Wiktor Wise
The flow of electricity is controlled by a switch, which allows or prevents current from flowing through a circuit. A circuit is a closed circle through which electricity can flow. Power stations generate electricity, which is then sent through transformers that increase voltage so that the electricity can be pushed over long distances. Transmission lines carry the electricity across the country to substations, where the voltage is lowered so that it can be sent on smaller power lines to homes, businesses, and schools.
| Characteristics | Values |
|---|---|
| Device used to control the flow of current in a circuit | Switch |
| Direction of current | Positive or negative |
| Current intensity | Depends on the circuit |
| Circuit | A closed circuit allows electricity to flow |
| Transformers | Increase or decrease voltage |
| Transmission lines | Carry high-voltage electrical current |
| Power source | Power stations, wind, coal, natural gas, hydropower, batteries, solar cells |
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What You'll Learn
Switches control the flow of electricity in a circuit
The flow of electricity is controlled by switches, which allow or prevent current from flowing through a circuit. A circuit is the path electricity flows through, and it must be closed for electricity to flow. When a switch is turned on, it closes the circuit, allowing electricity to flow through and power devices. For example, when you turn on a TV, you close the circuit, enabling electricity to flow and the TV to turn on.
Switches act as a gatekeeper, controlling the flow of electricity. They can cut off the electrical current when desired, ensuring safety and preventing overloading. This is particularly important in homes, businesses, and schools, where electricity is distributed through power lines and wires to power various devices.
Electricity is generated in power stations using spinning turbines powered by wind, coal, natural gas, or hydropower. It is then transmitted through transmission lines, which carry high-voltage electrical currents across vast distances. Transformers play a crucial role in increasing or decreasing voltage to facilitate efficient transmission.
The electrical current can flow in either direction in a circuit, depending on the type of charge carriers. In metals and wires, negatively charged electrons are the charge carriers, moving freely within the material. However, the conventional direction of current is defined as the direction in which positive charges flow.
Understanding the behaviour of electric current is essential for effectively controlling it. This includes knowledge of alternating current (AC) and direct current (DC) systems, where the former involves the periodic reversal of charge direction, while the latter features a unidirectional flow commonly produced by batteries and solar cells. By manipulating switches, we can harness the power of electricity and utilise it for our devices and appliances.
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Transformers increase or decrease voltage
The flow of electricity can be controlled in a variety of ways, one of which is through the use of transformers. Transformers are electrical devices that transform voltage from one value to another, either increasing or decreasing it. They are based on Faraday's law of induction, which states that a changing magnetic field will induce a voltage in a nearby coil of wire. This phenomenon is the fundamental principle behind how transformers work.
A transformer consists of two coils of wire, known as the primary coil and the secondary coil. The primary coil is connected to a source of voltage, which can be alternating current (AC) or direct current (DC). When the primary coil is energised, it creates a magnetic field. This magnetic field then induces a voltage in the secondary coil, which is connected to the load. By varying the number of turns or windings in the primary and secondary coils, the voltage can be transformed.
If the secondary coil has more windings than the primary coil, the transformer will step up the voltage. This means that the output voltage will be higher than the input voltage. For example, if the primary coil has a voltage of 100V and the secondary coil has 10 times as many windings, the output voltage will be 1,000V. Conversely, if the secondary coil has fewer windings than the primary coil, the transformer will step down the voltage, resulting in an output voltage lower than the input voltage.
The relationship between voltage and current in a transformer is inversely proportional. According to the Joule-Lenz Law, power (P) is equal to the product of voltage (V) and current (I) (P = VI). Therefore, if the voltage increases, the current must decrease, and vice versa. So, when a transformer steps up the voltage, it simultaneously steps down the current, and when it steps down the voltage, it steps up the current. This principle is utilised in various applications, such as in portable X-ray units, where a step-up transformer is used to increase the voltage to the required level.
Transformers play a crucial role in power distribution systems. Electricity is often transmitted over long distances at high voltages to reduce line losses. However, high voltages can be hazardous, so transformers are employed to step down the voltage to a safer level for use in homes and businesses. Additionally, transformers are commonly found in electronic devices such as cell phones, laptops, and power tools, where they convert standard AC voltages to the specific voltage requirements of the device.
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Transmission lines carry electricity across countries
Transmission lines are a crucial component of electrical power transmission, facilitating the long-distance transfer of electricity across countries. These lines, which can be overhead or underground, form part of a transmission grid, a complex network encompassing power stations, transmission lines, and substations. Efficient long-distance electricity transmission relies on high voltages, which help mitigate losses caused by strong currents.
