What is a Busbar?

An ampere is the standard unit of electric current in the International System of Units (SI). It measures the flow of electric charge in a circuit, with one ampere equal to one coulomb of charge passing through a point per second.

What is an Ampere?

The ampere (A) is one of several units used to measure the electromagnetic force between straight, parallel conductors carrying electric current.

✅ Measures electric current or flow of electric charge per second

✅ Defined as one coulomb of charge per second in a conductor

✅ Essential in circuit design, safety, and load calculations

Scientific Definition and Formula

The ampere is defined by the formula:

​

• V is voltage in volts

• R is resistance in ohms

• I is current in amperes

When you explore Ohm’s Law, you'll learn how voltage and resistance influence current using the formula I = V / R.

Safety Considerations

Electric current levels and their effects on the human body:

• 1 mA: barely perceptible

• 5–10 mA: painful shock

• 50 mA: can cause breathing difficulty

• 100 mA: potentially fatal if it passes through the chest

Even small currents, if applied in the wrong way, can be dangerous, especially in wet conditions.

Applications of Amperes

• Power system design: selecting proper wire gauges and protective devices

• Circuit protection: fuses and circuit breakers are rated in amperes

• Electronics: current limits are vital in component design

• Battery ratings: indicate how much current a battery can safely deliver

An ammeter is essential for measuring current directly in amperes within a circuit.

Although the ammeter can measure electric flow in coulombs per second, it is calibrated or marked in amperes. For most practical applications, the term amperes is used instead of coulombs per second when referring to the amount of current flow. Note the use of the prefixes micro and milli to represent very small amounts of current and kilo and mega to represent very large amounts.  The article on the ampere explains why one coulomb per second is foundational to electrical theory. Exploring power factor reveals how reactive energy and real power interact in systems with large currents.

A current of a few milliamperes will give you a startling shock. About 50 mA will jolt you severely, and 100 mA can cause death if it flows through your chest cavity.

An ordinary 100-watt light bulb draws a current of about 1 A. An electric iron draws approximately 10 A; an entire household normally uses between 10 A and 50 A, depending on the size of the house, the types of appliances it has, and also the time of day, week, or year. Learning about the watt helps readers see how power (watts) relates to current (amperes) and voltage.

The amount of current that flows in an electrical circuit depends on both the voltage and the resistance. There are some circuits in which extremely large currents, say 1000 A, flow; this might happen through a metal bar placed directly at the output of a massive electric generator. The resistance is extremely low in this case, and the gen­erator is capable of driving huge amounts of charge. In some semiconductor electronic devices, such as microcomputers, a few nanoamperes are often sufficient for many complex processes. Some electronic clocks draw so little current that their batteries last as long as they would if left on the shelf without being used at all. Reading about electricity safety shows why even small currents—measured in amperes—can pose serious hazards.

Related Articles

• Electricity Fundamentals

• Electricity Questions

• Electricity History

It is produced by converting various energy sources, such as fossil fuels, nuclear, solar, wind, or hydro, into electrical energy using turbines and generators. These systems harness mechanical or chemical energy and transform it into usable power.

How Is Electricity Generated?

✅ Converts energy sources like coal, gas, wind, or sunlight into power

✅ Uses generators driven by turbines to create electrical current

✅ Supports global power grids and industrial, commercial, and residential use

Understanding Electricity Generation

Electricity generation is the lifeblood of modern civilization, powering homes, industries, hospitals, transportation systems, and digital infrastructure. But behind the flip of a switch lies a vast and complex process that transforms raw energy into electrical power. At its core, electricity is generated by converting various forms of energy—mechanical, thermal, chemical, or radiant—into a flow of electric charge through systems engineered for efficiency and reliability.

Understanding the role of voltage is essential in this process, as it determines the electrical pressure that drives current through circuits.

