Trystan Casey
Electrical resistance is a fundamental concept in physics that describes how a material or component in a circuit resists or impedes the flow of electric current. It is denoted by the symbol 'R' and measured in ohms (Ω), named after German physicist Georg Simon Ohm. Ohm's Law establishes a relationship between voltage, current, and resistance, where the current is directly proportional to the potential difference or voltage. This property is influenced by factors such as the material's properties, length, cross-sectional area, temperature, and shape. Understanding electrical resistance is crucial for designing and troubleshooting electrical circuits, as it helps identify issues like open or short circuits, failed components, and overheating.
| Characteristics | Values |
|---|---|
| Definition | The property of an electrical conductor to oppose (resist) the flow of electric current. |
| Symbol | R |
| Unit | Ohm Ω (Greek letter Omega) |
| Relation to Ohm's Law | The current flowing through a conductor is directly proportional to the potential difference across it. |
| Relation to Voltage and Current | The electrical resistance of a circuit is the ratio between the voltage applied to the current flowing through it. |
| Factors Affecting Resistance | Material, Length, Cross-sectional Area, Temperature |
| Factors Not Affecting Resistance | Material alone (it depends on size and shape as well) |
| Examples of Low Resistance | Copper, aluminium, other metals |
| Examples of High Resistance | Rubber, paper, glass, wood, plastic, other insulators |
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What You'll Learn
Electrical resistance and conductance
Electrical resistance is a property of a material that describes how much it resists or obstructs the flow of electric current through it. It is denoted by the letter 'R' and measured in ohms (Ω). The higher the resistance, the lower the amount of current that can pass through.
Every material has a different electrical resistance, and this is why conductors give out heat when an electric current passes through them. Electrical resistance is dependent on the material's properties, length, cross-sectional area, and temperature. For example, a long, thin wire has higher resistance than a short, thick wire of the same material. Materials with high electrical resistance include rubber, paper, glass, wood, and plastic. These materials are known as insulators.
Substances that conduct electric current easily are called conductors and have very low electrical resistance. Common conductors include metals such as copper and aluminium. Resistors are made from a wide variety of materials depending on factors such as the desired resistance and energy requirements.
The reciprocal of electrical resistance is electrical conductance, which measures how easily a current passes through a material. Electrical conductance is measured in siemens (S). The higher the conductance, the lower the resistance, and vice versa. Similar to resistance, the conductance of a material depends on the material it is made of and its size and shape.
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Factors affecting resistance
Electrical resistance is the property of a substance or material that obstructs the flow of electric current. It is denoted by the letter 'R' and is measured in ohms (Ω).
The resistance of a conductor depends on several factors, including the length of the conductor, the nature of the material, the cross-sectional area, and the temperature.
Length of the Conductor
The resistance of a conductor is directly proportional to its length. In other words, longer conductors cause more resistance. For example, a long, thin copper wire has higher resistance than a short, thick copper wire of the same material.
Nature of the Material
The type of material used is another important factor influencing resistance. The nature of a material determines its resistivity, which is a measure of its ability to resist the flow of electric current. Materials with low resistivity, such as metals like copper and aluminium, are good conductors and have low resistance. On the other hand, materials with high resistivity, such as rubber, are insulators and have high resistance.
Cross-Sectional Area
The resistance of a conductor is inversely proportional to its cross-sectional area. This means that thicker wires have lower resistance than thinner wires of the same material.
Temperature
Temperature also affects resistance. When a wire is heated, its resistance increases.
Other factors that can influence resistance include the presence of impurities and the frequency of the current, as it affects the movement of electrons within the conductor.
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Ohm's Law
- Voltage (V) = Current (I) x Resistance (R)
- Volts (V) = Amps (A) x Ohms (Ω)
- V = A x Ω
In other words, one volt of pressure is required to push one amp of current through one ohm of resistance. The unit of electrical resistance, the ohm, is defined as:
Ohm = 1 volt / 1 ampere
It is important to note that Ohm's law is an empirical law based on generalizations from many experiments. It is not always obeyed, as some materials are non-ohmic and do not show a direct relationship between voltage and current.
