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.
Basics of electricity: what is electric current?
Let's get to grips with the basics of electricity starting with electric current.
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. \(I = \frac{4C}{500s} = 8 \cdot 10^{-3} A = 8 mA\)
B. \(I = \frac{8C}{100s} = 80 \cdot 10^{-3} A = 80 mA\)
C. \(I = \frac{1 C}{8s} = 125 \cdot 10^{-3}A= 125 mA\)
Option A is the right choice.
Basics of electricity: what is the potential difference?
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
\[V = \frac{W}{Q}\]
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:
\[V = \frac{4J}{4C} =1V\]
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.
Basics of electricity: what is resistance?
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:
\[R = \frac{V}{I}\]
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:
\[R = \frac{V}{I} = \frac{2V}{0.17 A} = 11.76 \Omega\]
Basics of electricity: what is electric power?
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:
\[P = I \cdot V = I^2 \cdot R = \frac{V^2}{R}\]
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.
Basic components of electric circuits
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.
Basic knowledge of circuits
We can describe circuits by using diagrams with standard symbols that indicate the role of each component in the circuit. The main components are:
- Sources, which supply voltage and/or current.
- Wires that transport the current.
- Resistors, which offer resistance to the flow of current.
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
Resistance association
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.
- Resistances in series: n resistors are on the same wire or conductor one after another, and the wire does not bifurcate (divide into branches). The resulting resistance of the set of resistors is the sum of their individual resistances:
\[R_{Total} = \sum_i R_i\]
Resistors connected in series
- Resistances in parallel: n resistors are on different branches that originated from a division of the same wire. The following equation gives the resulting resistance of the set of resistors.
\[R_{Total} = \frac{1}{\sum_i \frac{1}{R_i}}\]
Resistors connected in parallel
Kirchhoff’s Laws
Finally, when considering circuits, we have to take into account two laws of conservation that are named after the scientist Gustav Kirchhoff:
- Conservation of current: whenever we reach a node of wires/conductors, the current entering the node must equal the current leaving it (conservation of charge and, hence, current).
- Conservation of voltage: whenever we consider a closed circuit or a closed loop in a circuit, the sum of the voltage supplied by all batteries must equal the drop of potential caused by every element in the closed-loop. This is simply Ohm’s law applied to closed circuits.
Network of components
Basics of Electricity - Key takeaways
- Electric current is the number of charges that flow through a certain section of a conductor per unit of time. It is measured in amperes.
- The potential difference is the energy needed per unit of charge to move charges from one point to another. It is measured in volts. It is supplied by phenomena like chemical reactions, magnetic fields, non-renewable sources, etc.
- Resistance is the opposition of a substance to the flow of charge. It is measured in ohms. It is usually determined through Ohm’s law, a widely used approximation that relates it to the voltage and the current.
- In circuits, it is interesting to study how can we calculate the resistance of a set of resistors. The two usual positions are in series and parallel.
- In circuits, there are laws that capture the conservation of energy and charge that translate into the conservation of voltage and electric current. They are called Kirchhoff laws.
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