According to the Energy Information Administration, the United States relies on a diverse mix of technologies to produce electric power, including fossil fuels, nuclear power, and renewables. In recent years, the rapid growth of solar photovoltaic systems and the widespread deployment of wind turbines have significantly increased the share of clean energy in the national grid. These renewable systems often use turbines to generate electricity by converting natural energy sources—sunlight and wind—into mechanical motion and ultimately electrical power. This transition reflects broader efforts to reduce emissions while meeting rising electric power demand.

How Power Generation Works

Most electricity around the world is produced using turbines and generators. These devices are typically housed in large-scale power plants. The process begins with an energy source—such as fossil fuels, nuclear reactions, or renewable inputs like water, wind, or sunlight—which is used to create movement. This movement, in turn, drives a turbine, which spins a shaft connected to a generator. Inside the generator, magnetic fields rotate around conductive coils, inducing a voltage and producing alternating current (AC) electricity. This method, known as electromagnetic induction, is the fundamental mechanism by which nearly all electric power is made.

In designing and maintaining electrical systems, engineers must also consider voltage drop, which can reduce efficiency and power quality. You can evaluate system losses using our interactive voltage drop calculator, and better understand the math behind it using the voltage drop formula.

Energy Sources Used in Power Production

Steam turbines remain the dominant technology in global energy production. These are especially common in plants that burn coal, natural gas, or biomass, or that rely on nuclear fission. In a typical thermal power plant, water is heated to create high-pressure steam, which spins the turbine blades. In nuclear facilities, this steam is generated by the immense heat released when uranium atoms are split. While highly efficient, these systems face environmental and safety concerns—greenhouse gas emissions from fossil fuels, radioactive waste and accident risk from nuclear power.

Power quality in these plants can be impacted by voltage sag, which occurs when systems experience a temporary drop in electrical pressure, often due to sudden large loads or faults. Managing such variations is crucial to stable output.

The Rise of Renewable Energy in Electricity Generation

Alongside these large-scale thermal technologies, renewable sources have grown significantly. Hydroelectric power harnesses the kinetic energy of falling or flowing water, typically from a dam, to spin turbines. Wind energy captures the movement of air through large blades connected to horizontal-axis turbines. Solar power generates electricity in two distinct ways: photovoltaic cells convert sunlight directly into electric power using semiconductors, while solar thermal plants concentrate sunlight to heat fluids and produce steam. Geothermal systems tap into the Earth’s internal heat to generate steam directly or via heat exchangers.

These renewable systems offer major advantages in terms of sustainability and environmental impact. They produce no direct emissions and rely on natural, often abundant energy flows. However, they also face limitations. Solar and wind power are intermittent, meaning their output fluctuates with weather and time of day. Hydropower and geothermal are geographically constrained, only viable in certain regions. Despite these challenges, renewables now account for a growing share of global electricity generation and play a central role in efforts to decarbonize the energy sector.

In areas where water and electricity coexist—such as hydroelectric plants—understanding the risks associated with water and electricity is critical to ensure operational safety and prevent electrocution hazards.

Generators and Turbines: The Heart of Electricity Generation

Generators themselves are marvels of electromechanical engineering. They convert rotational kinetic energy into electrical energy through a system of magnets and copper windings. Their efficiency, durability, and capacity to synchronize with the grid are critical to a stable electric power supply. In large plants, multiple generators operate in parallel, contributing to a vast, interconnected grid that balances supply and demand in real-time.

Turbines, powered by steam, water, gas, or wind, generate the rotational force needed to drive the generator. Their design and performance have a significant impact on the overall efficiency and output of the plant. Measuring output accurately requires devices like a watthour meter or wattmeters, which are standard tools in generation stations.

Technicians often use formulas such as Watt’s Law to determine power consumption and verify performance. Understanding what ammeters measure also plays a role in monitoring electrical current flowing through generator systems.

Related Articles

• Electricity Fundamentals

• Electricity Questions

• Electricity History

Current electricity is the flow of electric charge through a conductor, usually as moving electrons. It powers devices in homes and industries. Driven by voltage, this flow enables the transfer of energy through wires, forming the basis of modern electrical systems.

