Electricity generation

Key questions

What are the sources of electricity?
What effects does electricity have and how are they utilised?
How is the electrical power of a device calculated?

Sources of electricity

The widespread use of electricity has fundamentally changed society. Electricity comes from power plants, where energy is released from natural energy sources for human use. The energy is converted into rotational motion of a turbine, and then into electricity in a generator for transmission. The most common types of power plants that generate electricity with a generator are nuclear power plants, hydroelectric power plants, wind power plants, and various thermal power plants. Alternating voltage is produced in the generator, which generates alternating current. Solar power plants also produce electricity, but their electricity production is based on converting solar radiation energy directly into electricity without a generator.

Power plants are connected to electricity distribution networks. These networks enable the transfer of energy in the form of electricity for societal use. The electricity produced in power plants is made available for household use through wall sockets.

Photograph of a battery and an electronic device.

Photograph of a person holding a small solar cell in his hand.

Not all electrical devices need to be connected to the mains voltage. Instead, the power source can be, for example, a battery or accumulator. They produce direct voltage and direct current. The battery and accumulator have a positive charge side (anode) and a negative charge side (cathode). The type of battery or accumulator determines the materials used for the anode and cathode. Both batteries and accumulators contain substances harmful to the environment, so they are recycled as hazardous waste.

The generation of voltage in a battery or accumulator is based on chemical redox reactions. Chemical energy is converted into other forms in devices via electric current. Unlike a battery, an accumulator can be recharged, restoring its chemical energy reserve.

A solar cell converts the energy of light particles into electrical energy of charged particles. A photon, or light particle, strikes the surface of the solar cell, freeing an electron. The solar cell is constructed from a semiconductor in such a way that the freed electrons move in only one direction. This generates direct voltage and direct current. Silicon is the most common semiconductor material used in solar cells. In addition to energy production, solar cells are used in a variety of devices, from space probes to pocket calculators. The photovoltaic phenomenon behind the solar cell is explored in more detail in the eight modules of high school physics.

Solar power (Wikipedia)

Number of electric vehicles in the world

The number of electric cars in traffic is continuously increasing. The chart shows the development of electric car sales in the world.

A bar chart showing the number of new cars sold by type in the World, divided into electric and non- electric cars. From the chart, we see that the number of cars sold has peaked in 2017 and 2018 (about 80 million cars per year), while the figure is now once again approaching that number after a slump in the years 2020 to 2022 (around 70 million cars per year). The share of electric cars from non car sales has grown from a very minimal share in 2017 to almost 15 million cars by 2024.

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Utilisation of electricity in devices

When talking about electricity, it often refers to electric current. The generation of electric current requires a charge difference, or voltage. Voltage is formed when electric charges are not evenly distributed within a substance, object, or between objects. Electric current is created when charges move in an attempt to equalise the charge differences.

Charge moves through a substance in a manner characteristic of that material. In metals, charge transfers efficiently, and they are electrical conductors. Electrical conductors are typically made of copper or aluminium. In addition to metals, conductors include liquids containing ions, such as non-distilled water. In metals, charge is transferred via electrons, and in ionic solutions via ions.

In plastics or rubber, charge transfer is poor. They are electrical insulators. Metal conductors are coated with insulation to prevent electric current from escaping the conductor. Insulators are materials in which electric charges cannot move easily. They have a solid lattice structure (for example, ionic compounds such as table salt), or they are solid materials composed of large molecules (such as plastics or paper).

Gases, such as air, are also insulators. In such materials, electrons are bound to individual atoms or molecules and cannot move between the structural components of the material. Electric charge differences also equalise through the insulator if the voltage is high enough. A dramatic example of this is lightning striking through the air.

Photograph of an electric cable showing various copper wires inside the cable.

In an electrical wire, the conductive material, such as copper, is protected by insulation, such as rubber or plastic. If the insulation is damaged and a person comes into contact with the conductor, the electric charges flowing through the wire can pass through the person. The person touching the wire would then receive an electric shock.

The properties of materials are utilised when constructing various components. Components allow electric current to flow in a controlled manner and produce desired effects of electric current. Structures made of components are called electric circuits. The field of science related to controlling electric current is called electronics.

Electric current causes various phenomena. As a result,

heat is generated.
light is generated.
mechanical energy is generated.
information is transferred.

Heat

All parts of electrical devices, including conductors, resist the formation of electric current to some extent. Moving charges collide with the particles of the material and transfer energy to it. As a result, the material heats up. This is often undesirable, but heating can also be utilised. A heating element is a component designed to generate heat through electric current. For example, an electric kettle contains a resistor that heats up. The heat from the resistors is transferred to the water by conduction and radiation.

Light

Electric current generates light in several different ways. If the temperature of a material rises high enough due to the electric current, the material starts to emit visible light in addition to thermal radiation. Light production can also be based on changes in the energy states of electrons. A light-emitting diode (LED) operates on this latter principle, and its operation is introduced in section 7.3. The operation of other light sources is discussed in later Resonance textbooks.

