Beatrice Lang
Electrical resistance is a fundamental property of conductive materials, such as metals and semiconductors, and is measured in units called ohms (Ω). It is the property of an electrical conductor to oppose (resist) the flow of electric current. All materials have their own electrical resistance, and it is represented by the letter R. The SI unit of resistance is Ohm Ω (the Greek letter Omega), named after German physicist Georg Simon Ohm, who gave Ohm's law and the relation between voltage, current, and resistance. Electrical resistance is an important concept to understand for designing, operating, and maintaining electronic devices and systems.
| Characteristics | Values |
|---|---|
| Definition | The property of an electrical conductor to oppose (resist) the flow of electric current |
| Symbol | R |
| SI unit | Ohm Ω (Greek letter Omega) |
| Reciprocal quantity | Electrical conductance |
| Factors that affect resistance | Material, length, cross-sectional area, temperature |
| Resistivity | The resistance offered by the material per unit length for unit cross-section |
| Resistivity symbol | ρ (Greek letter rho) |
| SI unit of electrical resistivity | Ohm-meter (Ω-m) or mho |
| Relation between resistance and resistivity | Resistivity is inversely proportional to resistance |
| Relation between voltage and current | Directly proportional |
| Types of electrical resistance | Static resistance, dynamic resistance |
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What You'll Learn
How to calculate electric resistance
Electric resistance is calculated using Ohm's Law, which states that voltage is equal to the product of current and resistance. Mathematically, this can be written as V = IR, where V stands for voltage, I for current, and R for resistance. Resistance is measured in ohms, while voltage is measured in volts and current in amps.
The formula to calculate resistance takes into account the material the resistor is made of, and can be expressed as R = ρLA, where R is resistance, ρ is the resistivity of the material, L is the length of the resistor, and A is the area of a cross-section of the resistor. Length and area are measured in meters.
To calculate the total resistance in a circuit, it is important to first determine whether the resistors are connected in series or in parallel. In a series circuit, where components are connected one after the other, the total resistance is equal to the sum of the resistances. In a parallel circuit, where components are connected along parallel branches, the inverse of the total resistance equals the sum of the inverse of each individual resistance.
For example, let's say we have a carbon resistor that is 0.005 meters long with a diameter of 0.001 meters. The resistivity of this resistor is 45 x 10-5 ohm-meter. To calculate the resistance, we multiply the resistivity by the length and divide that by pi times the square of half the diameter. This gives us a resistance of approximately 2.86 ohms.
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How the material's resistivity affects electric resistance
The resistivity of a material is a measure of how strongly it resists an electric current. It is an intrinsic property of a material, meaning it does not depend on the geometric properties of the material. Every material has its own characteristic resistivity, and this is measured in ohm-metres (Ω⋅m). The higher the resistivity, the more the material resists the flow of electric current.
Resistivity is related to the conductivity of a material. Conductivity is the inverse or reciprocal of resistivity. It measures how well a material conducts an electric current. The higher the resistivity, the lower the conductivity, and vice versa. Good conductors, such as gold, silver, copper, aluminium, and steel, have low resistivity and allow electrical currents to flow easily. On the other hand, insulators like rubber, glass, and plastic have high resistivity and are much more difficult for electricity to flow through.
The resistivity of a material can be influenced by several factors, including temperature, purity, and the current's frequency. As the temperature of a material increases, its atoms vibrate faster, making it more difficult for electric current to flow through it. Similarly, impurities in a material can hinder the flow of electrons, increasing its resistivity. The type of atoms in a material also determines its resistivity.
Resistance, which is related to resistivity, factors in the length and size of a conductor to measure its ability to allow electricity to flow through it. While resistivity is an intrinsic property of a material, resistance is an extensive property that gives the opposition of a specific object to electric current.
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How the shape of the material affects electric resistance
The shape of a material is a key factor in determining its electrical resistance. This is because the resistance of an object is influenced by its size and shape, as these properties are extensive rather than intensive.
For example, a wire's resistance is higher if it is long and thin, and lower if it is short and thick. This is because it is more difficult for electricity to flow through a long, thin wire than a short, thick one. This is similar to how water flows more easily through a wide, short pipe than a long, narrow one.