What is Current Electricity?

Current electricity refers to the continuous movement of electric charges (typically electrons) through a conductor, driven by an electromotive force (EMF), or voltage.

✅ Describes the movement of electric charge through a conductor

✅ Powers electrical devices in homes, industries, and electronics

✅ Exists as direct current (DC) or alternating current (AC)

Current is defined as an electrical charge in motion. It consists of a flow of negatively charged electrons from atom to atom through a conductor in an electrical circuit. The external force that causes the electron current flow of electric charge is called the electromotive force (emf) or voltage.

When charges move through these wires, they generate different forms of energy, which can power devices and systems. One common type of current is alternating current (AC), where the direction of the charge flow periodically reverses. This movement of charges can be compared to water flowing through a pipe, where the electrical potential pushes the flow, making it essential for transporting energy efficiently across distances.

The Quantitative Definition

Electric current is defined as the rate of flow of electric charge:

I = Q / t

Where:

• I is current in amperes (A)

• Q is charge in coulombs (C)

• t is time in seconds (s)

For example, a current of 1 ampere means 1 coulomb of charge flows per second through a given point in the circuit.

Types of Current: AC and DC

• Direct Current (DC): Charge flows in one constant direction. Example: Batteries.

• Alternating Current (AC): Charge reverses direction periodically. Example: Household wall outlets (typically 60 Hz in North America).

How Current Electricity Works

Electric current arises when free electrons move through a conductor. In metals, these electrons are loosely bound and can drift when a voltage is applied. This movement is like water flowing through a pipe — voltage acts as the pressure that pushes the electrons along.

Voltage sources such as batteries or generators create this pressure, known as electromotive force (emf). The electrons flow from the negative terminal (excess electrons) toward the positive terminal (electron deficiency).

Effects of Electric Current

Electric current produces multiple observable effects:

• Magnetic effect: Current creates magnetic fields (used in motors and transformers).

• Thermal effect: Electric current generates heat (used in heaters and toasters).

• Chemical effect: In electrolysis, current causes chemical reactions.

These effects make current electricity essential for thousands of real-world applications.

Ohm’s Law and Resistance

Ohm’s Law defines the relationship between voltage (V), current (I), and resistance (R):

V = I × R

Where:

• V is voltage in volts

• I is current in amperes

• R is resistance in ohms (Ω)

This relationship helps calculate circuit behavior and select the right components for safe and effective electrical design.

Measuring Current

Current is measured using devices such as:

• Ammeter: Installed in series with a circuit to measure current directly.

• Clamp meter: Measures current without disconnecting wires, using magnetic field sensing.

• Multimeter: A general-purpose tool that can measure voltage, resistance, and current.

Real-World Applications

Current electricity is present in virtually every aspect of modern life:

• Lighting systems

• Household appliances

• Industrial machinery

• Electric vehicles

• Cell phones and computers

• Medical equipment

• Power grids and transmission lines

It also flows in biological systems, such as ion currents in nerve cells and plasma currents in scientific and industrial applications.

Static vs. Current Electricity

Although both involve electric charges, static electricity refers to charges at rest, whereas current electricity refers to the motion of those charges. Static electricity may build up and discharge suddenly, but only current electricity performs continuous work, such as powering a device or generating heat.

Frequently Asked Questions

What is an example?

Examples of current electricity are starting a car, turning on a light, cooking on an electric stove, watching TV, shaving with an electric razor, playing video games, using a phone, charging a cell phone and more. 

Is current and electricity the same?

Current is a general characteristic of alternating current AC, like voltage and resistance. It's a name given to the flow of electrons (charge) through a medium. Electricity is a generic term used to describe the common flow of energy that powers all electrical devices. Current, in general, refers to any flow.

What are the two types of current electricity?

There are two different types of current in widespread use today. They are direct current, abbreviated DC, and alternating current, which is abbreviated AC. In a direct current, the electrons flow in one direction.

