# Advice 1: How to measure the resistance of the resistor

The resistor is one of the main elements of any electrical circuit. Its main purpose is to create a certain resistance. Resistance can be measured with special devices or to identify a special marking on the body of the resistor.
You will need
• tester;
• calculator;
• table of markings.
Instruction
1
Take a tester who can work in ohmmeter mode. Connect it to the contacts of the resistor and perform measurement. Since the resistance of the resistors is very different, adjust the sensitivity of the instrument. If the tester can only measure current and resistance, take the current source and assemble electric circuit including a resistor. When connecting the chain, be sure to monitor the current which passes through it, so as not to cause a short circuit. After the change of current in amperes, switch the tester to measure voltage. Connect it in parallel with the resistor and take readings in volts. Then find the resistance of the resistorby dividing the voltage value U and the current I (R=U/I). If you are using a constant current source, connecting devices
2
If the resistor marked to find its resistance, without resorting to additional operations. Resistors are marked or digits or combination of numbers with letters, or a set of color bars.
3
If the resistor is indicated with three digits, the first two numbers identify tens and ones number, and the third digit is the degree 10 to which it should be raised to obtain the correct values. For example, if the resistor numbered 482, it means that its resistance is equal to 48∙102=4800 Ohms.
4
When the resistor marking SMD, the first two figures are taken as the coefficient, and the letter corresponds to the degree 10 to which it needs to multiply. All the values of the coefficients and the letters get in the table of marking of the SMD resistors EIA. The resistor may be the fourth letter of its accuracy. For example, if a resistor is marked 21ВF, its resistance will be equal to 162∙10=1620 Ohms ±1%.
5
If the drawn resistor color bands, use the table to determine the resistance of the resistor by the colored marking. The first three labels correspond to the numbers that make up a coefficient, and the fourth – power of 10 by which to multiply the resulting factor.

# Advice 2: What is measured by the resistance

If included in the electrical circuit of a single current source to different conductors and the ammeter can be observed that the readings of the ammeter when different conductors are different. This is due to the electric resistance of the portion, from which, like voltage, depends on current.

## Resistance as a physical quantity

The electrical resistance of a conductor is a physical quantity, denoted by the letter R. the unit of resistance adopted 1 Ohm – the resistance of the conductor in which the current strength is 1 ampere at a voltage of 1 volt at the ends. Briefly write down this formula:

1 Ohm=1V/1A.

The units of resistance can be and multiple. So 1 milliohm (mω) – is a 0.001 Ohm, 1 kilo Ohm (kω) 1000 Ohms, 1 Megohm (MW) Is 1,000,000 Ohms.

## What is the cause of electrical resistance in conductors

If orderly moving in a conductor the electrons have not experienced any obstacles in their path, they could move by inertia indefinitely. But in reality this does not happen, because the electrons interact with the ions arranged in the crystal lattice of the metal. Their movement from this slows down and in 1 second through a cross section of the conductor is smaller number of charged particles. Therefore, the charge carried by the electrons in 1 second decreases, i.e., decreases the current strength. Thus, every conductor as though resists moving in this current, opposing him.
The reason for the resistance – collision of moving electrons with the ions of the crystal lattice.

## What is expressed in Ohm's law for subcircuit

In any electrical circuit, a physicist has to deal with three physical quantities – current, voltage and resistance. These values do not exist separately in themselves, but are linked by a certain ratio. Experiments show that the strength of the current in the circuit is directly proportional to the voltage at the ends of this area and inversely proportional to the resistance of the conductor. This is Ohm's law, outdoor German scientist Georg Ohm in 1827:

I=U/R,

where I is the amperage on the circuit, U is the voltage applied to the ends of the plot, R is the phase resistance.

Ohm's law is one of the fundamental laws of physics. Knowing the resistance and current, you can calculate the voltage circuit (U=IR), and knowing the amperage and voltage, you can calculate the phase resistance (R=U/I).
The resistance depends on the length of the conductor, the cross sectional area and the nature of the material. The least resistance is typical of silver and copper and ebonite and porcelain almost do not conduct electricity.

It is important to understand that the resistance of the conductor, expressed from Ohm's law formula R=U/I, is a constant. It does not depend on current or voltage. If the voltage at this area will increase several times, at the same time increase the strength of the current, and their ratio will remain unchanged.
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