Electric Circuits: I-V Characteristics

The essential parts of a circuit are a source of energy (the cell or power supply), a load (the component or components that energy is transferred to), and conducting pathways to and from the load.
An electric circuit provides a complete path for the flow of electric charge. This flow of charge is what we refer to as electric current.
In order to cause a current to flow, a source of electromotive force (emf) is required.
A potential difference (voltage) is required to maintain the flow of charge through a resistor or other component in a circuit.
Current (I) is measured in amperes (A), potential difference (V) is measured in volts (V), and resistance (R) is measured in ohms (Ω).

Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.
It can be mathematically expressed as V=IR where V is the potential difference, I is the current, and R is the resistance.

Components that obey Ohm’s law over a range of voltages have an I-V characteristic that is a straight line through the origin. These are known as ohmic conductors.
For non-ohmic conductors the Resistance (R) value changes with different values of I and/or V. Examples include filament lamps and diodes.
While an ohmic conductor’s graph is a straight line, the filament bulb (non-ohmic) has a curve. This curve indicates that as the current increases, the temperature of the bulb filament increases, causing increased resistance.
For a diode, the I-V characteristics show that it conducts in one direction only. It has a very high resistance in the reverse direction and a low resistance in the forward direction.

Power (P) in an electric circuit is the rate at which energy is transferred. The unit of power is the watt (W).
Power can be calculated by the expression P = IV, where P is the power, I is the current, and V is the potential difference.

Energy transferred in a circuit is given by the equation E = QV, where E is the energy, Q is the charge, and V is the potential difference.
Energy transferred can also be calculated using the formula E = Pt, where E is the energy, P is the power, and t is the time.

In a series circuit, the same current flows through each component. The sum of the potential differences across each component is equal to the total emf supplied.
In a parallel circuit, the potential difference across each component is the same. The current through each component depends on the resistance of the component. The total current supplied is the sum of the currents through each component.