The electromotive force (emf) or voltage, as explained by Ohm's law, is created by a battery that consists of one positively charged and one negatively charged terminal. The negative terminal has an excess of electrons, while the positive terminal has a deficiency of electrons. When a conductor, in this case an electric light bulb, is connected to the two terminals of the battery, a flow of electrons occurs, producing both magnetic fields and an electric field. The positive terminal of the battery has a shortage of electrons and thus attracts electrons from the conductor. The negative terminal has an excess of free electrons to move, which repels electrons into the conductor.

Although "static" and "current" may seem different, they are really the same—both consist of electrical charges. Static electricity consists of electrons at rest on an insulated object and does little work. CE charge flows and does useful work. When static electricity is discharged, it is no longer static electricity — it is actually current electricity.

CE may also be classified as either direct current (DC) or alternating current (AC) based on the voltage source. Direct current voltage produces a constant flow of electrons in one direction only. Alternating current voltage produces a flow of positive and negative charge electrons in both directions, with varying magnitude. A battery is a common DC voltage source, while an electrical wall outlet is the most common AC voltage source (Figure 1).

Polarity identification (+ or -) and magnetic fields are one way to distinguish between a voltage source and a current source. Polarity can be identified on direct current circuits, but in alternating current circuits, the current continuously reverses direction; therefore, polarity cannot be identified. Additionally, it is essential to determine whether an electrical energy source generates alternating current or direct current. Many control and load components are designed to operate with a specific type of current. Operating the components with the wrong type of current or DC polarity can result in improper operation and/or permanent damage to the component.

Fig. 1. Current Electricity. Electrical charges moves.

Related Articles

• Electricity Fundamentals

• Electricity Questions

• Electricity History

A capacitor is an electrical component that stores and releases energy in a circuit. It consists of two conductive plates separated by an insulator and is commonly used for filtering, power conditioning, and energy storage in electronic and electrical systems.

What is a Capacitor?

A capacitor is a key component in electronics and power systems. It temporarily stores electrical energy and is widely used in both AC and DC circuits.

✅ Stores and discharges electrical energy efficiently

✅ Used in filtering, timing, and power factor correction

✅ Found in electronics, motors, and power supplies

It is designed for energy storage and can store electric charges, which can be released when needed. In this article, we will delve into the fundamentals of capacitors, including their functions, types, and applications. To better understand how capacitors support overall system performance, explore our Power Quality overview covering the fundamentals of voltage stability and energy flow.

Power Quality Analysis Training

Power Factor Training

Request a Free Power Quality Training Quotation

A capacitor consists of two metallic plates separated by an insulating material known as the dielectric. The dielectric can be made from various materials, such as mica, paper, or ceramic. When voltage is applied across the plates, positive charges accumulate on one plate, while negative charges accumulate on the opposite plate. The amount of capacitor charge that can be stored depends on several factors, including plate area, plate separation, dielectric material, and voltage ratings. Capacitors are often used in capacitor banks to improve power factor and reduce energy losses in electrical systems.

How does a capacitor work? The primary function of a capacitor in an electronic circuit is to store electrical energy. Capacitors can be used for various purposes, such as filtering, timing, and coupling or decoupling signals. In addition, they play a crucial role in power supplies, ensuring that the output voltage remains stable even when there are fluctuations in the input voltage. Learn how capacitive loads influence circuit behavior and why they require precise capacitor selection for optimal performance.

A capacitor stores energy through the electrostatic field created between its plates. The stored energy can be calculated using the formula E = 0.5 * C * V^2, where E is the stored energy, C is the capacitance, and V is the voltage across the capacitor. Capacitance, measured in Farads, is a measure of a capacitor's ability to store charge. The capacitor voltage rating is crucial for ensuring safe operation and preventing dielectric breakdown during voltage spikes.

