Wiktor Wise
Resistance is a material's tendency to resist the flow of electric charge or current in a circuit. It is measured in ohms (Ω) and is dependent on the size of the voltage acting across it and the
| Characteristics | Values |
|---|---|
| Definition | A material's tendency to resist the flow of charge (current) |
| SI Unit | Ohm (Ω) |
| Relation to Voltage and Current | Resistance = Voltage/Current |
| Relation to Conductance | Reciprocal of electrical conductance |
| Relation to Temperature | Resistance is affected by temperature. |
| Relation to Material | Materials with high resistance are called insulators, while those with low resistance are called conductors. |
| Examples of Conductors | Silver, copper, gold, and aluminum |
| Examples of Insulators | Rubber, paper, glass, wood, and plastic |
| Relation to Power | Power = Voltage x Current = Current2 x Resistance = Voltage2/Resistance |
| Calculation | Resistance of a sample of material = Resistivity x Length/Cross-sectional Area |
| Measurement | Digital multimeters are used to measure resistance. |
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What You'll Learn
How resistance is measured
Measuring resistance can help identify whether components are operating properly and is a useful way to troubleshoot electrical problems. Multimeters are often used to measure resistance, and they can be either analog or digital. Both types of multimeters can measure resistance, current, voltage, and other parameters.
To measure resistance with an analog multimeter, first, switch off the power to the circuit under measurement. Then, plug the red test lead into the positive input terminal with the "+" mark and the black test lead into the COM input terminal. Next, switch the instrument to Ω mode and set the range button as appropriate based on the circuit's anticipated resistance. Finally, place the red and black test pins in contact with both ends of the circuit and read the value indicated by the meter.
Digital multimeters can measure resistance using the two-terminal measurement method, which applies a constant current and measures the resistance value using the instrument's voltmeter. This method is the same as that used by analog multimeters, but it has the disadvantage of yielding resistance values that include the wiring between the instrument and the circuit. To address this issue, four-terminal measurement uses four test leads and separate voltmeter and ammeter circuits.
It is important to note that applying a voltage to the test leads while the instrument is set to resistance mode could damage the tester. Additionally, always conduct a zero adjustment when measuring resistance to ensure accurate results.
Another way to measure resistance is by using an ohmmeter, a tool that only measures resistance. Ohmmeters measure resistance by putting a known (constant) current through the resistor and measuring the resulting voltage to display the resistance using Ohm's law.
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The role of resistors
Resistance is a material's tendency to resist the flow of electric charge (current). It is measured in ohms (Ω). Resistors are electrical components that create resistance in the flow of electric current. They are used in almost all electrical networks and electronic circuits.
Resistors can also be used to regulate voltage or current, although they are rarely used to control power. In high-frequency circuits, such as radio electronics, resistors with low parasitic reactance, such as foil resistors, are used to minimise unwanted effects. In audio amplifiers, the electric noise of the resistor must be as low as possible for accurate performance.
Resistors can also be used to generate heat, such as in the heating element of an electric kettle, which has a resistance of about 50 Ω. They can also be used in gain control and setting time constants. In cars, a special resistor is used to control the fan speed of the air ventilation system.
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The relationship between voltage, current, and resistance
Resistance is a material's tendency to resist the flow of charge (current). The higher the resistance, the lower the current flow. Resistance can be measured using a multimeter or an ohmmeter. 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 is applied across the points, resulting in a current of one ampere, or amp.
Voltage is the driving force behind the flow of a current and is measured in volts. It is also referred to as the potential difference or electromotive force. The force motivating charge carriers to "flow" in a circuit is called voltage. Voltage is a specific measure of potential energy that is always relative between two points. When we speak of a certain amount of voltage being present in a circuit, we refer to how much potential energy exists to move charge carriers from one particular point in that circuit to another particular point.
Current refers to the amount of charge that flows in any part of a conductor per time interval. It is measured in amps (A). If the current is very small, it is described in milliamps (mA), with 1000 mA equalling 1 A.
