Trystan Casey
Resistance is a force that counteracts the flow of electricity. It is an important property of electricity that controls the flow of current in electrical systems. Electrical 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 applied across the points results in a current of one ampere, or amp. Resistance measurements are normally taken to indicate the condition of a component or a circuit. The higher the resistance, the lower the current flow. Measuring resistance can be done with specialized tools like a multimeter or ohmmeter.
| Characteristics | Values |
|---|---|
| Definition | Resistance is a force that counteracts the flow of electricity. |
| Formula | Ohm's Law: V = I x R (Volts = Current x Resistance) |
| Unit | Ohm (Ω) |
| Factors Affecting Resistance | Type and temperature of the substance, length over which current must travel, size and shape of the object, and nature of the material. |
| Measuring Instruments | Multimeter, ohmmeter, digital multimeter, voltmeter, ammeter, Kelvin Double Bridge, low resistance ohmmeter. |
| Applications | Troubleshooting electrical problems, testing failed components, measuring joint resistance, power distribution cable joints, and rail track joints. |
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What You'll Learn
Measuring resistance with a multimeter or ohmmeter
Resistance to electricity is measured in ohms, represented by the Ω symbol. Multimeters and ohmmeters are tools used to measure resistance. A multimeter is multifunctional and can measure voltage, current, resistance, and other electrical measurements. On the other hand, an ohmmeter is a more specialized tool that only measures resistance.
Using a Multimeter
- Ensure the circuit's power is turned off before taking any resistance measurements.
- If using an analog multimeter, calibrate it for 0 Ω of resistance using the adjustment knob or wheel. Touch the test probes together and adjust until the needle points to zero.
- Insert the black test lead into the COM or common input jack.
- Insert the red lead into the VΩ jack (for digital multimeters) or the resistance input jack (for analog multimeters).
- Turn the dial to the resistance setting (often denoted as Ω).
- Establish good contact between the test leads and the circuit.
- Connect the test leads across the component being tested.
- View the resistance reading on the display.
- Note the unit of measurement, which may be ohms (Ω), kilohms (kΩ), or megohms (MΩ).
- Capture a stable measurement by pressing the HOLD button on the multimeter.
- Press the MIN/MAX button to record the lowest and highest measurements.
- For very low-resistance measurements, use the relative mode (REL) to subtract test lead resistance.
Using an Ohmmeter
- Turn off the power to the circuit before using the ohmmeter.
- Plug in the test leads: insert the black test lead into the common input jack and the red or yellow lead into the resistance input jack.
- Select the resistance setting on the ohmmeter.
- Touch the probe tips across the component or portion of the circuit you want to measure.
- View the reading on the display and note the unit of measurement (ohms, kilohms, or megohms).
- Remember to turn off the ohmmeter when testing is complete to conserve battery life.
By following these steps, you can effectively measure resistance using a multimeter or ohmmeter, allowing you to identify and troubleshoot electrical problems in circuits and components.
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Troubleshooting electrical problems
Resistance is an important property of electricity that controls the flow of current in electrical systems. It is measured in ohms, represented by the symbol Ω.
There are several tools that can be used to measure resistance, including a multimeter or an ohmmeter. A multimeter is a multifunctional tool that can measure voltage, current, resistance, and other types of electrical measurements, while an ohmmeter only measures resistance.
It is important to note that resistance cannot be measured in an operating circuit. Instead, technicians often determine resistance by taking voltage and current measurements and applying Ohm's Law.
Some common issues that can cause problems with electrical resistance measurements include:
- Loose or intermittent connections
- Using an auto-ranging ohmmeter with a large inductance in the circuit
- Voltage or power supply in the circuit being measured
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Resistivity and conductivity
Resistance to electricity is measured in ohms (Ω).
Resistivity is a fundamental property of a material that measures its electrical resistance or how strongly it resists electric current. It is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-metre (Ω⋅m). The lower the resistivity, the more readily the material permits the flow of electric charge.
Conductivity is the reciprocal of electrical resistivity. It represents a material's ability to conduct electric current. It is commonly signified by the Greek letter σ (sigma), but κ (kappa) (especially in electrical engineering) and γ (gamma) are sometimes used. The SI unit of electrical conductivity is Siemens per metre (S/m).
The amount of resistance is caused by two factors: the temperature and thus the amount of vibration of the crystal lattice, and the purity of the metal. Higher temperatures cause bigger vibrations, which act as irregularities in the lattice. A mixture of different ions is also an irregularity.
Silver is the most electrically conductive element, followed by copper and gold. However, copper and gold are used more often in electrical applications because copper is more affordable and gold has superior corrosion resistance.
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Resistance in different materials
Resistance is the measure of a material's opposition to the flow of electric current. It is quantified by resistivity or conductivity, with the former being more commonly used. Resistivity is the measure of how much a substance resists the flow of electric current. It is the reciprocal of conductivity, which measures how easily current passes through a substance. Resistivity is normally measured in ohm metres.
The resistance of an object depends largely on the material it is made of. Objects made of electrical insulators like rubber, glass, wood, and plastic tend to have very high resistance and low conductivity. Materials that have high resistance restrict the flow of electrons. On the other hand, objects made of electrical conductors like metals tend to have very low resistance and high conductivity. Copper, aluminium, and silver are commonly used as conductors due to their low resistivity.
The resistivity of different materials varies significantly. For example, the conductivity of Teflon is about 1030 times lower than that of copper. This is because metals have a large number of "delocalized" electrons that are free to move across large distances, while in an insulator, each electron is tightly bound to a single molecule. Semiconductors lie between these two extremes, with their conductivity depending on various factors such as temperature, composition, and exposure to light.
Other factors also influence resistance, such as the size and shape of an object. For example, a wire's resistance is higher if it is long and thin, and lower if it is short and thick. Placing a conductor under tension also increases its resistance, as the length of the conductor increases and its cross-sectional area decreases. Similarly, temperature affects resistance, with resistivity typically decreasing at higher temperatures.
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Ohm's Law
Resistance to electricity is measured in ohms, represented by the Ω symbol. Ohm's Law is a formula used to calculate the relationship between voltage, current, and resistance in an electrical circuit.
- V = I x R
- I = V / R
- R = V / I
Where:
- V = voltage
- I = current
- R = resistance
Technicians can also use Ohm's Law to detect what part of a circuit is faltering and determine where a problem may lie. It is especially useful when resistance needs to be calculated in an operating circuit, as resistance cannot be measured directly in such cases.
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Frequently asked questions
Resistance to electricity is measured in ohms, represented by the symbol Ω.
The relationship between these three quantities is defined by Ohm's Law, which states that V = I x R (Volts = Current x Resistance).
Resistance can be determined using Ohm's Law, or it can be measured with specialized tools like a multimeter or ohmmeter.