So, when I am asked what is a capacitor? I tell readers about several types of capacitors, each with unique applications. Common types include ceramic, electrolytic, film, and tantalum capacitors. Ceramic capacitors are widely used due to their low cost and small size. They are ideal for high-frequency applications and decoupling in power supply circuits. On the other hand, Electrolytic capacitors are popular for their high capacitance values and are commonly used in filtering and energy storage applications. Capacitors play a crucial role in power factor correction, enabling industrial systems to reduce demand charges and enhance energy efficiency.

Dielectric materials used in capacitors can be organic (such as paper) or inorganic (such as ceramic). The choice of dielectric material depends on factors like the desired capacitance value, voltage rating, and operating temperature range. Additionally, different dielectric materials exhibit varying properties, making them suitable for specific applications. For a deeper understanding of energy relationships, see how apparent power differs from real and reactive power in systems using capacitors.

A capacitor can be classified as polarized or non-polarized based on the presence or absence of polarity. Polarized capacitors, like electrolytic capacitors, have a positive and a negative terminal and must be connected correctly in a circuit to function properly. Non-polarized capacitors, like ceramic capacitors, do not have a specific polarity and can be connected in any orientation.

A Capacitor behaves differently in AC and DC voltage circuits. In DC circuits, once a capacitor is charged, it blocks the flow of current, essentially acting as an open circuit. However, in ac voltage circuits, capacitors allow the flow of alternating current. This phenomenon is known as displacement current, which occurs due to the continuous charging and discharging of charges.

So, what is a capacitor? Understanding what a capacitor is and how it works is essential for anyone interested in electronics. The Capacitor plays a vital role in a wide range of applications, from energy storage and filtering to signal coupling and decoupling. Understanding the various types of capacitors and their specific applications enables you to make informed decisions when designing or troubleshooting electronic circuits. Explore how an automatic power factor controller dynamically adjusts capacitor usage to maintain an efficient power factor in real-time.

Related Articles

• Power Quality Channel

An arc fault is a dangerous electrical discharge between conductors or to ground. It generates intense heat and light, often caused by damaged insulation, frayed wires, or loose connections, posing major electrical safety and fire hazards.

What is an Arc Fault?

An arc fault is an unintended electrical discharge that occurs when insulation or wiring fails, producing dangerous heat that can ignite fires and damage circuits.

✅ Caused by frayed wires or loose connections

✅ Produces intense heat and light energy

✅ Prevented by Arc Fault Circuit Interrupters (AFCIs)

Basic Protection Relay Training

Short Circuit Study Training

Request a Free Training Quotation

Understanding Arc Faults and Electrical Safety

An arc fault is a hazardous electrical event that can lead to severe consequences, including fires and substantial property damage. Understanding how faults occur, how to prevent them, and why protective measures like Arc Fault Circuit Interrupters (AFCIs) are essential can significantly improve home and workplace safety.

When electrical current jumps across a gap or an unintended path, it forms an electric arc. This arc generates extremely high temperatures—often exceeding 10,000°F—capable of igniting nearby insulation, wood framing, or other combustible materials. Faults are typically caused by damaged, frayed, or aging wiring, loose terminal connections, or punctured cables from nails and screws during construction. For more insight into advanced safety devices, learn how an arc fault interrupter breaker detects hazardous arcing and disconnects power before a fire can start.

Arc fault protection is especially important in areas where people live and spend time, such as family rooms, dining rooms, and living rooms, where electrical wiring runs behind walls containing materials such as wood framing or insulation that can easily ignite. Modern safety standards, as mandated by the National Electrical Code, require the installation of Arc Fault Circuit Interrupters (AFCIs) in these spaces to prevent fires caused by faults. When combined with Ground Fault Circuit Interrupters, which protect against electrical shock, AFCIs provide comprehensive protection against both fire and shock hazards in residential and commercial environments.

Types of Arc Faults

Arc faults can appear in different forms, each with its own risks and detection requirements:

• Series Faults – Occur along a single conductor, usually from a broken wire or loose terminal. These arcs produce less current but can still ignite fires.

