Key Points

Electric current is the rate of flow of electric charge. It is calculated as $I = \frac{Q}{t}$, where Q is charge and t is time. The SI unit of current is the ampere (A).

An electric circuit is a continuous and closed path along which an electric current can flow. If the circuit is broken anywhere, the current stops flowing.

Potential difference between two points is the work done to move a unit charge from one point to another, $V = \frac{W}{Q}$. Its SI unit is the volt (V).

Ohm's law states that the potential difference (V) across a conductor is directly proportional to the current (I) through it, provided temperature remains constant: $V = IR$. R is the resistance, measured in ohms ($\Omega$).

The resistance (R) of a conductor depends on its length (l), area of cross-section (A), and the material's resistivity ($\rho$). The relationship is given by $R = \rho \frac{l}{A}$.

Resistivity ($\rho$) is a characteristic property of a material that quantifies how strongly it resists electric current. Its SI unit is the ohm-meter ($\Omega \text{ m})$.

When resistors are connected in series, the total equivalent resistance ($R_s$) is the sum of their individual resistances: $R_s = R_1 + R_2 + R_3 + \dots$. The current is the same through each resistor.

For resistors in parallel, the reciprocal of the equivalent resistance ($R_p$) is the sum of the reciprocals of individual resistances: $\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \dots$. The potential difference is the same across each resistor.

The heat (H) produced in a resistor is directly proportional to the square of the current (I), the resistance (R), and the time (t) for which the current flows. The formula is $H = I^2Rt$.

The heating effect of current is used in devices like electric irons, toasters, and heaters. It is also used in electric bulbs with high melting point filaments like tungsten and in safety fuses.

Electric power (P) is the rate at which electrical energy is consumed. It can be calculated using the formulas $P = VI$, $P = I^2R$, or $P = \frac{V^2}{R}$. The SI unit is the watt (W).

The commercial unit of electrical energy is the kilowatt-hour (kWh), also known as a 'unit'. One kilowatt-hour is the energy consumed by a 1 kW device in 1 hour, where $1 \text{ kWh} = 3.6 \times 10^6 \text{ J}$.

Conventionally, the direction of electric current is taken as the direction of flow of positive charge. This is opposite to the actual direction of the flow of electrons, which are negatively charged.

An ammeter measures electric current and is always connected in series in a circuit. A voltmeter measures potential difference (voltage) and is always connected in parallel across the component.