Siemens
Resistance in electricity is analogous to friction in mechanical physics. Due to friction, mechanical energy is lost in the form of heat. Similarly, due to resistance, the electrical energy is lost as heat. The reciprocal of resistance is electrical conductance and is represented by G = 1/R, where G is the electrical conductance and R is the resistance.
Electrical conductance denotes how easily electricity can pass through an electrical component for a given voltage difference. Electrical conductance is the property of a specific electrical element (for example a particular wire) and conductance is closely related to electrical conductivity (which is the property of the material for example silver wire).
The older SI unit of conductance used to be mho which has been replaced by Siemens (S). This article will look into the basics of electrical conductivity, Ohm’s law and discuss what is unit of conductance.
Ohm’s Law
Ohm’s law expresses the relation between voltage (potential difference V), electric current (I), and resistance (R) in mathematical form. A material can obey Ohm’s law if the resistance of the system is a constant i.e. the voltage and current are proportional to each other. As per Ohm’s law:
I = V/R, where I (current in the circuit) is measured in amperes, V (potential difference) is measured in volts, and R (resistance) is measured in ohms. If this ratio of voltage to current remains constant over a wide range of voltages then the conducting material is said to be an “ohmic” material.
Electrical Conductivity
The property of a material that determines how well it can conduct electricity is referred to as its electrical conductivity. Electrical conductivity and resistivity are closely related so first let us understand what is the resistance of an object.
In a qualitative sense, resistance defines how difficult it is for electricity to pass through a component. In quantitative terms, the resistance between two points is defined as the difference in voltage between the points to carry a unit of current across those points.
When a current of 1 A (ampere) passes through an electric component with a potential difference of 1 V (volts) then the component is said to have a conductance of 1 S (Siemens).
The equation of electrical conductance is G = I/E, Where G denotes electrical conductance, I is the current in ampere and E is the voltage in volts.
Factors Affecting Electrical Conductivity
Electrical conductivity is determined largely by the number of electrons in the outermost shell of the conducting material. These electrons determine how easily mobile electrons are generated. The number of atoms per unit volume is also a small factor in determining the electrical conductivity of a material. These atoms give an idea about how many electrons would readily move when placed in an electric field.
like copper and aluminium have high conductivity and are called conductors while materials like rubber and glass have low conductivity and are called electric insulators. There is another special category of materials, for example, germanium and silicon, which are called semiconductors whose conductivity is in between conductors and insulators.
In general, metals have high conductivity and tend to be conductors of electricity since their outermost shell’s electrons can move easily. Non-metals usually have low conductivity.
What is the Unit of Conductance?
A Siemens unit in a direct current (DC) is equivalent to 1 Ampere per volt (reciprocal of resistance in ohms) while in the case of alternating current (AC), the Siemens conductance unit is reciprocal of impedance. If the applied voltage is constant then the current in DC is directly proportional to conductance i.e. if the current is doubled, conductance is also doubled, and similarly, if conductance is cut to 1/5th, current also reduces to 1/5th of its value. The same rule applies to many low-frequency AC systems for example any household utility circuits. But, in some AC circuits with high frequency, the situation gets more complex since some components in these systems store and release energy which is accompanied by dissipation and conversion of energy. Now, since we know what is the unit of conductance, let us take a look at the old units that were used before the use of Siemens.
History of the Unit of Electrical Conductance
The symbol of the unit of conductance was created to depict conductance and it is the capital letter “G”. The unit of conductance was mho (which is ohm spelt backwards, since conductance is the reciprocal of resistance). But in later years, mho was replaced by the unit of Siemens as the SI unit of electrical conductance which is similar to the change in the unit of temperature from Centigrade to Celsius (even the unit of frequency used to be cps (cycles per second) which later changed to Hertz.). You can see a pattern here since Siemens, Hertz, and Celsius are all surnames of famous scientists.
Conductance and Resistance Relationship
Since conductance is reciprocal of resistance we can figure out that the greater the resistance in a component, the lower would be its electrical conductance (and vice versa). So if there are two components “C1” and “C2” and if the resistance of “C1” is half of that of “C2” then we can easily conclude that “C1” is twice more conductive than “C2”.
How Conductance is Calculated?
