قوانين الدارة ووحدات قياسها .. الإلكترونيات

Circuit Laws and Units of Measurement .. Electronics

Circuit Laws and Units of Measurement

The volt (V) is the unit of voltage or pressure resulting from a battery or any other electrical source. The voltage of a dry cell is equal to 1.5 volts. If two cells are connected in series, a voltage of 1.5 + 1.5 = 3 volts can be obtained. To explain the effect of increasing the voltage on increasing the current that can be obtained, suppose that there is a container filled to half, with a depth of water in it of V1 (17), while the depth of water in the second container is V2. If a pipe is connected to the bottom of the second container, it supplies us with water with a pressure twice that of the water coming out of the first container.

Some circuits operate on low voltage. Others require hundreds of thousands of volts (1000 volts = 1 kilovolt or (kV)). On the other hand, very low voltages are measured in millivolts or mV (1000 mV = 1 volt), while extremely low voltages are measured in microvolts or mV (1000 mV = 1 mV).

The ampere (1) is the unit of current intensity passing through the circuit. An example of this is the electric lamp that has one ampere passing through it as in Figure (118), and accordingly, two lamps connected in parallel need two amperes, each of which has one ampere passing through it (18 B).

The intensity of small currents is measured in milliamperes (1000 milliamperes = (amperes) and currents that are zero are measured in microamperes (1000 microamperes = 1 milliampere).

The ohm is the unit of electrical resistance. The electrical resistance of the wire decreases as its thickness increases, allowing the current to pass easily (19), just as happens in a wide pipe that allows water to pass through it easily. As for thin wires, they resist the passage of current just like a narrow pipe that resists the passage of water in it. The more the resistance of the circuit or one of its elements increases, the more difficult the passage of current becomes. Copper wires are characterized by a small resistance that allows the current to pass through them easily, but we may need high resistance in many electrical circuits, so we use elements with high resistance made of iron or its various alloys.
The longer the wire and the thinner it is, the greater its electrical resistance. As for carbon compounds, they resist the passage of current and can be formed into rods with different resistances for use in Electrical circuits. Such rods are commonly used. The component that resists the passage of current is called a "resistor" (20). The variable resistor includes a sliding contact, by which any part of the resistance can be inserted into the circuit as needed. The voltage meter includes two connections, one on each end; and a sliding contact by which the voltage can be controlled.

High-value resistors are measured in kilo-ohms (k-ohm = 1000 ohms). Very high-value resistors are measured in megohms (mega-ohms = 1000 kilo-ohms).

Resistors are usually colored conventionally, each color indicating a specific number or a specific number of zeros.