DC Electrics - Basic Principles — Page 10, Lesson 16

I want to walk you through the basic principles of DC Electrics, starting with the concept of resistance. For a current to flow, there must be a complete path or circuit. The fewer obstructions in that circuit, the greater the current flow will be. Also, the higher the voltage, the greater the current flow. The obstruction in the circuit that opposes current flow is called resistance. Different materials have different numbers of free electrons. Materials with more free electrons have a lower resistance than those with few free electrons, so materials with more free electrons are better conductors of electricity. For a fixed voltage, the smaller the resistance, the larger the current flow, and the larger the resistance, the smaller the current flow. So, the current in a circuit can be adjusted by altering the resistance. Now, let's look at the factors that affect resistance. There are four main ones: - Type of material: For example, silver is a better conductor than copper. - Length: The longer the wire, the greater the resistance. - Cross-sectional area: The thicker the wire, the smaller the resistance. - Temperature: The symbol for temperature coefficient is the Greek letter α (alpha). If resistance increases with an increase in temperature, the resistor is said to have a Positive Temperature Coefficient (PTC). If resistance decreases with an increase in temperature, the resistor is said to have a Negative Temperature Coefficient (NTC). Resistors with these characteristics are used in aircraft systems for temperature measurement. Next, the units of resistance. The unit of resistance is the ohm, with the symbol Ω. A material has a resistance of one ohm if an applied voltage of one volt produces a current flow of one ampere. For larger and smaller values, we use metric prefixes: - One millionth of an ohm is one microhm (1 µΩ) - One thousandth of an ohm is one milliohm (1 mΩ) - One thousand ohms is one kilohm (1 kΩ) - One million ohms is one megohm (1 MΩ) Sometimes, resistance is used to adjust the current flow in a circuit by fitting resistors of known value. These can be either fixed or variable, and they are drawn with specific symbols in circuit diagrams. Now, let's move to Ohm's Law. In a closed circuit, there is a relationship between voltage, current, and resistance. If the voltage remains constant, any increase in resistance will cause a decrease in current, and vice versa — current is inversely proportional to resistance. If the resistance remains the same, any increase in voltage will cause an increase in current, and vice versa — current is directly proportional to voltage. This relationship is expressed as Ohm's Law: V = I × R And by transposition, we can also write: I = V / R or R = V / I Finally, let's talk about power. When a force produces movement, work is said to have been done, and the rate at which work is done is called power. In an electric circuit, work is done by the voltage causing the current to flow through a resistance, creating heat, magnetism, or chemical action. The rate at which work is done is called power, and it is measured in watts. The formula is: Watts (W) = Voltage (V) × Amperes (I) So, power equals voltage multiplied by current. That covers the basic principles of resistance, Ohm's Law, and power in DC circuits.