Otis Singleton
Electrical resistance is a measure of the opposition to the flow of current in an electrical circuit. It is influenced by the material's properties, length, cross-sectional area, and temperature. The unit of electrical resistance is ohms, represented by the symbol Ω. Georg Ohm discovered the relationship between voltage, current, and resistance, and Ohm's Law can be used to calculate the values of circuit components, current levels, voltage supplies, and voltage drops around a circuit. For example, in a series circuit, the required resistance can be calculated by dividing the voltage by the current. In a parallel circuit, the total resistance is the reciprocal of the sum of the reciprocals of the individual resistances. Measuring resistance can help identify electrical problems, such as open or short circuits, failed or overheating components, or damaged insulation.
| Characteristics | Values |
|---|---|
| Definition | Opposition to the flow of current in an electrical circuit |
| Measurement unit | Ohm (Ω) |
| Factors influencing resistance | Material's properties, length, cross-sectional area, and temperature |
| Relationship with current | Higher resistance results in lower current flow |
| Relationship with voltage | Higher resistance requires higher voltage |
| Relationship with temperature | Resistance of pure metals increases with temperature; resistance of insulators decreases with temperature |
| Troubleshooting | Measuring resistance at different points can help identify issues such as open or short circuits, failed or overheating components, and damaged insulation |
| Ohm's Law | Relates electrical current (I), voltage difference (V), and resistance (R); used to calculate unknown values in a circuit |
| Ohmmeter | A tool specifically used to measure resistance |
| Multimeter | A multifunctional tool that can measure voltage, current, resistance, and other electrical parameters |
| Calculation in series circuit | Resistor value (R) = Voltage (V) / Current (I) |
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What You'll Learn
Ohm's Law
The law can be used to calculate any one of the three variables (voltage, current, or resistance) when the other two are known. For example, if the voltage and current are known, the technician can calculate the resistance by rearranging the equation and plugging in the known values. This is particularly useful when trying to identify issues in a circuit, as abnormal values can indicate a problem with the circuit or its components.
It is important to note that Ohm's Law assumes that physical conditions and temperatures remain constant, as certain components, such as lightbulb filaments, increase in temperature as the current is increased, which would violate the law.
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Measuring resistance
When measuring resistance, it is important to note that the resistance of a component can vary over time and from component to component. For example, the resistance of switches and relay contacts usually starts very low and increases over time due to wear and dirt. In contrast, motors and solenoids may experience a decrease in resistance over time due to insulation breakdown and moisture.
To measure resistance accurately, it is recommended to remove the component from the circuit before taking measurements. This is because other components in the circuit can affect the readings. When measuring resistance with a multimeter, ensure good contact between the test leads and the circuit. For very low-resistance measurements, use the relative mode (REL) or zero/Delta (Δ) mode, which automatically subtracts the test lead resistance.
Additionally, factors such as foreign substances (dirt, solder flux, oil), body contact with the metal ends of the test leads, or parallel circuit paths can influence resistance readings. To avoid errors, it is recommended to avoid touching the metal parts of the test leads. By following these steps and considerations, one can effectively measure resistance in electrical circuits using a multimeter.
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Conductors and insulators
When it comes to electrical circuits, conductors and insulators are essential components. A conductor is a material that offers very little resistance to the flow of electric current, allowing electrons to move easily through it. Metals are well-known conductors, with copper being the most commonly used due to its excellent conductivity and relatively low cost compared to other metals. Other metals that can be used as conductors include silver, gold, and aluminum, although they may not conduct electricity as efficiently as copper. Interestingly, graphite, the human body, and the earth are also good conductors of electricity.
On the other hand, insulators are materials that have high resistance and restrict the flow of electrons. Examples of insulators include rubber, paper, glass, wood, and plastic. Insulators are necessary to protect us from the dangerous effects of electricity flowing through conductors. For instance, the rubber coating on electrical wires acts as an insulator, shielding us from the conductor inside.
The distinction between conductors and insulators becomes crucial when dealing with electrical circuits. Conductors, with their low resistance, are essential for facilitating the flow of electric current. In contrast, insulators, with their high resistance, are strategically placed to control and contain the flow of electricity.
The electrical resistance of a material is influenced by its properties, length, cross-sectional area, and temperature. As temperature increases, the resistance of pure metals also increases due to a higher number of electrons in the conduction band, which reduces their mobility. Conversely, the resistance of insulators decreases with increasing temperature because the movement of electrons from the conduction band to the valence band increases, leading to enhanced conductance.
It is important to note that while conductors generally have low resistance, they still offer some obstruction to the flow of electric current, known as electrical resistance. This resistance can be measured in ohms, denoted by the Greek letter omega (Ω). Ohm's law relates the electrical current to the voltage difference between two points of an electrical conductor, providing a formula for calculating resistance.
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Series circuits
Kirchhoff’s Voltage Law states that the sum of all voltages in a loop must equal zero. Therefore, if there is 9V of electromotive force between points 1 and 4, there must also be a 9V drop between points 2 and 3. This means that we can apply Ohm's Law (I = V/R) to the current through the resistor, as we know the voltage (V) and resistance (R).
In a series circuit, the total voltage drop is equal to the sum of the individual voltage drops. The voltage across each component is the electrical energy converted by each component. Therefore, the energy supplied equals the energy converted – energy has not been created or destroyed in the circuit.
The current through a series connection of any number of resistors will always be lower than the current into a parallel connection of the same resistors, as the equivalent resistance of the series circuit will be higher.
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Parallel circuits
In a parallel circuit, resistors are connected to the same two points (or nodes) and have more than one path for current to flow through. This means that the circuit current can take multiple paths, and the resistors are classed as current dividers. The voltage drop across all resistors in a parallel circuit is the same, and this voltage can be calculated using Ohm's law: $V = IR$, where $I$ is the current in amps and $R$ is the resistance in ohms.
The total current in a parallel circuit is equal to the sum of the individual branch currents. This is because, as the total current exits the positive battery terminal, some of the flow splits off to go through each resistor, and these individual currents then rejoin to flow back to the negative terminal.
The total resistance of a parallel circuit is calculated differently from a series circuit. In a series circuit, the total resistance is equal to the sum of the individual resistances, whereas in a parallel circuit, the reciprocal ($1/R$) value of the individual resistances is added together to give the equivalent resistance. This results in the total resistance of a parallel circuit being lower than that of a series circuit. Each parallel resistor added to a circuit reduces the total equivalent resistance, and this can be useful when a specific resistance value is needed but not available.
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Frequently asked questions
Resistance is a measure of the opposition to the flow of current in an electrical circuit. It is influenced by the material's properties, length, cross-sectional area, and temperature.
Resistance is measured in ohms, represented by the symbol Ω. One Ω represents the electrical resistance between two points when a voltage of one volt is applied across the points, resulting in a current of one ampere, or amp.
In a series circuit, the current stays fixed, so to calculate the required resistance, divide the volt (V) by the current (I) to get the resistance in ohms (R).
