The Basics of Electric Circuits

SI Units

The system of units used in engineering and science is the (International System of Units), usually abbreviated as SI units, and is based on the metric system.

The basic units in the SI system are listed with their symbols in Table 1

Table 1: Basic SI units

Quantity	Unit
Length	Meter, m
Mass	Kilogram, Kg
Time	Seconds, s
Electric current	Ampere, A
Thermodynamic temperature	Kelvin, K
Luminous intensity	Candela, cd
Amount of substance	Mole, mol

Derived Units

Derived units use combination of basic units for example: Velocity {Meters per Second (m/s)}, Acceleration-{Meters per Second Square (m/s²)}

Charge

The unit of charge is the coulomb (C), where one coulomb is one ampere second (1 coulomb = 6.24 x 10¹⁸ electrons). The coulomb is defined as the quantity of electricity that flows past given point in an electric circuit when a current of one ampere is maintained for one second. Therefore, charge in coulombs is given by, Q = It where I is the current in amperes and t is the time in seconds.

Force

The unit of force is the newton (N) where one newton is one kilogram meter per second squared. The newton is defined as the force which, when applied to a mass of one kilogram gives it an acceleration of one meter per second squared. Therefore, force, in newtons is given by:

F = ma

Where, m is the mass in kilograms and a, is the acceleration in meters per second squared.

For gravitational force, or weight we use gravitational acceleration (gravity) of 9.81 m/s² in the calculation i.e. F = mg (where g = 9.8 m/s²).

Work

The unit of work or energy is the joules (J) where one joule is one Newton meter. The joule is defined as the work done or energy transferred when a force of one newton is exerted through a distance of one meter in the direction of the force. So work done on a body, in joules W = FS where F is the force in Newtons and S is the distance in meters moved by the body in the direction of the force. Energy is the capacity for doing work.

Power

Power is defined as the rate of doing work or transferring energy. The unit of power is the Watt (W) where one Watt is one joule per second. Hence power in Watts is given by:

Where, w it the work done or energy transferred in joules and t is the time in seconds, therefore energy in joules, w = pt.

Electrical Potential and emf

A charge in electrical potential between two points in an electric circuit is called a potential difference. The electromotive force (emf) provided by a source of energy such as a battery or generator is measured in volts.

The volt (V) is the unit of electric potential where one volt is one joule per coulomb. One volt is defined as the difference in potential between two points in a conductor which, when carrying a current of one ampere, dissipates a power of one watt, thus:

Resistance and Conductance

The unit of electric resistance is the ohm (𝝮) where one ohm is one volt per ampere. It is defined as the resistance between two points in a conductor when a constant electric potential of one volt applied at the two points produces a current flow of one ampere in the conductor, hence:

Where, V is the potential difference across the two points in volts and I is the current flowing between the two points in amperes.

The reciprocal of resistance is called conductance, and is measured in siemens (S), so conductance in siemens G is given by:

Where, R is the resistance in ohms.

Electrical Power and Energy

When a direct current of I amperes is flowing in an electric circuit and the voltage across the circuit is V volts, then power, in watts P = VI

Electrical energy = Power x time = VIt joules

Although the unit of energy is joule, when dealing with large amounts of energy, the unit used is the kilowatt hour (kWh) where,

1 kWh = 1000 Watt hour = 1000 x 36,000 watt second or joules = 3600,000 J

Table 2: Electrical terms, units, and symbols

Quantity	Symbol	Unit	Unit Symbol
Length	l	meter	m
Mass	m	kilogram	kg
Time	t	second	s
Velocity	v	meters per second	m/s or ms-1
Acceleration	a	meters per second squared	m/s2 or ms-2
Force	F	newton	N
Electrical charge or quantity	Q	coulomb	C
Electric current	I	ampere	A
Resistance	R	ohm	ꭥ
Conductance	G	siemen	S
Electromotive force	E	volt	V
Potential difference	V	volt	V
Work	W	joule	J
Energy	E or W	joule	J
Power	P	watt	W

The Electric Current and Quantity of Electricity

Atoms consist of protons, neutrons and electrons. The protons have positive electrical charges and the neutrons have no electrical charge. Protons and neutrons are contained within the nucleus. Removed from the nucleus are negatively charged particles called electrons. Atoms of different material differ from one another by having different numbers of protons, neutrons and electrons.

When an equal number of protons and electrons exist within an atom, it is said to be electrically balanced, as the positive and negative charges cancel each other out. When there are more than two electrons in an atom, the electrons are arranged into shells at various distances from the nucleus.

All atoms are bound together by powerful forces of attraction existing between the nucleus and its electrons. However electrons in the outer shell of an atom are attracted to their nucleus less powerfully than electrons whose shells are nearer the nucleus.

When an atom loses an electron, it becomes an ion, and it is electrically imbalanced. It becomes positively charged and hence able to attract an electron to itself from another atom. Electrons that move from one atom to another are called free electrons and such random motion can continue indefinitely. However, if an electric pressure or voltage is applied across any material, there is a tendency for electrons to move in a particular direction. This movement of free electrons, known as drift, constitutes an electric current flow. Hence current is the rate of movement of charge

Conductors are materials that contain electrons that are loosely connected to the nucleus and can easily move through the material from one atom to another. Conductors have low resistance and allow electric current flow through them e.g. copper, brass, silver, etc.

Insulators are materials whose electrons are held firmly to their nucleus. Insulators have high resistance and do not allow electric current to flow through it e.g. plastic, rubber, glass, among other insulators.

The unit used to measure the quantity of electrical charge Q is called the coulomb C (where 1 coulomb = 6.24 x 10¹⁸ electrons).

If the drift in electrons in a conductor takes place at the rate of one coulomb per second, the resulting current is said to be a current of one ampere.

Hence, 1 ampere = 1 coulomb per second or 1 A = 1 c/s. Thus, 1 coulomb = 1 ampere second or 1 c = 1 As. Generally, if I is the current in amperes, and t, the time in seconds during which the current flows, the quantity of charge in coulombs, Q = It (Quantity of electrical charge transferred).

You can also read: Principles of Control Systems

Basic Electrical measuring Instruments

An Ammeter is used to measure current and must be connected in series with the circuit.

A Voltmeter is used to measure voltage and must be connected in parallel with the part of the circuit whose voltage is required.

An Ohmmeter is used to measure resistance.

A Multimeter is a universal instrument used to measure voltage, current, and resistance.

The Oscilloscope maybe used to observe waveforms, and to measure voltages and currents.

Ohm’s Law

Ohm’s law states that the current flowing in circuit is directly proportional to the applied voltage V and inversely proportional to the resistance R, provided the temperature remains constant.

Electrical Power and Energy

Power (P) in an electrical circuit is given by the product of potential difference V and Current I. The unit of power is the watt (W). Therefore, P = VI watts.

From ohm’s law V = IR

Substituting for V in this equation P = VI = (IR)I = I²R watts

Practical applications of electric current effects

• Magnetic effect: relays, bells, motors, generators, transformers, car ignition etc.
• Heating effect: water heaters, electric fires, cookers, irons, furnaces etc.
• Chemical effect: primary and secondary cells, electroplating, etc.

Related articles:

• Basic Electric Circuits Questions and Answers
• AC Circuits with Resistors, Inductors and Capacitors
• Clamper Circuits – Features & Applications
• Basic Electronics – Multiple Choice Questions & Answers (MCQ1)
• Series and Parallel Circuits