AC Current

Electric Current

Electric current is the flow of electric charge through a material. It is the rate at which electric charge flows past a point in a circuit. The flow of electric charge is typically carried by electrons, which are negatively charged particles.

The SI unit for current is the coulomb per second, or the ampere (A), which is an SI base unit: 

1 ampere = 1A = 1 coulomb per second = 1 C/s.

AC Current

AC, or Alternating Current, is a type of electric current that periodically reverses direction, oscillating between positive and negative values. In an AC circuit, the electric charge flows first in one direction and then in the opposite direction, constantly reversing its direction at a certain frequency.

The most common example of AC current is the electricity that is delivered to our homes and businesses by the power grid. AC current is preferred for long-distance power transmission because it can be easily stepped up to high voltages for transmission over long distances with less power loss, and then stepped down to lower voltages for use in homes and businesses.

The frequency of AC current is typically measured in Hertz (Hz), which refers to the number of times the current oscillates back and forth in one second. In the United States, the standard frequency of AC power is 60 Hz, while in some other countries it is 50 Hz.

In addition to power transmission, AC current is also used in a wide range of electrical and electronic devices, from small appliances to industrial machinery. AC current can be converted to DC (Direct Current) using devices like rectifiers, which are commonly used in electronic devices like computers and mobile phones.

AC Current – Measurement

Ammeters are used to measure the electric current flowing through a circuit. There are three basic types of ammeters for measuring AC (Alternating Current) current:

1. Moving Coil Ammeters: Moving coil ammeters use a coil that moves in a magnetic field to measure the current flowing through a circuit. When current flows through the coil, it creates a magnetic field that interacts with the magnetic field of a permanent magnet, causing the coil to move. The movement of the coil is proportional to the current flowing through the circuit, and this movement is converted into a current reading. When an AC with a very low frequency is passed through a PMMC, the pointer tends to follow the instantaneous level of the AC. Therefore, a PMMC instrument connected directly to measure 50Hz AC indicates zero average value. It is important to note that although a PMMC instrument connected to an ac supply may indicating zero, there can actually be very large rms current flowing in its coils. To convert alternating current (AC) to unidirectional current flow, which produces positive deflection when passed through a PMMC, the diode rectifier is used.
2. Digital Ammeters: Digital AC ammeters measure AC current passing through a circuit by converting it into a proportional digital signal. The AC current is passed through a current transformer to step down the current to a safe level for measurement. The output of the current transformer is then rectified to a DC voltage using a rectifier circuit. The DC voltage is then converted into a digital signal using an Analog to Digital Converter (ADC) circuit. The microcontroller receives the digital signal and processes it to display the current value on the screen. The final step is to display the current value on an LED or LCD display.
3. Clamp-On Ammeters: Clamp-on ammeters are designed to measure the current flowing through a wire without having to physically disconnect the wire. The ammeter has a clamp that can be opened and placed around the wire to measure the current flowing through it. Clamp-on ammeters are often used in industrial settings where large wires and cables need to be measured.

When selecting an ammeter for measuring AC current, it is important to choose one that is rated for the maximum current that will be measured. Ammeters that are rated for higher currents will typically have a lower resistance, which can cause a voltage drop in the circuit being measured. Therefore, it is important to select an ammeter that has a low resistance compared to the circuit being measured. It is also important to select an ammeter with a frequency range that matches the frequency of the AC current being measured.

Rectification and AC Ammeters

Rectification is the process of converting an AC (alternating current) signal into a DC (direct current) signal. AC current alternates in polarity, meaning it flows in one direction and then the other, constantly changing direction. This can be visualized as a wave that oscillates both above and below a zero point, which is also known as the “zero crossing.”

In order to convert an AC signal to DC, the negative half of the AC waveform must be eliminated or “rectified.” This is typically done by using a diode, which is a one-way valve for current flow. A diode allows current to flow in one direction, but blocks it from flowing in the other direction. By placing a diode in series with the AC signal, the negative half of the waveform is blocked, effectively turning the AC signal into a unidirectional or DC signal.

There are two types of rectifiers: half-wave rectifiers and full-wave rectifiers. A half-wave rectifier uses one diode to rectify the AC signal, while a full-wave rectifier uses four diodes arranged in a specific pattern to rectify both the positive and negative halves of the AC signal.

Rectification is an important process in electronics because many electronic devices require DC power to operate, while AC power is commonly available in the power grid. Rectification allows AC power to be converted into DC power for use in electronic circuits and devices.

