Trystan Casey
The ohm, represented by the Greek letter Ω, is the standard unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm, who, in 1827, published a treatise describing measurements of applied voltage and current through simple electrical circuits. The ohm is used to measure resistance in both direct current (DC) and alternating current (AC) circuits. It is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces a current of one ampere (A).
| Characteristics | Values |
|---|---|
| Full Form | ohm (symbol: Ω, the uppercase Greek letter omega) |
| Named After | German physicist Georg Ohm (1789–1854) |
| Definition | Electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A), the conductor not being the seat of any electromotive force |
| Application | Both direct current (DC) and alternating current (AC) |
| Related Concepts | Voltage (V), Current (I), Resistance (R) |
| Formula | Ohm's Law (E = IR), V = IR, or V/I = R |
| Use Cases | Used to calculate the relationship between voltage, current and resistance in an electrical circuit |
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What You'll Learn
Ohm's Law
The ohm (Ω), named after German physicist Georg Ohm, is the unit of electrical resistance in the International System of Units (SI). It is used to measure resistance in both direct current (DC) and alternating current (AC) circuits.
The law states that there is a proportional relationship between voltage, current, and resistance in an electrical circuit. In other words, it defines the voltage or potential difference in a circuit as the product of the current flowing in the circuit and the resistance offered by it. Mathematically, it is represented as:
- Voltage (V) = Current (I) x Resistance (R)
- Volts (V) = Amps (A) x Ohms (Ω)
- V = A x Ω
If two of the values are known, the third can be calculated using Ohm's Law. For example, if the voltage (V) and current (I) are known, the resistance (R) can be calculated by rearranging the equation as V/I = R.
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Georg Ohm
The ohm, the unit of electrical resistance in the International System of Units (SI), is named after German physicist Georg Ohm. Born in 1789 in Erlangen, Bavaria (now part of Germany), Georg Ohm came from humble roots and struggled financially for most of his life. He was the son of a locksmith and was initially educated by his father, who had a vast knowledge of various subjects despite his lack of formal schooling. At the age of eleven, Georg and his brother Martin started attending Erlangen Gymnasium, where they received little scientific training. This prompted their father to send Georg to Switzerland, where he took up a position as a mathematics teacher in Gottstadt bei Nidau in 1806.
In 1805, Georg Ohm began studying at the University of Erlangen, where he demonstrated a strong understanding of advanced mathematics. However, he did not dedicate enough time to his studies, which displeased his father. As a result, Ohm dropped out after three semesters and moved to Switzerland, marking the start of a long line of teaching positions that he would hold throughout his life. During this time in Switzerland, he worked as a mathematics teacher and later as a private tutor in Neuchâtel from 1809 to 1811, while continuing his private studies in mathematics.
In 1811, Georg Ohm returned to the University of Erlangen, obtaining his doctorate a year later. He joined the faculty as a lecturer in mathematics but left after three semesters due to low pay. In 1813, he took up a teaching position in Bamberg, Bavaria, where he taught mathematics and physics. Despite his dissatisfaction with the job, it was during this period that he penned an elementary textbook on geometry, showcasing his abilities. After the school's closure in 1816, Ohm moved to another teaching post in Bamberg to teach mathematics.
In 1817, Ohm became a professor of mathematics at the Jesuits' College in Cologne, where he had access to a well-equipped physics laboratory. This allowed him to begin experimenting with physics, building on his practical experience with mechanical devices gained from his father's work as a locksmith. During this time, he published "Die galvanische Kette, mathematisch bearbeitet" ("The Galvanic Circuit Investigated Mathematically") in 1827, which included his electromagnetic theories and the components of what would become known as Ohm's Law.
Ohm's Law is a fundamental formula used to calculate the relationship between voltage, current, and resistance in an electrical circuit. It states that there is a proportional relationship between voltage, current, and resistance, and it is expressed as I=V/R. This law is applicable to both direct current (DC) and alternating current (AC) circuits.
The unit of electrical resistance, the ohm, was named in honour of Georg Ohm, recognising his significant contributions to the field of electricity.
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Electrical resistance
The ohm (Ω), named after German physicist Georg Ohm, is the unit of electrical resistance in the International System of Units (SI). It is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A), provided that the conductor is not the seat of any electromotive force. In other words, resistance is the measure of how much an object or substance "resists" an electric current.