• Parallel Faults – Form between two conductors (hot-to-neutral or hot-to-ground). These faults create higher current levels and more intense arcing.

• Ground Faults – Happen when current leaks or shorts to a grounded surface, such as a metal outlet box or appliance casing. Explore how ground fault protection complements AFCIs by guarding against current leakage that could cause electric shock or parallel arc conditions.

Recognizing these types helps electricians and inspectors identify the right protection strategies and select appropriate AFCI devices. To see how fault current behavior impacts fault risks, review our explanation of available fault current and why accurate short-circuit studies are essential for system safety.

How AFCI Detection Works

AFCIs are intelligent safety devices designed to detect the unique electrical signatures of faults. They continuously monitor current waveforms and frequencies, distinguishing dangerous arcs from normal switching arcs (such as those produced by light switches or vacuum cleaners).

When an AFCI identifies an abnormal frequency pattern consistent with arcing, it trips the circuit within milliseconds—disconnecting power before the fault can ignite a fire. This advanced “signature detection” technology is required by modern safety codes and has saved countless lives and properties. For more insight into advanced safety devices, learn how an arc fault interrupter breaker detects hazardous arcing and disconnects power before a fire can start.

Limitations and Nuisance Tripping

While AFCIs are highly effective, they can occasionally cause nuisance tripping. This occurs when the device misinterprets harmless electrical noise as a fault, typically triggered by motors, dimmers, or other electronic devices. Regular inspection, proper grounding, and updated AFCI models help minimize these false positives. If nuisance tripping persists, it’s advisable to have an electrician verify circuit wiring and device compatibility. To understand how electrical systems respond to fault conditions, refer to our detailed explanation of protective relay coordination, which ensures that circuit breakers isolate faults without disrupting unaffected circuits.

Code Requirements and Standards

Arc fault protection is mandated by both U.S. and Canadian electrical codes:

• National Electrical Code (NEC 210.12) requires AFCI protection for all 120-volt, single-phase, 15- and 20-amp branch circuits supplying living areas such as bedrooms, family rooms, dining rooms, and similar spaces.

• Canadian Electrical Code (CEC Section 26) similarly mandates AFCI in dwelling units.

• IEEE 1584 provides calculation guidelines for flash hazards in industrial power systems, complementing residential and commercial fault safety standards.

Following these standards ensures compliance and dramatically reduces fire risks across residential, commercial, and industrial applications.

Statistics and Case Studies

According to the U.S. Consumer Product Safety Commission (CPSC), electrical fires cause over 51,000 residential fires annually, resulting in more than 500 deaths and $1.3 billion in property damage. Studies show that AFCI protection can prevent more than half of these incidents, highlighting its critical role in modern electrical safety systems.

Emerging Technologies in Arc Fault Detection

New generations of AFCIs utilize microprocessors and artificial intelligence to enhance accuracy and minimize false trips. These smart devices analyze waveform patterns with greater precision, detecting high-impedance arcs and subtle irregularities. Future technologies may integrate predictive analytics and IoT monitoring to diagnose potential faults before they become hazards. Finally, explore comprehensive methods of electrical surge protection, which safeguard sensitive equipment from voltage spikes often linked to lightning events.

Common Causes of Arc Faults

• Damaged or aging electrical wiring

• Loose terminal connections in outlets or switches

• Overloaded circuits or faulty appliances

• Nails or screws penetrating electrical cables

• Deteriorated insulation from heat, moisture, or rodents

Regular maintenance and periodic inspections by a licensed electrician are essential preventive measures.

Arc Fault vs Ground Fault vs Short Circuit

Understanding these differences helps ensure that electrical protection systems are properly matched to the specific hazards they are intended to address.

Frequently Asked Questions

Why does my AFCI keep tripping?

Often due to electronic interference, shared neutrals, or actual wiring issues. Replace outdated AFCIs and consult a professional if tripping persists.

Can I retrofit AFCIs into older panels?

