the electricity supply, it will be more profitable. But the increase in voltage leads to a decrease in electrical safety. To improve safety, increase load resistance, respectively, increase of voltage.

Also, the law of Joule affects the choice of cables for circuits. With the wrong selection of wires possible strong heating of the conductor, as well as his fire. This occurs when the current exceeds the maximum permissible value and stands out too much energy. With proper selection of cables for electrical circuits is to follow the regulations.

# Advice 2: How does the current from the voltage

Between amperage and voltage there is directly proportional dependence described by Ohm's law. This law defines the relationship of current, voltage and resistance at the area of electrical circuits.
Instruction
1
Remember, what is current and voltage.
Electric current — is orderly for charged particles (electrons). For the quantitative determination in physics, we use the index I, called current.
- The voltage U is the potential difference at the ends of an electrical circuit. It is this difference that makes the electrons move, like a stream of liquid.
2
Current is measured in amperes. In electrical circuits the current is determined by ammeter. The unit of voltage is volts, measure the voltage using the voltmeter. Assemble a simple electrical circuit from a power source, a resistor, ammeter and voltmeter.
3
On closing the circuit and passing current through it, write down the readings. Change the voltage at the ends of the resistance. You will see that the ammeter reading will increase as the voltage increases and Vice versa. This experience demonstrates a directly proportional relationship between current and voltage.
4
Electric current is like the flow of the fluid. But charged particles moving in empty pipe and in the conductor. Conductor material has a significant impact on the character of this movement. To quantify this effect, the value of R is the resistance of an electric circuit. The unit of resistance is Ohm.
5
The amperage the more voltage and less than the resistance of the chain. This dependence is described by Ohm's law:
I = U/R.
6
For alternating current is maintained directly proportional dependence on voltage. Alternating current is an electromagnetic oscillation of a harmonic (sinusoidal) with a frequency determined by the voltage source. To complete an electrical circuit with a different resistance relationship between current and voltage is described by Ohm's law.
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