Wiktor Wise
Electrical resistance is a measure of how much an object resists the flow of an electric current. The SI unit of electrical resistance is the ohm, represented by the uppercase Greek letter omega (Ω). The ohm is named after German physicist Georg Ohm, who studied the relationship between voltage, current, and resistance. Resistance is influenced by the material's properties, length, cross-sectional area, and temperature. It is the reciprocal of electrical conductance, which is measured in siemens. Resistivity, measured in ohm-meters, is a measure of a material's ability to oppose the flow of electric current. The higher the resistance, the lower the current flow, and vice versa.
| Characteristics | Values |
|---|---|
| What is electrical resistance? | A measure of the degree to which an object opposes the passage of an electric current. |
| What is the SI unit of electrical resistance? | Ohm (Ω) |
| What is the reciprocal quantity of electrical resistance? | Electrical conductance measured in Siemens (S) |
| What is the formula for electrical resistance? | R (resistance) is usually measured in ohms, V (potential difference) is usually measured in volts, and I (current) is usually measured in amperes |
| What is resistivity? | A measure of the material's ability to oppose electric current. |
| What is the unit of resistivity? | Ohm-metre |
| What is the reciprocal of resistivity? | Conductivity |
| How is resistance influenced? | By the material's properties, length, cross-sectional area, and temperature |
| What is a resistance thermometer or thermistor? | A component that uses temperature-dependent resistance to infer the temperature of the environment. |
| What is PTB? | Physikalisch-Technische Bundesanstalt, the national metrology institute providing scientific and technical services. |
| What is the resistance unit at PTB realised from? | The quantum Hall resistance |
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What You'll Learn
The unit of electrical resistance is ohms
Ohm's Law states that there is a proportional relationship between voltage, current, and resistance in an electrical circuit. In other words, the ohm is the measurement of resistance between two points of a conductor when a constant potential difference of one volt (V) is applied to those points and a current of one ampere (A) is produced. This can be simplified as one volt per one ampere (V/A).
The ohm is a derived unit in the International System of Units (SI), meaning it is based directly or indirectly on the standard's fixed constants. The rapid rise of electrotechnology in the 19th century led to the demand for a coherent and international system of units for electrical quantities, resulting in the creation of the ohm as the standard unit of electrical resistance.
It is important to note that resistance values can vary depending on the applied voltage or current. For example, in alternating current (AC) circuits, the electric current can reverse direction, resulting in different methods for measuring resistance compared to direct current (DC) circuits. Additionally, the resistance of an object depends largely on the material it is made of, with electrical insulators like rubber tending to have high resistance and electrical conductors like metals having low resistance.
To maintain the accuracy and dissemination of the ohm as the unit of electrical resistance, calibration and comparisons are regularly conducted by national metrology institutes such as the Physikalisch-Technische Bundesanstalt (PTB). These efforts ensure worldwide uniformity in the resistance unit ohm.
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Resistance is influenced by the material's properties, length, and temperature
The electrical resistance of an object is a measure of its opposition to the flow of electric current. The SI unit of electrical resistance is the ohm (Ω). The resistance of an object depends on several factors, including the material it is made of, its length, and its temperature.
Material Properties
The material an object is made of plays a significant role in determining its resistance. Objects made of electrical insulators like rubber tend to have very high resistance, while conductors like metals tend to have very low resistance. This relationship is quantified by resistivity or conductivity. Resistivity refers to a material's ability to oppose electric current, and it varies greatly between different materials. For example, the conductivity of Teflon is about 1030 times lower than that of copper. This is because metals have many "delocalized" electrons that can move freely, whereas in an insulator like Teflon, electrons are tightly bound to individual molecules.
Length
Resistance is directly proportional to the length of the conductor. This means that as the length of a material increases, so does its resistance, and vice versa. The relationship between resistance and length can be expressed mathematically using the resistivity formula:
> R = ρL/A
Where:
- R is the resistance measured in ohms
- L is the length of the material in meters
- A is the cross-sectional area in m^2
- Ρ is the resistivity of the material in ohm-meters
Temperature
Resistance is also influenced by temperature. As the temperature of a conductor increases, so does its resistance, as higher temperatures cause atoms and molecules to bounce around more, impeding the flow of electrons. However, this relationship is not linear, and some materials, like silicon, exhibit a negative temperature coefficient of resistivity, meaning their resistance decreases as temperature increases.