Yes. AFCI breakers can replace standard breakers in most modern panels. Have a qualified electrician confirm compatibility before installation.

Are AFCIs required everywhere?

While required in most living spaces, some regions exempt areas like garages or unfinished basements. Check the NEC or CEC requirements for your jurisdiction.

Related Articles

• Electrical Protection Channel

Electrical resistance is the measure of how much a material opposes the flow of electric current. Measured in ohms (Ω), it affects voltage, limits current, and plays a vital role in circuit function, energy loss, and power distribution.

What is Electrical Resistance?

Electrical resistance is a key concept in electronics that limits the flow of electricity through a conductor.

✅ Measured in ohms (Ω) to indicate opposition to current flow

✅ Affects voltage, current, and overall power efficiency

✅ Essential in designing safe and effective electrical circuits

Electrical Resistance is an electrical quantity that measures how a device or material reduces the flow of electric current through it. The resistance is measured in units of ohms (Ω). If we make an analogy to water flow in pipes, the resistance is greater when the pipe is thinner, so the water flow is decreased.

Electrical Resistance is a measure of the opposition that a circuit offers to the flow of electric current. You might compare it to the diameter of a hose. In fact, for metal wire, this is an excellent analogy: small-diameter wire has high resistance (a lot of opposition to current flow), while large-diameter wire has low resistance (relatively little opposition to electric currents). Of course, the type of metal makes a difference, too. Iron wire has higher resistance for a given diameter than copper wire. Nichrome wire has still more resistance.

Electrical resistance is the property of a material that opposes the flow of electric current. The resistance of a conductor depends on factors such as the conducting material and its cross-sectional area. A larger cross-sectional area allows more current to flow, reducing resistance, while a smaller area increases it. The unit of electrical resistance is the ohm (Ω), which measures the degree to which a material impedes the flow of electric charge. Conductors with low resistance are essential for efficient electrical systems.

What causes electrical resistance?

An electric current flows when electrons move through a conductor, such as a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance.

Why is electrical resistance important?

Therefore, it is sometimes useful to add components called resistors into an electrical circuit to restrict the flow of electricity and protect the components in the circuit. Resistance is also beneficial because it allows us to shield ourselves from the harmful effects of electricity.

The standard unit of resistance is the ohm. This is sometimes abbreviated by the upper-case Greek letter omega, resembling an upside-down capital U (Ω). In this article, we'll write it out as "ohm" or "ohms."

You'll sometimes hear about kilohms, where 1 kilohm = 1,000 ohms, or about megohms, where 1 megohm = 1,000 kilohms = 1,000,000 ohms.

Electric wire is sometimes rated for resistivity. The standard unit for this purpose is the ohm per foot (ohm/ft) or the ohm per meter (ohm/m). You may also encounter the unit of ohms per kilometre (ohm/km). 

When an IV is placed across 1 ohm of resistance, assuming the power supply can deliver an unlimited number of charge carriers, there will be a current of 1 A. If the re­sistance is doubled, the current is halved. If the resistance is cut in half, the current doubles. Therefore, the current flow, for a constant voltage, is inversely proportional to the resistance. 

Typically, four-terminal resistors are used to measure current by measuring the voltage drop between the voltage terminals with current flowing through the current terminals. These standards, designed for use with potentiometers for precision current measurement, correspond in structure to the shunts used with millivoltmeters for current measurement with indicating instruments. Current standards must be designed to dissipate the heat they develop at rated current, with only a small temperature rise. They may be oil- or air-cooled; the latter design has a much greater surface area, as heat transfer to still air is less efficient than to oil. An air-cooled current standard with a 20 μω resistance and 2000 A capacity has an accuracy of 0.04%. Very low-resistance oil-cooled standards are mounted in individual oil-filled containers, provided with copper coils through which cooling water is circulated and with propellers to provide continuous oil motion.

Related Articles

• Electricity Fundamentals

• Electricity Questions

• Electricity History