Sources of AC Current

There are several sources of AC (alternating current) current, including:

1. Power Grid: AC current is commonly used for power distribution in the electrical power grid. The electrical power grid is a network of power stations, transformers, and transmission lines that deliver AC power to homes, businesses, and other facilities. The AC current used in the power grid typically has a frequency of 50 or 60 Hz, depending on the country.
2. Generators: AC generators are machines that convert mechanical energy into electrical energy. They generate AC current by rotating a coil of wire inside a magnetic field, which induces an electrical current in the coil. AC generators are used in a variety of industrial applications, such as in power plants, wind turbines, and transportation.
3. Inverters: Inverters are devices that convert DC (direct current) voltage into AC voltage. They are commonly used in renewable energy systems, such as solar and wind power systems, to convert the DC voltage generated by the solar panels or wind turbines into AC voltage for powering homes and businesses.
4. Transformers: Transformers are devices that are used to change the voltage of AC power. They are commonly used in power distribution networks to step up or step down the voltage of AC power for transmission over long distances.

Types of Electric Current

There are three types of electric current:

1. Direct Current (DC): A flow of electric charge that flows in one direction is called direct current. The magnitude and direction of DC remains constant over time.
2. Alternating Current (AC): Alternating current is the flow of electric charge that changes direction periodically. The magnitude and direction of AC varies with time, usually in a sinusoidal pattern.
3. Pulsed DC: Pulsed DC is a type of current that flows in pulses or brief bursts. The pulses may be unidirectional or bidirectional, but they are not continuous like DC or AC currents. This type of current is often used in specialized applications such as welding and electroplating.

How does electric current flow – Mechanisms of the current flow

In electrostatic situations, the electric field is zero everywhere within the conductor, and there is no current. However, this does not mean that all charges within the conductor are at rest. In an ordinary metal such as copper or alumium, some of the electrons are free to move within the conducting material. These free electrons move randomly in all directions, somewhat like the molecules of a gas but with much greater speeds, of the order of 10⁶ m/s. The electrons nonetheless do not escape from the conducting material, because they are attracted to the positive ions of the material. The motion of the electrons is random, so there is no net flow of charge in any direction and hence no current.

When a voltage difference is applied across a conductor, it creates an electric field within the material. The electric field exerts a force on the free electrons within the conductor, causing them to move from areas of high potential energy to areas of lower potential energy. The flow of electrons in response to the applied electric field is what we refer to as an electric current.

In conductors, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons, which propagates “domino fashion” through the conductor.

In everyday electrical and electronic devices, the signals travel as electromagnetic waves typically at 50%–99% of the speed of light in vacuum, while the electrons themselves move much more slowly.

The drift velocity of electrons in a conductor is typically quite slow, on the order of a few millimeters per second, even though the current in the conductor may be quite high.

AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field (over a distance of a few micrometers.

Why do we use AC current at home?

The main reason we use AC (alternating current) current at home instead of DC (direct current) is that AC current is much more efficient to transmit over long distances. AC current can be easily transformed into different voltage levels using transformers, which allows for high voltage transmission and low current, resulting in less power loss due to resistance in the wires.

Additionally, AC generators are more efficient and cost-effective to build and maintain compared to DC generators. AC generators use a simple design of rotating magnets and stationary coils, whereas DC generators require a commutator and brushes to convert the rotating magnetic field into a unidirectional current.

Another reason for using AC current is that AC generators can produce a wide range of frequencies, which makes it suitable for powering different types of devices and appliances. For example, high-frequency AC current is used for electric motors, while low-frequency AC current is used for lighting and electronics.

Finally, AC current is also safer than DC current at higher voltage levels. AC current periodically reverses direction, which reduces the risk of electrocution and allows for the use of safety devices such as circuit breakers and fuses to protect against electrical hazards.

Electric Current and Hydraulic Analogy

The hydraulic analogy, or the electric-fluid analogy, is a widely used analogy between hydraulics and electricity, which is a useful tool for teaching and for those who are struggling to understand how circuits work. it can also be applied to heat transfer problems. 

Current is equivalent to a hydraulic volume flow rate; that is, the volumetric quantity of flowing water over time. Usually measured in amperes. The wider pipe is, the more water will flow through. It is measured in amps (I or A).

Resistance is like pipe diameter or obstacles in the hose that slow down the water flow. It is measured in ohms (Ω). In hydraulics, resistance is associated with the pressure loss coefficient.