Transmission lines can carry either alternating current (AC) or direct current (DC), and the voltage levels are adjusted using transformers. Before transmission, the voltage is increased, then lowered for local distribution. This process reduces current levels in the lines, minimising energy wastage and ensuring that a substantial amount of power reaches consumers. The voltage transformation enhances the efficiency of conductors in delivering power.
Overhead AC transmission lines are characterised by their use of three-phase current, with voltages varying based on the specific grid system. Transmission voltages typically range from 69 kV to 765 kV. In contrast, DC voltage transmission towers utilise line pairs, consisting of a positive and negative current line, differing from the three-line configuration in AC voltage systems.
Underground transmission lines are more prevalent in populated areas for safety reasons. These lines may be unprotected and buried, or they may be placed within conduits, trenches, or tunnels. While insulation is typically not used for long-distance power lines due to cost considerations, insulation may be provided through air gaps or by maintaining sufficient spacing between components.
The expansion of power plants away from populated areas has heightened the importance of efficiently transporting electricity over long distances. Power lines, or conductors, play a pivotal role in this process, enabling the bulk transport of electricity across vast rural areas. The ongoing electrification and renewable energy transition have prompted the adoption of reconductoring, which involves replacing existing transmission lines with higher-capacity lines to augment transmission capacity.
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Alternating current systems: the charge periodically reverses direction
Alternating current (AC) is an electric current that periodically reverses its direction. This is in contrast to direct current (DC), which only flows in a single direction and cannot change sporadically. The prototypical example of a direct current source is a discharging battery or capacitor, such as those used to recharge mobile phones, power drills, or electric cars. Most railroad systems are also DC-powered.
The voltage of an alternating current source varies sinusoidally (like a sine function). Since sine and cosine functions have the same shape but with a phase offset, it could also be described as varying cosinusoidally (like a cosine function). The average of a sinusoidally varying quantity is zero, as it is positive half of the time and negative the other half. However, the average power value is always greater than zero in any electrical system that is turned on.
AC voltage and current may average out to zero, but power is different. A positive voltage produces a positive current, and a negative voltage produces a negative current. However, when multiplied, both result in positive power.
AC is most commonly found in mains-wired buildings such as homes and offices. This is because it is relatively easy to generate and transport over long distances.
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Direct current systems: the charge moves in one direction
Direct current (DC) is a unidirectional flow of electric charge, always moving in the same direction from an area of high electron density to an area of low electron density. This is in contrast to alternating current (AC), where the direction and amperage can change. In a DC circuit, a power source like a battery has a positive and negative terminal, and the load also has corresponding positive and negative terminals. To complete the circuit, positive charges must flow from the power source to the load. These charges then return to the negative terminal of the load and flow back to the negative terminal of the battery. If either the positive or negative terminal is disconnected, the circuit is broken, and the charges cannot flow.
The direction of current flow in a DC circuit is important, and in most applications, the polarity matters. Connecting the circuit backwards will result in the load not functioning properly. However, there are some DC circuit applications where polarity does not matter, and the circuit will still work even if the positive and negative terminals are connected backwards.
Direct current is commonly found in extra-low voltage and low-voltage applications, especially those powered by batteries or solar power systems, as these sources produce DC. It is used in many household electronics and devices that use batteries, including rechargeable devices like laptops and cell phones. DC is also used in automotive batteries, providing power for engine starting, lighting, ignition systems, climate controls, and infotainment systems.
While AC is more commonly used for power delivery due to its advantages in voltage transformation, DC has its own advantages for long-distance transmission. High-voltage direct current (HVDC) transmission can be more efficient than AC for very long distances or undersea cables. In some cases, AC is converted to DC using a rectifier for specific applications that require direct current, such as third rail power systems.
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Frequently asked questions
A switch controls the flow of electricity in a circuit. When the switch is turned on, the circuit is closed and electricity is free to flow.
The direction of the flow of electricity, also known as conventional current, is defined as the direction in which positive charges flow. In a metal wire, the negatively charged electrons are the charge carriers and they flow in the opposite direction of conventional current.
There are two main types of electric current: alternating current (AC) and direct current (DC). In AC, the movement of electric charge periodically reverses direction. In DC, the movement of electric charge is in only one direction.
Electricity is generated in power stations using large spinning turbines powered by wind, coal, natural gas, or water (hydropower). The electrical current is then sent through transformers, which increase the voltage so that the power can be transmitted over long distances through transmission lines.
