The opposition a conductor offers to the flow of electric current is called resistance. This resistance to electric current is denoted by the letter R, the first letter of the word "resistance." The unit of resistance is the ohm (Ω) and is represented by the symbol Ω (omega). In the example of the water-filled container in Figure 2.6, the holes of different diameters on the container show different resistances to the flow of water. The larger diameter hole offers less resistance to water flow. From this point, the following analogy can be made in electricity: as the cross-section of a conductor increases, its resistance to electric current decreases.
The resistance of a conductor depends on the following properties:
1. The conductor depends on the material's structure. In materials with a stable atomic structure, the number of electrons that can become free is very small, so the path for the current is quite limited. The material will exhibit resistance due to this structure.
2. It depends on the cross-section of the conductor wire. As the cross-section decreases, the resistance increases.
3. It depends on the length of the conductor wire. As the length of the conductor increases, the friction of the electrons conducting electric current within the conductor will increase, and the resistance will also increase in proportion to this length.
4. It depends on the temperature of the conductor. As the temperature increases, the resistance increases. For example, the resistance of a 40-watt light bulb with a tungsten filament is 110 Ω at 20°C, but when the bulb is energized, the temperature of the tungsten filament rises to 2400°C, and its resistance increases to 1100 Ω. An electrical circuit can have more than one receiver. The connections of these receivers on the circuit can be series or parallel, or complex, meaning both series and parallel connections can be present. To find the total resistance of such a circuit:
Ohm's Law
It is the fundamental law of electricity that expresses the relationship between current intensity, voltage, and resistance in an electrical circuit. As in all movements, electrons encounter friction in their movement within the conductor. This opposition to electron flow is called resistance. The unit of electrical resistance is the ohm. As resistance increases, the number of electrons passing through the circuit decreases. In other words, there is an inverse relationship. As resistance decreases, the circuit current will increase. This ratio is named after the famous German physicist George S. OHM, who discovered this relationship in 1827, and is expressed as OHM's law. According to the law, in an electrical circuit, the current intensity is directly proportional to the voltage and inversely proportional to the resistance.
Mathematically, the relationship is expressed as follows:
Current (I) is equal to the ratio of voltage (E) to resistance (R), and is expressed as I = E/R.
To summarize this information:
– Electromotive force (EMF) is expressed in volts. It is denoted by E.
– Electrical resistance is expressed in ohms and is denoted by the symbol Ω (omega).
– Current, the rate of electrical flow, is expressed in amperes. Current intensity is denoted by I.
Let's apply this information to the simple electrical circuit in Figure 2.8. The voltmeter in the circuit shows 12 volts, and the ammeter shows 0.8 amperes. Let's find the resistance of this circuit.
According to Ohm's Law:
I = E/R; 0.8 = 12/R, so R = 15Ω.
Current intensity (I) in amperes = Voltage (E, in volts) / Resistance (R, in ohms)
Based on this formula, the relationships between current, voltage, and resistance can also be used as:
I = E/R, E = I × R, or R = E/I.