Motion

In an electric motor, the energy transmitted by electric current is converted into mechanical energy. The magnetic effect of the electric current generates a force that drives the motor. Electric motors come in various designs and are found in many types of devices, such as power tools, computers, vacuum cleaners, or fans. Especially electric cars and other electrically powered vehicles have become more common in the 2020s.

Photograph of a Tesla electric car being charged in a charging station.

Information transfer

Electronics has spread to a wide range of applications since the early 20th century and is an essential part of modern society and its applications. Electronics are needed to control the operation of devices, such as starting a washing machine or using a computer. Wireless data transmission also occurs through electricity. The movement of charges can generate electromagnetic waves, in which the transmitted message is encoded. For example, radio waves and WLAN connections operate on this principle.

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Electrical power and energy

The energy consumption of electrical devices was examined in the textbook Resonance 2. Since devices can be used for arbitrary periods of time, their power is specified. The power of a device, $P$ , is the ratio of the change in energy $Δ E$ to the usage time $t$ .

$P = \frac{Δ E}{t}$

Energy consumption

Since power represents the rate of energy consumption of an electrical device, the energy consumed by the device, $E$ , is obtained by multiplying the power and the usage time $t$ .

$E = P t$ .

Electrical power

The power of an electrical device is usually indicated on the rating plate found on the device, which also includes information about the operating voltage. The power $P$ of the electrical device is calculated using the device's voltage $U$ and the electric current $I$ in it. This law is derived in section 5.3, where the generation of electric current is discussed in more detail.

$P = U I$

If the device uses alternating current, the power is calculated using the average values of voltage and current in terms of energy consumption. These values are referred to as the effective voltage and current. For example, the 230 V voltage from a socket is the effective value of the mains voltage.

Efficiency

Electrical devices cannot convert all the energy associated with their use into a useful form. The efficiency $η$ indicates how much of the energy transferred with the electricity is converted into the energy form required for the device's intended purpose.

$η = \frac{E_{output}}{E_{input}} = \frac{P_{output}}{P_{input}}$

Price

Electricity has a price, which consists of sales and transmission costs as well as taxes. The price of electricity varies according to factors related to consumption and production. The price of electricity ranges from almost zero in some countries to the order of magnitude of 0.5 US$/KWh in some areas. Prices tend to fluctuate significantly.

When calculating the cost of electricity, the energy unit is converted into kWh. In this case, power is expressed in kilowatts (kW) and the device's usage time in hours (h).

1. What kind of voltage is obtained from the following sources of electricity?

a. Rechargeable battery

Alternating voltage
Direct current voltage

b. Mains socket

Alternating voltage
Direct current voltage

c. Solar cell

Alternating voltage
Direct current voltage

2. In the cable of a mobile phone charger, the core is

a conductor
an insulator.

3. In the cable of a mobile phone charger, the outer layer is

a conductor
an insulator.

4. The power of an electrical device is calculated as the product of the voltage and the electric current.

Yes
No

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Examples

Example 1

The power of the electrical device is 55 W. The maximum electric current in it may be 2.5 A.

What is the maximum voltage the device can have?
The electrical device is used on average 30 minutes per day. How much energy does the device consume in a year?

Example 1 solution

a. From the definition of electric power, voltage can be determined.

$\begin{aligned} P & = U I \\ U & = \frac{P}{I} \\ = \frac{55 W}{2.5 A} = 22 V \end{aligned}$

The voltage of the electrical device can be a maximum of 22 V.

b. From the definition of power, energy can be determined.

$\begin{aligned} P & = \frac{E}{t} \\ E & = P t \\ = 55 W \cdot 365 \cdot 30 \cdot 60 s = 36.135 \cdot 10^{6} J \approx 36 MJ \end{aligned}$

Energy can just as well be expressed in kilowatt-hours:

$E = 0.055 kW \cdot 365 \cdot 0.5 h = 10.037 5 kWh \approx 10 kWh$

The device consumes approximately 10 kWh of energy per year.

Example 2

The efficiency of the electric kettle is 89%. It is connected to a 230 V mains voltage. The kettle heats 1.2 litres of water to boiling from an initial temperature of 12 °C in 4 minutes 10 seconds.

What is the power of the kettle?
What is the electric current in the kettle?

Example 2 solution

a. 89% of the energy transferred by electricity increases the internal energy of the water, raising its temperature.

$\begin{aligned} Δ U & = E_{electricity} \\ c m Δ T & = η P t \\ P & = \frac{c m Δ T}{η t} \\ P & = \frac{4 190 J/(kg^{\circ} C) \cdot 1.2 kg \cdot (100^{\circ} C - 12^{\circ} C)}{0.89 \cdot 250 s} = 1 988.6 \dots W \approx 2.0 kW \end{aligned}$