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The movement of electric charges is known as electricity. It is a secondary energy source, meaning it is obtained by converting primary energy sources, such as coal, natural gas, oil, nuclear power, and other natural sources. Electricity can be generated using renewable or non-renewable energy sources. To understand how the basics of electricity are connected, we must study both the fundamental quantities and the basic components of electric circuits.
The rate of charge flow past a particular point in an electric circuit is known as electric current.
Negatively charged electrons or positive charge carriers, such as protons or positive ions, can carry the charge. The magnitude of an electric current is measured in coulombs per second, which receives a special name: amperes (A).
The equation that captures the relationship between charge, current, and time is:
In this equation, Q is the charge in coulombs (C), I is the current in amperes (A), and t is the time in seconds (s).
In how many of the below options will 8mA of current pass through an electric circuit?
A. When a charge of 4C passes in 500s.
B. When a charge of 8C passes in 100s.
C. When a charge of 1C passes in 8s.
Using the current equation, we solve for I:
A. 
B. 
C. 
Option A is the right choice.
The potential difference or voltage is the amount of energy per unit of charge needed to move charges from a certain space point to another.
This definition is equivalent to a difference in electric potential between two spatial points (indicated by its name). The way to achieve this difference of electric potential is usually through starting chemical reactions (which move electrons that generate a potential difference), moving magnets, water-powered turbines, etc.
The voltage difference will create a direction for the charges to flow. The voltage can be described mathematically as
In this equation, V is the potential difference in volts (V), W is the energy in joules (J), and Q is the charge in coulombs (C). The device used to measure potential difference or voltage is a voltmeter.
Find the potential difference in a circuit where the energy invested in carrying a charge of 4 coulombs is 4 joules.
Simply use the equation for the potential difference:
Hence the voltage is 1 volt for a circuit where the work done (energy invested) in carrying a charge of 4 coulombs is 4 joules.
Resistance is a measure of the opposition of a component to the flow of electric current.
The lower the resistance of a component, the higher the current flows through the component. Materials with lower resistance make better conductors (so the lower the resistance of a particular material, the better it is as a conductor). That’s why most wires are made up of copper because this material has a low resistance value. We can calculate the resistance thanks to Ohm’s law, which defines resistance as the ratio of the potential difference and the current:
R is the resistance measured in Ohms (Ω), V is the potential difference measured in volts (V), and I is the current measured in amperes (A).
Ohm’s law is an experimental law that is only fulfilled by some materials called ohmic materials. However, in general, resistance is a measure of a substance’s opposition to the flow of a current. This relationship can be as complex as we desire and depends on many variables, such as the specific material, temperature, etc.
How much resistance is offered by a circuit, which has a total potential difference of 2 volts and a total current of 0.17 amperes?
Given the voltage and current as V = 2 Volts and I = 0.17 amperes, we can use the equation above:
Since electric circuits with a current involve the movement of charges, there is an associated power (energy per unit of time) that can be calculated in the quantities we have already studied. The expression for the electric power is the following:
In this equation, the power P is measured in watts (W), and we have used Ohm’s law several times to obtain the power in terms of two of the three basic quantities.
The basic electrical systems where we care about voltage, current, and resistance are electric circuits. They are structures made out of electric devices, such as cables, resistors, switches, power sources, etc., where the potential difference is established and a current is formed. The setup determines the specific characteristics of each circuit and its possible applications.
We can describe circuits by using diagrams with standard symbols that indicate the role of each component in the circuit. The main components are:
The majority of other complex devices are usually described in terms of their resistance and extra conditions. For instance, a lightbulb is a resistor that shines when a current flows through it. Below is an example of a basic circuit (don't worry about the cix symbol).
Diagram of a simple circuit, commons.wikimedia.org
The laws that resistors obey when there is more than one in a circuit are important because they allow us to simplify circuits or achieve certain effects. The two main forms of association of resistors we use are resistances in series and resistances in parallel.
Resistors connected in series, commons.wikimedia.org
Resistors connected in parallel, commons.wikimedia.org
Finally, when considering circuits, we have to take into account two laws of conservation that are named after the scientist Gustav Kirchhoff:
Network of components, commons.wikimedia.org
When a potential difference is established between two points, a flow of charges appears. The rate of this flow of charge is the electric current, which is determined by the potential difference and the resistance, i.e. the opposition of the medium to the movement of charges.
The basic unit of electric charge is the Coulomb (C).
The basic components of an electric circuit are wires, batteries, and resistors.
Voltage is the work needed per unit of charge to move charges between two points. Electric power is the rate of energy generated by a potential difference per unit time when charges are allowed to move.
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