If we know the resistance, voltage, current, conductivity, we can easily calculate the conductance of a specific electrical component. Let us see how we can calculate conductance from the parameters provided.
Resistance is known - If a circuit element has a resistance of 1.2 * 103 ohms, we can find its conductance as G = 1/R = 1/(1.2 * 103) = 0.8 * 103 Siemens.
Voltage and current are known - Let us say in an electric circuit a voltage of 5 volts generates a current of 0.3 amperes in a unit length of wire then from Ohm’s law we know V = I * R or R = V/I. Since G = 1/R, G = I/V = 0.3/5 = 0.06 siemens.
The resistivity of the material and area of cross-section of conducting material is known - The relationship between resistivity and G (conductance) is G = (A * σ)/L, where A is the area of cross-section of the wire and σ is its resistivity. If there is an iron rod whose radius is 0.001 meters and length is 0.1 meters we can calculate the conductance as follows:
The value of σ for iron is 1.03 * 107 Siemens/m
Area of the rod = 𝛑 * r2 = 𝛑 * 0.0012 = 3.14 * 10-6.
Conductance = (1.03 * 107 * 3.14 * 10-6)/0.1 = 324 Siemens
FAQs on Siemens - Unit of Energy Measurement
1. What is electrical conductance and what is its SI unit?
Electrical conductance is a measure of how easily electric current can flow through a material or component. It is the reciprocal of electrical resistance. The SI unit for electrical conductance is the siemens, represented by the symbol S. A higher siemens value indicates that the material is a better conductor of electricity.
2. How is one siemen (S) officially defined?
One siemen is defined as the electrical conductance of a component through which a current of one ampere (A) flows when a potential difference of one volt (V) is applied across it. Mathematically, it is the reciprocal of the ohm (Ω), the unit of resistance. Therefore, 1 S = 1 A/V = 1 Ω⁻¹.
3. Is the siemens a unit for measuring energy?
No, this is a common misconception. The siemens (S) is the SI unit of electrical conductance, which measures the ease of current flow. The SI unit for energy is the Joule (J). Conductance and energy are fundamentally different physical quantities; one describes current flow for a given voltage, while the other describes the capacity to do work.
4. What is the key difference between electrical conductance and conductivity?
While related, conductance and conductivity describe different aspects of electrical flow.
- Conductance (G) is a property of a specific object, like a particular wire. It depends on the material's nature as well as its dimensions (length and area). It is measured in siemens (S).
- Conductivity (σ) is an intrinsic property of a material itself, like copper or glass. It describes how well that material conducts electricity, regardless of its shape or size. It is measured in siemens per metre (S/m).
5. How can you calculate the conductance of a component using Ohm's Law?
According to Ohm's Law, resistance is the ratio of voltage to current (R = V/I). Since conductance (G) is the reciprocal of resistance (G = 1/R), you can calculate it directly from voltage and current using the formula G = I/V. For example, if a 6-volt battery causes a current of 2 amperes to flow through a resistor, its conductance is 2 A / 6 V = 0.333 siemens.
6. What is the relationship between the units 'siemens' and 'mho'?
The 'mho' is the older, non-SI unit for conductance. The name 'mho' is simply 'ohm' spelled backwards, creatively highlighting that conductance is the reciprocal of resistance. The unit was officially replaced by the siemens (S) to standardise units within the SI system and to honour the scientist Ernst Werner von Siemens. Both units represent the same quantity, so 1 mho = 1 siemens.
7. Why does the total conductance increase when components are connected in parallel?
When components are connected in parallel, you are essentially providing multiple pathways for the electric current to flow. Each new parallel branch reduces the overall opposition (total resistance) of the circuit. Because conductance is the measure of the ease of flow, adding more paths increases the total ease of flow. Therefore, the total conductance in a parallel circuit is the sum of the individual conductances (G_Total = G₁ + G₂ + ...).
8. How does the concept of conductance apply to ideal insulators and superconductors?
In theoretical physics, ideal materials illustrate the limits of conductance:
- An ideal insulator would have infinite resistance, completely blocking the flow of current. Consequently, its conductance would be zero siemens. Real-world insulators have extremely low, but non-zero, conductance.
- A superconductor is a material that exhibits zero electrical resistance below a certain critical temperature. Since conductance is the reciprocal of resistance (1/0), a superconductor theoretically has infinite conductance.