Ohm's Law can be expressed in three different forms depending on the unknown and given quantities:
- Voltage is equal to current multiplied by resistance (V = IR)
- Resistance is equal to voltage divided by current (R = V/I)
- Current is equal to voltage divided by resistance (I = V/R)
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Materials that conduct electricity
Conductive materials are those that can conduct electricity to varying degrees. These materials allow electrons to flow freely and fluidly from one point to another if they are connected to a power source.
Metals are generally very good conductors of electricity, with some metals like copper, iron, gold, aluminium, and silver being the best materials for conducting electricity. Silver is considered the best conductor of electricity, but due to its high cost, it is often only used in specific cases. Hardened copper is the most widely used conductive material, as its high conductivity and low cost make it ideal for wiring systems in machinery and appliances. Gold, like silver, is used in specific applications like phones or watches. Steel and aluminium are also highly conductive and have the added benefit of being low-cost, making them popular in industrial settings.
Other materials that conduct electricity include alloys, which are a fusion of two or more metals, and gaseous conductors like hydrogen. Electrolytic conductors are another type of conductive material where electrical conduction occurs through a chemical reaction that divides a charge-carrying substance into positive and negative poles. Graphite, an organic material made of carbon chains, is also used to conduct electrical circuits.
The shape, size, and temperature of a material also affect its conductivity. A thick piece of matter will conduct better than a thin piece of the same size and length, and a shorter piece will conduct better than a longer one of the same thickness. As temperature increases, atoms and their electrons gain energy, which can cause some insulators like glass to become good conductors.
Materials that offer very little resistance to the flow of electrons are called conductors, while materials that have high resistance and restrict the flow of electrons are called insulators.
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How resistance is affected by temperature
Temperature has a significant impact on resistance in an electrical circuit. The relationship between temperature and resistance is influenced by the type of material and its role within the circuit.
Conductors, semiconductors, and insulators respond differently to changes in temperature. Conductors, such as pure metals, generally experience an increase in resistance as temperature rises. This is because the atoms in the conductor vibrate more at higher temperatures, causing more collisions with the electrons flowing through the conductor. These collisions hinder the flow of electrons, increasing resistance. The speed at which electrons travel through a conductor is also influenced by temperature, with higher temperatures reducing their speed, which further affects the flow of electricity.
On the other hand, insulators typically exhibit a decrease in resistance as temperature rises. At high temperatures, the atoms in an insulator vibrate violently, shaking some electrons free from their atoms. These free electrons then become carriers of current, reducing resistance. Materials like glass and plastic, commonly used as practical insulators, maintain their insulating properties over a wide range of temperatures and only show a significant drop in resistance at extremely high temperatures.
Semiconductors, such as carbon, silicon, and germanium, demonstrate a negative temperature coefficient of resistance, meaning their resistance decreases as temperature increases. This is due to an increase in free electrons available for conducting electricity as the temperature rises.
The impact of temperature on resistance is quantified by the temperature coefficient of resistance, which represents the rate of change in resistance per degree Celsius change in temperature. Materials with a positive temperature coefficient experience an increase in resistance with increasing temperature, while those with a negative coefficient show a decrease in resistance. The temperature coefficient of resistance is influenced by the specific material and its properties.
In summary, the relationship between temperature and resistance is complex and depends on the material's inherent characteristics and its role within the circuit. Understanding these interactions is crucial for designing and predicting the behaviour of electrical circuits, especially in applications where temperature variations may occur.
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Frequently asked questions
Resistance in an electrical circuit is a force that counteracts the flow of current. It is measured in ohms (Ω).
The relationship between these three factors is shown by the equation: V = the potential difference and is measured in volts (V). The equation can be rearranged to find the resistance: R = V ÷ I.
As resistance increases, it becomes harder for electricity to move through the component, and therefore the rate of flow of electricity (i.e., the current) decreases. Conversely, as resistance decreases, it becomes easier for electricity to flow, and the rate of flow increases.
Resistors are electronic components that resist the flow of electricity in a circuit. They are made of materials that resist the flow of electricity as it passes through them. Resistors are essential to ensure that current flows at the appropriate level in circuits.
Resistance can be measured using a digital multimeter. These instruments can measure not only resistance but also voltage, current, and other parameters.