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is electrical conductance, which measures how easily an electric current passes. Electrical resistance shares some conceptual parallels with mechanical friction. The SI unit of electrical conductance is the siemens (S). Objects made of electrical insulators like rubber tend to have very high resistance and low conductance, while objects made of electrical conductors like metals tend to have very low resistance and high conductance.
Resistance is influenced by several factors, including the length over which a current must travel, the cross-sectional area of the substance, the type and temperature of the substance, and the voltage. Conductors with a large cross-sectional area have low resistance since electricity flows more readily through them, while conductors with a small cross-sectional area have higher resistance. Resistance decreases with temperature, while increased temperature results in increased resistance. The relationship between voltage and current is described by Ohm's Law, which states that there is a proportional relationship between voltage, current, and resistance in an electrical circuit.
Resistance is also dependent on the material the object is made of. This relationship is quantified by resistivity or conductivity. Materials with high resistivity, such as rubber, have high resistance, while materials with high conductivity, such as metals, have low resistance. Examples of ohmic materials that follow Ohm's Law include wires and resistors. Resistors are electronic components that resist the flow of electricity in a circuit, allowing for the control of current and voltage.
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DC and AC circuits
The ohm (Ω) is the unit of electrical resistance in the International System of Units (SI). It is used to measure the standard unit of electrical resistance in both direct current (DC) and alternating current (AC) circuits.
DC Circuits
In DC circuits, electric charge flows in only one direction and does not oscillate back and forth. Voltage (E), current (I), and resistance (R) are the three important quantities used to measure a DC charge. Voltage is the difference in charge between two points, caused by the pressure that forces the current to flow, and it is measured in volts. Current is the rate at which the current flows, measured in amperes or amps. Resistance is the rate at which a material resists the current's flow, measured in ohms.
AC Circuits
In AC circuits, the electric current can reverse direction, and the voltage periodically reverses as well. This is in contrast to DC circuits, where the voltage and current remain constant. Measuring resistance in AC circuits is more complex than in DC circuits because other components can inhibit the current's flow. Instead of solely measuring resistance, the circuit's impedance must be measured, which includes inductance and capacitance. Inductance (XL) is the amount of impedance that can occur when an AC charge generates an electromagnetic field (EMF) that opposes the current, and it is often generated by components such as inductors. Capacitance (XC) is the amount of electrical charge stored in the circuit, and it is often generated by components such as capacitors. Both inductance and capacitance are measured in ohms, and together they are referred to as the circuit's reactance (X), also measured in ohms.
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Symbol Ω
The ohm, represented by the uppercase Greek letter omega (Ω), is the unit of electrical resistance in the International System of Units (SI). It is used to measure resistance in both direct current (DC) and alternating current (AC) circuits.
In a DC circuit, the electric charge flows in only one direction and does not oscillate, unlike in an AC circuit. In a circuit with a 9-volt battery and a small lightbulb, for example, the resistance created by the lightbulb can be calculated using Ohm's Law, which states that voltage = current x resistance, or volts = amps x ohms (V = A x Ω).
Ohm's Law, named after German physicist Georg Ohm, is a fundamental formula in electronics that describes the relationship between voltage, current, and resistance in an electrical circuit. It allows technicians to calculate the unknown value of one of these three quantities if the other two are known. For instance, in the previous example, the 9-volt circuit with a 0.5-amp current has a resistance of 18 ohms.
The ohm is also used in AC circuits to measure impedance, which includes inductance and capacitance. Inductance occurs when an AC charge generates an electromagnetic field (EMF) that opposes the current, and capacitance refers to the amount of electrical charge stored in the circuit. Both inductance and capacitance are measured in ohms and together are referred to as the circuit's reactance, also measured in ohms.
The use of the symbol Ω for ohm is recommended by the IEEE 260.1 standard when the character set is limited to ASCII. In the electronics industry, it is common to use the character "R" to represent ohms, so a 10-ohm resistor may be written as 10R.
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Frequently asked questions
The ohm (Ω) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm.
Ohm's Law is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit. The formula is E = IR, where E is voltage, I is current, and R is resistance.
Resistance is calculated by dividing voltage by current. In other words, if you know the voltage (V) and current (I) of a component, you can calculate its resistance (R) using the equation R = V/I.
DC circuits carry a direct current, meaning the electric charge flows in only one direction. In AC circuits, the electric current can reverse direction, resulting in a sinusoidal waveform. As a result, measuring resistance in AC circuits requires considering the circuit's impedance, which includes inductance and capacitance.
