Ohm's law

The basic law of current flow is Ohm's law, named for its discoverer, the German physicist Georg Ohm. Ohm's law states that the amount of current flowing in a circuit made up of pure resistances is directly proportional to the electromotive force impressed on the circuit and inversely proportional to the total resistance of the circuit.
The law is usually expressed by the formula I = V / R, where I is the current in amperes, V or ( E ) is the electromotive force in volts, and R is the resistance in ohms (see Electrical Units). Ohm's law applies to all electric circuits for both direct current (DC) and alternating current (AC), but additional principles must be invoked for the analysis of complex circuits and for AC circuits also involving inductances and capacitances.

Now take a look to a simple circuit :

fig.1 - Ohm's Law

A potential difference of 10 V (volts) is needed to make a current of 2A (amps) flow through a conductor with a resistance of 5 ohms.

Kirchhoff's Laws

If a circuit has a number of interconnected branches, two other laws are applied in order to find the current flowing in the various branches. These laws, discovered by the German physicist Gustav Robert Kirchhoff, are known as Kirchhoff's laws of networks.

First Kirchhoff Law

fig.2 - The first Kirchhoff Law

The first of Kirchhoff's laws states that at any junction in a circuit through which a steady current is flowing, the sum of the currents flowing to the point is equal to the sum of the currents flowing away from that point.

Now take a look to the equation in each point of the circuit :

• A: I2 =I1 + I3
• B: I6 = I2 + I4
• C: I6 = I 3 + I 5
• D: I5 = I1 + I4

 

Second Kirchhoff Law

fig.3 - The second Kirchhoff Law

The second law states that, starting at any point in a network and following any closed path back to the starting point, the net sum of the electromotive forces encountered will be equal to the net sum of the products of the resistances encountered and the currents flowing through them. This second law is simply an extension of Ohm's law.

Now take a look to the equation for each path :

Starting B point :

• E3 = R2 * I6

Starting D point :

• E2 = R1 * I5

Starting A point:

• E1 =R3 * I4

Measuring Voltage

fig.4 - Measuring Voltage

The instrument most generally used to measure potential difference, or voltage, is a galvanometer with a high resistance in series with the coil. When such a meter is connected across a battery, or to two points in an electrical circuit between which a potential difference exists, a small current (limited by the series resistor) will pass through the meter. The current is proportional to the voltage, and the latter quantity can be measured if the galvanometer is calibrated appropriately. By using the proper values of series resistors, one galvanometer can be used to measure a large range of voltages. The most accurate instrument for the determination of voltage, resistance, or direct current is the potentiometer, which indicates an unknown electromotive force by comparing it with a known value.

For the measurement of AC voltages, AC meters having high internal resistance, or similar meters with high series resistance, are employed.
Other methods for measuring the value of voltages depend on vacuum tubes and electronic circuits (see Electronics) and are especially useful in measurements at high frequencies. One such device is the vacuum-tube voltmeter. In the simplest form of this meter an AC voltage is rectified by a diode tube, and the rectified current is measured by an ordinary galvanometer. Other such voltmeters employ the amplifying characteristics of electronic tubes to measure extremely small voltages. The cathode-ray oscilloscope can also be used for voltage measurements because the deflection of the electron beam is proportional to the voltage impressed on the deflection plates or coils.

Measuring Currents

fig.5 - Measuring Current

Galvanometers are the main instruments used to detect and measure current. They depend on the fact that force is generated by an electric current flowing in a magnetic field. The mechanism of the galvanometer is so arranged that a small permanent magnet or electromagnet sets up a magnetic field that generates a force when current flows in a wire coil adjacent to the magnet. Either the magnet or the adjacent coil may be movable. The force deflects the movable member by an amount proportional to the strength of the current. The movable member may have a pointer or some other device to enable the amount of deflection to be read on a calibrated scale.

In the D'Arsonval galvanometer, a small mirror attached to a movable coil reflects a beam of light on a scale about 1 m (about 3 ft) away from the instrument. This arrangement involves less inertia and friction than does a pointer, and consequently, greater accuracy is achieved. The instrument is named after the French biologist and physicist Jacques d'Arsonval, who devised the first reflecting galvanometer. He also conducted experiments with the mechanical equivalent of heat and in the high-frequency oscillating current of low voltage and high amperage, D'Arsonval current, used in the treatment of certain diseases (diathermy treatment). The addition of a scale and proper calibration converts a galvanometer into an ammeter, the instrument used for measuring electric current in amperes; D'Arsonval was also responsible for inventing a direct-current (DC) ammeter.
Only a limited amount of current can be passed through the fine wire of a galvanometer coil. When large currents must be measured, a shunt of low resistance is attached across the terminals of the meter. Most of the current is bypassed through this shunt resistance, but the small current flowing through the meter is still proportional to the total current. By taking advantage of this proportionality, a galvanometer can be used to measure currents of hundreds of amperes. Galvanometers are usually named according to the magnitude of the currents they will measure. A microammeter is calibrated in millionths of an ampere and a milliammeter in thousandths of an ampere.
Ordinary galvanometers cannot be used for the measurement of an alternating current (AC), because the alternation of the current would produce deflection in both directions. An adaptation of the galvanometer, however, called an electrodynamometer, can be used to measure alternating currents by means of electromagnetic deflection. In this meter a fixed coil, in series with the moving coil, is employed in place of the permanent magnet of the galvanometer. Because the current in the fixed and moving coils reverses at the same instant, the deflection of the moving coil is always in the same direction, and the meter gives a constant current reading. Meters of this type can also be used to measure direct currents. Another form of electromagnetic meter is the iron-vane meter or soft-iron meter. In this device two vanes of soft iron, one fixed and one pivoted, are placed between the poles of a long, cylindrical coil through which is passed the current to be measured. The current induces magnetism in the two vanes, causing the same deflection no matter what the direction of the current. The amount of the current is ascertained by measuring the deflection of the moving vane.
Meters that depend on the heating effect of an electric current are used to measure alternating current of high frequency. In thermocouple meters the current passes through a fine wire that heats a thermocouple junction; the electricity generated by the thermocouple is measured by an ordinary galvanometer. In hot-wire meters the current passes through a thin wire that heats and stretches. This wire is mechanically linked to a pointer that moves over a scale calibrated in terms of current.

Direct Measuring

fig.6 - Digital Multimeter

We can also measure voltage or current by using a digital meter.

There are a few rules in measuring with digital meters :

1.First move the switch to the area that specifies what do you want to measure.
2.Then choose a measuring range (Ex: If you are trying to measure 220 V AC first move the switch to the alternative voltage area ,
then in this area move the switch to a number greater then the voltage you want to measure (in your case you can try 250 ).
3.The next step is the connection of the digital meter to the circuit. (rules about how you connect the digital meter are specified above ).
4.Last thing that you have to do is to read the digital screen when the number displayed becomes steady.

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