Basic Properties of Electrical Circuits: Voltage, Current, Resistance, Power [The Mountain Analogy]

Current, Voltage, Resistance, and Power are the four basic properties of electrical circuits. The mountain analogy in this article will help you to understand these properties. You'll master:
  • What is current
  • What is voltage
  • What is resistance
  • What is power

The mountain analogy

Alex and Clark are enjoying their vacations on a hill station. Clark is standing on the top of the mountain. On the top of the mountain is a box filled with tennis balls. Clark rolls down the balls to Alex who is lodged in the bottom of the mountain. There are two forces which facilitated the movement of the ball. The force of repulsion by Clark and the force of gravity at the bottom which pulled the ball.

Let's consider a battery which is connected to a conductor. An electrical battery has two terminals namely the Anode (Alex) and the Cathode (Clark). The conductor (mountain) has a large number of free electrons (tennis balls) in it. The cathode (which is negatively charged) repels these electrons and the anode attracts them. A beginner often feels it difficult to remember which terminal is positive and which one is negative. A pro tip is to remember the word PANiC. Positive is Anode and Negative is Cathode.


The flow of electrical charge per unit time is known as current.

The electrical charge is measured in coulombs, whereas the time is measured in seconds. We can define electric current as the coulombs per second.

The SI unit of current is ampere.
Mathematically, I = Q/t
where [I = current in amperes] [Q = charge in coulombs] [t = time in seconds]

The example below further clears the concept of current.

Example # 1: A charge of 1000 coulombs passes through a point for 5 minutes. What is the current?
Solution: I = 1000 / {5 * 60} = 3.3 A


The potential difference between two or more than two points.

We previously observed how a ball rolls down under the force of gravity. If there was no gravity the ball would not have been displaced from top to bottom. The electrical charges in battery move from -ve to +ve terminal of the battery due to the difference of potential. If there is no potential, there is no movement of charges.
Technically the voltage between any two points is defined as the amount of energy which is required to move a charge between two points.

The SI unit of voltage is volts.
Mathematically, V = W/Q
where [V = voltage in volts] [ W = energy in joules] [Q = charge in coulombs]

Example # 2: 100 J of energy is required to move a 10 C charge between two points. Find the voltage between two points.
Solution: V = 100 J / 10 C = 10 V


The opposition offered to the current flow is termed as resistance.

We previously observed two forces which facilitate the rolling of the ball. However, there is a third force which opposes the motion of tennis balls. The force of friction tends to stop or reduce the motion of balls. Let's compare this friction with atoms of electrical conductors.

Alongside with the conduction electrons, the conductors also have atoms and other electrons.

During their motion, the free charges strike with these atoms which results in the reduction of motion of free electrons. This reduction in movement is measured in terms of resistance.

The SI unit of resistance is ohms.
The actual resistance of the material depends on three major factors. Mathematically it is defined as:
R = (ρl)/A
[ρ= resistivity in ohm-meters] [ l = length of material] [A = area of cross section]


The product of voltage and current is termed as power.

Generally, the power is defined as the amount of work done.
Its SI unit is watt.
P = W/t
[P = Power in watts] [W = Energy in joules] [t = time in seconds]

In electrical engineering, the power is defined as the product of voltage and current.

P = VI
[P = Power in watts] [V = Voltage in volts] [I = current in amperes]
A tabulated summary of above discussed is provided here for reference:

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