CBSE Class 10 Science Electricity Notes Set 01

These CBSE Class 10 Science notes for Chapter 11, Electricity, cover electric current and potential difference, circuit symbols, Ohm's law, resistance and resistivity, series and parallel combinations of resistors, the heating effect of current, electric power and energy, and the safety devices used in household circuits. The formulas, units and reasons given here are the ones asked in board examinations.

Important Notes and Formulas

1. Electric Current and Circuit

Electric current is the rate of flow of electric charge through a conductor: I = Q/t. Its SI unit is the ampere (A). One ampere is the current when 1 coulomb of charge flows through a conductor in 1 second. Smaller units are 1 mA = 10⁻³ A and 1 μA = 10⁻⁶ A.

The charge on one electron is 1.6 × 10⁻¹⁹ C, so 1 coulomb is the charge of 6.25 × 10¹⁸ electrons. The direction of conventional current is taken as the direction of flow of positive charge, which is opposite to the direction of flow of electrons, that is, from the positive terminal to the negative terminal of the cell outside it.

An electric circuit is a continuous closed path of conducting wires containing a source of electricity, the load and a switch (key). When the key is on, current flows (closed circuit); when it is off, no current flows (open circuit). Current is measured by an ammeter, which is always connected in series.

2. Electric Potential and Potential Difference

The potential difference between two points is the work done to move a unit positive charge from one point to the other: V = W/Q. Its SI unit is the volt (V). The potential difference between two points is 1 volt if 1 joule of work is done in moving 1 coulomb of charge from one point to the other.

A cell or battery maintains the potential difference needed for the flow of charge; the chemical reactions inside the cell produce it. A battery is a combination of cells. Potential difference is measured by a voltmeter, which is always connected in parallel across the two points.

3. Circuit Diagram

A circuit diagram is a schematic diagram in which the components of a circuit are shown by conventional symbols.

4. Ohm's Law and Resistance

Ohm's law: the potential difference V across the ends of a given metallic wire is directly proportional to the current I flowing through it, provided its temperature remains the same. Thus V ∝ I, or V/I = constant = R, or V = IR. The constant R is the resistance of the wire.

Resistance is the property of a conductor to resist the flow of charge through it. Its SI unit is the ohm (Ω). The resistance of a conductor is 1 Ω if a current of 1 A flows through it when a potential difference of 1 V is applied across its ends. For a given voltage, I = V/R: if the resistance is doubled, the current is halved.

The V–I graph of a metallic conductor at constant temperature is a straight line through the origin; its slope gives the resistance. A rheostat (variable resistance) is used to change the current in a circuit without changing the voltage source.

A good conductor has a low resistance, an insulator has a very high resistance, and the motion of electrons through a conductor is opposed by the atoms among which they move.

5. Factors on which the Resistance of a Conductor Depends

1. Length: R ∝ l (doubling the length doubles the resistance).
2. Area of cross-section: R ∝ 1/A (a thin wire has more resistance than a thick wire).
3. Nature of the material.
4. Temperature: the resistance of a metallic conductor increases when the temperature rises.

Combining the first two: R = ρl/A, where ρ (rho) is the resistivity of the material. Resistivity is the resistance of a wire of length 1 m and area of cross-section 1 m²; its SI unit is ohm metre (Ω m). It is a characteristic property of the material: it does not depend on the length or the area, but changes with temperature.

Metals and alloys have low resistivity (about 10⁻⁸ to 10⁻⁶ Ω m) and are good conductors; insulators such as rubber and glass have very high resistivity (about 10¹⁰ to 10¹⁷ Ω m). The resistivity of an alloy is generally higher than that of its constituent metals, and alloys do not oxidise (burn) readily at high temperature, so they are used in heating devices such as electric irons, toasters and heaters. Tungsten is used for the filament of bulbs because of its high resistivity and very high melting point (3380 °C). Copper and aluminium are used for transmission wires because of their low resistivity.

If a wire is stretched to double its length, its volume remains constant, so its area becomes half and its resistance becomes four times, while its resistivity does not change.

6. Resistors in Series

When resistors are joined end to end, the same current flows through each of them, and the total potential difference is the sum of the potential differences across the individual resistors: V = V₁ + V₂ + V₃. Using V = IR, the equivalent resistance is R_s = R₁ + R₂ + R₃, which is greater than the largest individual resistance.

7. Resistors in Parallel

When resistors are connected between the same two points, the potential difference across each is the same, and the total current is the sum of the currents in the branches: I = I₁ + I₂ + I₃. The equivalent resistance is given by 1/R_p = 1/R₁ + 1/R₂ + 1/R₃, which is less than the smallest individual resistance. For two resistors R_p = R₁R₂/(R₁ + R₂).

8. Series and Parallel Circuits Compared

Disadvantages of a series circuit: the current is the same everywhere, so a bulb and a heater, which need very different currents, cannot work properly together; and if one component fails, the circuit is broken and none of the others works. Advantages of a parallel circuit: every appliance gets the full voltage, each has its own switch, a fault in one appliance does not affect the others, and the total resistance is smaller so more current can be drawn. That is why household appliances are connected in parallel.

9. Heating Effect of Electric Current

When a current flows through a resistor, electrical energy is converted into heat because the moving electrons collide with the atoms of the conductor. The heat produced is H = I²Rt = VIt (Joule's law of heating): it is directly proportional to the square of the current, to the resistance and to the time for which the current flows.

Applications: electric heater, iron, toaster, geyser, the filament of an electric bulb (which glows because it becomes very hot; the bulb is filled with an inert gas such as nitrogen or argon to prevent oxidation of the filament) and the electric fuse. The fuse is a safety device connected in series; it is made of an alloy of lead and tin with a low melting point and melts when the current exceeds its rating, breaking the circuit and protecting the appliances from overloading and short circuit.

10. Electric Power and Energy

Electric power is the rate at which electrical energy is consumed: P = VI = I²R = V²/R. The SI unit is the watt (W): 1 W = 1 volt × 1 ampere = 1 joule per second. A bulb marked 100 W, 220 V consumes 100 J every second when used on 220 V.

Electrical energy = power × time. The SI unit is the joule; the commercial unit is the kilowatt hour (kWh), called one 'unit': 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J. For a bulb of the same voltage rating, resistance R = V²/P, so the lamp of higher wattage has the lower resistance.

11. Domestic Electric Circuits and Safety

The mains supply is brought to a house through a live wire (red or brown), a neutral wire (black or blue) and an earth wire (green or green-yellow). The potential difference between the live and neutral wires is 220 V. Switches and fuses are connected in the live wire. The metal body of an appliance is connected to the earth wire, which carries any leakage current to the ground and protects the user from electric shock. A short circuit occurs when the live and neutral wires touch, so that the resistance becomes very small and a very large current flows.

Summary of Formulas

Common Errors to Avoid

1. Write the SI unit with every answer.
2. Show the direction of current by an arrowhead in circuit diagrams.
3. Connect the ammeter in series and the voltmeter in parallel.
4. Write the formula before substituting values in a numerical.
5. Write the parallel formula as 1/R_p = 1/R₁ + 1/R₂ + 1/R₃, not R_p = 1/R₁ + 1/R₂ + 1/R₃.
6. Do not confuse resistance (depends on the dimensions) with resistivity (depends on the material and temperature).