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Measuring resistance
Resistance can be measured using either a multimeter or an ohmmeter. A multimeter is a multifunctional tool that can measure voltage, current, resistance, and other electrical measurements, while an ohmmeter only measures resistance. Resistance measurements are normally taken to indicate the condition of a component or a circuit. The higher the resistance, the lower the current flow, and vice versa.
When measuring resistance with a digital multimeter, the display should show OLΩ because, in Resistance mode, the multimeter automatically begins taking a resistance measurement even before the test leads are connected to a component. The MΩ symbol may appear on the display because the resistance of open (unattached) test leads is very high. When the leads are connected, the multimeter automatically uses the Autorange mode to adjust to the best range. Pressing the Range button allows for a manual range selection.
For very low-resistance measurements, the relative mode (REL) may be used. This mode automatically subtracts test lead resistance, which is typically between 0.2 Ω and 0.5 Ω. If the test leads touch, the display should show 0 Ω. Other factors that can affect resistance readings include foreign substances (dirt, solder flux, oil), body contact with the metal ends of the test leads, or parallel circuit paths.
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. This involves using the formula V = I x R, where V is voltage, I is current, and R is resistance. If resistance is unknown, the formula can be rearranged to R = V/I.
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Applications of resistance
Resistance is a fundamental concept in electricity, and it has numerous applications in our daily lives. Here are some detailed examples of the applications of resistance in electricity:
Heating Devices:
The heating effect of electric current is a direct result of resistance. When an electric current passes through a conductor, the resistance offered by the conductor converts electrical energy into heat energy. This principle is widely used in heating appliances such as electric irons, room heaters, water heaters, and electric toasters. Electric bulbs and lamps also utilize the heating effect of current to produce light.
Fuses:
An electric fuse is another important application of resistance. A fuse wire has a specific resistance and melting point. When a high current flows through the fuse, exceeding its rated value, the heat generated melts the wire, breaking the circuit. This prevents damage to electrical devices and protects against electrical fires.
Electrical Troubleshooting and Diagnostics:
Resistance measurements are used to identify and diagnose issues in electrical circuits. By measuring resistance at different points, technicians can locate problems such as open circuits, short circuits, failed components, weak connections, or damaged insulation. This helps in quickly restoring proper circuit operation.
Variable Resistance Devices:
Devices like rheostats provide variable resistance in a circuit, allowing the regulation of current without changing the voltage from the source. This is particularly useful in applications where precise control of current is required, such as in dimmer switches for lighting or speed control in electric motors.
Superconductivity:
Superconductors are materials with zero electrical resistance, which means they allow the flow of current without any opposition. Superconductivity has important applications in technologies such as superconducting magnets, which are used in medical equipment like MRI machines, as well as particle accelerators and high-field magnets for research.
In summary, resistance is a critical aspect of electricity, and its applications are diverse, impacting our daily lives and various industries. Understanding and utilizing resistance is essential for the proper functioning of electrical systems and devices.
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Frequently asked questions
Electrical resistance is the obstruction offered by the material in the flowing of the current or charge through the material. It is influenced by the material's properties, length, cross-sectional area, and temperature.
The electrical resistance of a conductor depends on its length and cross-sectional area. It is also influenced by the material's resistivity, which is a measure of its ability to resist flowing electric current.
The SI unit of electrical resistance is the ohm (Ω), named after German physicist Georg Simon Ohm.
The relationship between voltage, current, and resistance is given by Ohm's law, which states that the current flowing through a conductor is directly proportional to the potential difference across it.
Electrical resistance can be measured using a multimeter or an ohmmeter. Measurements are typically taken in ohms (Ω), kiloohms (kΩ), or another unit.