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Resistors are electronic components that regulate current in circuits
Resistors are typically small, worm-like components with two connections, allowing them to be hooked into a circuit. They can be fixed or variable. Fixed resistors have resistances that change only slightly with temperature, time, or operating voltage. Variable resistors, on the other hand, can be adjusted using a dial, lever, or control to change the resistance.
The amount of resistance introduced into a circuit depends on the type of resistor used and its specific characteristics. For example, the number of copper turns in a wire-wound resistor controls the resistance, with more turns resulting in higher resistance.
The presence of a resistor in a circuit affects the overall current flow. According to Ohm's law, the current in a circuit is given by the equation I = V/R, where V is the voltage and R is the resistance. Increasing the total resistance in a circuit decreases the overall current, while the current within the resistor itself remains constant.
Resistors are commonly used to limit the current flowing through sensitive components, prevent overcurrent situations, and reduce power consumption in circuits. They also find applications in motor controls, power distribution systems, and test loads for generators. Additionally, variable resistors are used in devices like transistor radios and TVs for volume control, and in lamp dimmers.
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Measuring resistance can help identify electrical problems
Electrical resistance is a measure of how much an object resists the passage of an electric current. It is measured in ohms (Ω), and its reciprocal quantity is electrical conductance, measured in siemens (S).
Measuring resistance can be a straightforward way to identify and troubleshoot electrical problems. By taking measurements at different points in a circuit, it is possible to pinpoint the location of a failure and restore the circuit. For example, high or infinite resistance may indicate an open circuit, while very low or zero resistance may indicate a short circuit.
Failed or faulty components can also be identified through resistance measurements. Components such as resistors have a known resistance, and testing can indicate if they are malfunctioning. Overheating components often have higher resistance than normal, signalling potential issues.
Additionally, resistance measurements can help identify installation errors and poor connections. It is important to note that resistance cannot be measured in an operating circuit. Therefore, troubleshooting technicians often determine resistance by taking voltage and current measurements and applying Ohm's Law.
Specialized tools such as a multimeter or ohmmeter can be used to measure resistance and help diagnose problems with electrical systems, ensuring their reliability and safety. When using these tools, it is important to follow safety precautions and properly set up the device to obtain accurate readings.
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Resistance is calculated using Ohm's Law
Resistance is a measure of how much an object resists or opposes the flow of an electric current. The SI unit of electrical resistance is the ohm (Ω). The reciprocal of resistance is electrical conductance, which is measured in siemens (S).
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage. This means that the higher the voltage, the higher the current, and vice versa. This relationship holds true for many materials and over a wide range of voltages and currents.
Ohm's Law can be used to calculate the relationship between voltage, current, and resistance in an electrical circuit. By knowing any two values of voltage, current, or resistance, Ohm's Law can be used to find the missing value. The formula for Ohm's Law is:
V = I * R
Where:
- V is the voltage (measured in volts)
- I is the current (measured in amperes)
- R is the resistance (measured in ohms)
For example, if you have a circuit with a voltage of 12 volts and a current of 2 amperes, you can calculate the resistance as follows:
R = V / I
R = 12V / 2A
R = 6 ohms
Ohm's Law is a fundamental concept in electrical engineering and is used extensively in electrical calculations and formulas. It is important to understand and accurately remember the basic formula to work with electrical circuits and components.
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Frequently asked questions
Electrical resistance is a measure of how much an object resists or opposes the flow of an electric current.
The unit of electrical resistance is the ohm, represented by the Greek letter omega (Ω).
Electrical resistance is calculated by measuring circuit voltage and current, and then applying Ohm's Law.
The reciprocal of electrical resistance is electrical conductance, which is measured in siemens (S).
Electrical resistance is influenced by the material's properties, length, cross-sectional area, and temperature. Materials with high resistance include rubber, paper, glass, wood, and plastic.
