Fundamental Principles of Electric Circuits and Resistance

Electric Current (I)

In this formula, I represents the current (measured in amperes, A) and Q is the quantity of charge (measured in coulombs, C) flowing past a specific point in the circuit within a time interval t (measured in seconds).

One coulomb of charge is equivalent to the charge carried by 6.24 × 10¹⁸ electrons. Conversely, the charge carried by a single electron is equal to -1.602 × 10^-19 C.

Voltage (V)

Where V is the voltage drop (measured in volts, V) across a device, E is the amount of energy (measured in joules, J) transformed, and Q is the amount of charge (measured in coulombs, C) that passed through the device.

Energy (E)

Electric circuits are designed to transform electrical energy into other functional forms. The amount of energy (E) transformed by a charge (Q) passing through a load in a time interval (t) is expressed by the following relationship:

Power (P)

Power is defined as the rate of doing work, or the rate at which energy is transformed from one form to another. Power is equal to the amount of energy transformed per second, expressed as:

In this equation, P is the power (measured in watts, W) delivered when an amount of energy E is transformed in a time interval t.

1 watt = 1 joule per second = 1 J s^-1

Electromotive Force or EMF (ε)

The Electromotive Force (EMF) is a measure of the energy supplied to a circuit for each coulomb of charge passing through the power supply. Contrary to its name, it is not a force. The standard circuit symbol for EMF is ε.

It represents the source voltage of the circuit, which is the maximum potential difference across the source. In contrast, the term voltage is typically used when calculating the voltage drop between different points within a circuit.

Resistance (R)

The resistance (R) of a substance is defined as the ratio of the voltage drop (V) across it to the current (I) flowing through it:

Resistance is measured in ohms (symbol Ω) and serves as a measure of how difficult it is for an electric current to pass through a material.

Factors Affecting Resistance

The resistance of a conductor with a uniform cross-section (such as a wire) depends on three primary factors:

• Length (l): The greater the length of the conductor, the greater the resistance.
• Cross-sectional Area (A): The greater the area, the lower the resistance.
• Material Type: This intrinsic property is called resistivity (ρ), measured in ohm-metres (Ω m).

The general formula for resistance is:

Resistor Color Code Reference Table