The resistance of an object is also influenced by the material it is made of. Objects made of electrical insulators like rubber tend to have very high resistance, while objects made of electrical conductors like copper or aluminium tend to have very low resistance. This is because insulators have a very low concentration of free charges, while conductors have a high concentration of free electrons, allowing them to conduct electricity more easily.
The relationship between the resistance of an object and its shape and material composition is quantified by its resistivity or conductivity. Resistivity is an intrinsic property of a material that measures how strongly it resists electric current. It is represented by the Greek letter rho (ρ). Conductivity, on the other hand, measures a material's ability to conduct electric current and is the reciprocal of resistivity. It is represented by the Greek letter sigma (σ).
In addition to the shape and material of an object, other factors such as temperature can also influence its resistance. For example, the resistivity of conductors increases with increasing temperature, as the atoms vibrate more rapidly and over larger distances, causing more collisions with the electrons and increasing resistivity.
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How temperature impacts electric resistance
Temperature has a significant impact on electric resistance. As a rule, resistance increases with temperature. This is because as the temperature rises, the vibrations of the atoms within the metal lattice increase, which increases resistance to electron flow. The more the atoms jostle around, the more collisions occur, leading to greater resistance to current flow. This is particularly noticeable in power lines, where small changes in voltage can become significant over long distances.
However, this is not always the case. Materials like silicon have a negative temperature coefficient of resistance, meaning that as the temperature rises, resistance decreases. This is because an increase in temperature can free more charge carriers, which would be associated with an increase in current.
The impact of temperature on resistance is also dependent on the type of material. Conductors, insulators, and semiconductors all behave differently. Conductors, which already have a large number of free electrons flowing through them, experience increased resistance with higher temperatures due to the increased atomic vibrations causing more collisions. Insulators, on the other hand, have very few free electrons, so heating them can actually cause their atoms to vibrate violently enough to free some electrons, thereby decreasing resistance. Semiconductors, which are not conductors under normal conditions, can exhibit increased conductivity and decreased resistance with higher temperatures as their molecules start to align better.
Engineers can exploit these varying relationships between temperature and resistance by using materials with different temperature coefficients in the construction of resistors. For example, by placing one resistor with a positive temperature coefficient in series with another that has a negative temperature coefficient, they can create a resistor with a resistance that is almost independent of temperature.
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How electric resistance is used in electronics
Electric resistance is a fundamental property of electrical conductors, which describes their opposition to the flow of electric current. It is measured in ohms (Ω).
The electrical resistance of a material is caused by the movement of electrons. When an electric current is applied to a material, the electrons within that material begin to move. However, they collide with other electrons and the fixed atoms of the material, causing them to lose energy. This energy is converted into heat, which results in a reduction in the flow of electrons and a decrease in the electrical current. This process creates electrical resistance.
Resistance is used in electrical circuits to control the amount of current flowing through them. The resistance of a given object depends on two main factors: the material it is made of and its shape. For a given material, the resistance is inversely proportional to the cross-sectional area, and proportional to the length. For example, a thick copper wire has lower resistance than a thin copper wire, and a long copper wire has higher resistance than a short copper wire.
Resistance is also used to convert electrical energy to heat energy, according to Joule's law of heating. Devices such as incandescent light bulbs, electric ovens, hair dryers, space heaters, clothes dryers, and irons use heating elements, which are conductors with high resistance that convert electrical energy into heat energy.
Understanding electrical resistance is crucial for the design and operation of electronic devices and circuits. By considering factors like material, length, cross-sectional area, and temperature, engineers can optimise resistance to enhance energy efficiency, safety, and control in various applications.
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Frequently asked questions
Electrical resistance is a material's opposition to the flow of electric current. It is caused by the movement of electrons and is essential for controlling and managing electrical energy.
Electrical resistance is calculated using the potential difference (voltage) across a device or material, and the current flowing through it. The formula for this is R=V/I, where R is resistance, V is voltage, and I is the current.
The factors that influence electrical resistance include the material, length, cross-sectional area, and temperature. For example, a long, thin wire will have higher resistance than a short, thick wire of the same material.
