Electricity

11.1 Electric Current and Circuit:

• Electric Current: Think of electric current like a river, but instead of water, it's electric charge that's flowing. In most materials, these charges are electrons, tiny negatively charged particles. They move through materials that conduct electricity, like copper wires.

• Electric Circuit: Just like a river needs a channel to flow, electric current needs a path. This path is called an electric circuit. It must be a closed loop for the current to flow continuously. Imagine a light bulb connected to a battery by wires; that's a simple circuit.

• Direction of Electric Current: Interestingly, the direction of electric current is conventionally defined as the opposite direction to the flow of electrons. This is a historical quirk, but it's important to keep in mind.

• Electric Current Formula: The amount of current flowing is measured in amperes (A), and it's calculated by the formula I = Q/t, where Q is the amount of charge passing a point in time t.

• Ammeter: To measure the "flow rate" of electric charge (the current), we use a device called an ammeter. It's always connected in series within the circuit, so the current flows through it.

11.2 Electric Potential and Potential Difference:

• Electric Potential Difference: Imagine you're pushing a boulder uphill. You need to do work to move it against gravity. Similarly, it takes work to move electric charges against an electric field. This work done per unit charge is called electric potential difference, measured in volts (V).

• Formula: The potential difference (V) is calculated as V = W/Q, where W is the work done and Q is the charge moved.

• Voltmeter: To measure the potential difference between two points in a circuit, we use a voltmeter. It's connected in parallel across those points.

11.3 Circuit Diagram:

• Schematic Diagram: Instead of drawing realistic pictures of circuits, we use simplified symbols to represent the components. This is like a map of the circuit, making it easier to understand.

• Symbols: You'll need to learn the symbols for common components like cells, batteries (which are combinations of cells), switches (to open and close the circuit), wires, bulbs, resistors (which limit current flow), ammeters, and voltmeters.

11.4 Ohm's Law:

11.5 Factors Ohm's Law: This is a fundamental law in electricity. It states that the current (I) flowing through a conductor is directly proportional to the voltage (V) across it, as long as the temperature stays constant. This means if you double the voltage, you double the current.

Formula: Ohm's Law is expressed as V = IR, where R is the resistance of the conductor.

Resistance: Resistance is like the "friction" in an electrical circuit. It opposes the flow of current. It's measured in ohms (Ω).

Variable Resistance (Rheostat): A rheostat is a component that allows you to change the resistance in a circuit, which in turn controls the current. Think of it like a volume knob for electricity.

on Which the Resistance of a Conductor Depends:

• Factors: The resistance of a wire depends on its length, its thickness (cross-sectional area), and the material it's made from. A longer wire has more resistance, a thicker wire has less resistance, and different materials have different inherent resistances.

• Resistivity: Resistivity (ρ) is a property of the material itself. It tells you how much a particular material resists the flow of current. Copper, for example, has low resistivity, which is why it's used for electrical wiring.

• Formula: The resistance (R) is calculated as R = ρ l/A, where l is the length and A is the cross-sectional area.

11.6 Resistance of a System of Resistors:

• Series Combination: When resistors are connected in a line, end-to-end, they are in series. The same current flows through each resistor.

• Equivalent Resistance in Series: The total resistance in a series circuit is simply the sum of the individual resistances: Rs = R1 + R2 + R3 +...

• Parallel Combination: When resistors are connected across the same two points, they are in parallel. The current splits up, with some flowing through each resistor.

• Equivalent Resistance in Parallel: Calculating the total resistance in a parallel circuit is a bit trickier: 1/Rp = 1/R1 + 1/R2 + 1/R3 +... The total resistance in parallel is always less than the smallest individual resistance.

11.7 Heating Effect of Electric Current:

• Joule's Law of Heating: Whenever current flows through a resistor, some electrical energy is converted into heat. This is why appliances get hot when you use them.

• Formula: The amount of heat produced (H) is given by H = I^2Rt, where I is the current, R is the resistance, and t is the time.

• Applications: This heating effect is used in many devices, such as electric heaters, toasters, and even the filament in an incandescent light bulb. Fuses also rely on this principle to protect circuits; they melt and break the circuit if the current gets too high.

• 11.8 Electric Power:

  • Electric Power: Power is the rate at which energy is used. In electrical circuits, it's the rate at which electrical energy is converted into other forms of energy, like heat or light.

• Formulas: Electric power (P) can be calculated using different formulas: P = VI = I^2R = V^2/R.

• Units: Power is measured in watts (W) or kilowatts (kW).

• Commercial Unit of Energy: When you pay your electricity bill, you're paying for the energy used, which is measured in kilowatt-hours (kWh). This is the amount of energy used by a 1 kW appliance running for 1 hour.