Power, Current and Voltage

Definitions and Basic Concepts

Power (P) is the rate at which energy is transferred, used, or transformed, measured in watts (W). Can be calculated using the formula P = IV, where I is the current and V is the voltage.
Current (I) is a flow of electrical charge, measured in amperes (A). Electrons moving through a conductor such as a wire constitute an electric current.
Voltage (V), also known as potential difference, is the electric potential energy per unit charge, measured in volts (V). It is the force that drives an electric current through a circuit.
Resistance (R) is a measure of the difficulty to pass an electric current through a conductor. It is measured in ohms (Ω).

Ohm’s Law: Current (I) equals voltage (V) divided by resistance (R). It can be written as I = V/R.
Power Relationship: Power (P) equals voltage (V) multiplied by current (I). The formula can be written as P = VI.
The Electrical Energy transferred (E), measured in joules (J) equals power (P) multiplied by time (t), measured in seconds (s). The formula can be written as E = Pt.

In a series circuit, the current remains the same across all components. However, the voltage is divided across the components based on their resistance.
In a parallel circuit, the voltage remains the same across all components, but the current is divided based on their resistance.
Always make sure to convert units appropriately when using these formulae to solve problems. For instance, if power is given in kilowatts (kW), it should be converted to watts (W) by multiplying by 1000.
Be careful when differentiating between the symbols for power (P), resistance (R), current (I), and voltage (V) when solving problems – they may look similar, but changing one significantly changes the result.
The direction of current is defined as the direction in which positive charges would move. In metallic wires, actually the negatively charged electrons move.
Power loss: In real circuits, wires have a small resistance which causes some power loss in the form of heat. This power loss is determined by the formula P = I²R, where I is the current and R is the resistance. The power loss is greater when the current is high or the resistance is high.
When we say a device ‘uses’ a certain power, what we mean is that it converts electrical energy into some other form at this rate. We often talk about energy being ‘used’ but actually it’s conserved – it just changes from one form into another.