Everyday Science Notes for Competitive Exams

Heat, sound and electricity are part of our daily life: we feel the warmth of the Sun, listen to music, switch on a fan and pay an electricity bill. These topics are asked regularly in the general science section of SSC, railways, banking, defence, state-level examinations and the UPSC Prelims, in the form of simple concept questions, units and small numericals. This note explains heat and temperature, modes of heat transfer, sound and its properties, and electricity with circuits, Ohm's law, power, safety and magnetism, along with solved examples, in simple language.

Heat and Temperature

Heat is a form of energy that flows from a hot body to a cold body, and it is measured in joules (or calories; 1 calorie is about 4.18 joules). Temperature measures how hot or cold a body is. It is measured in degrees Celsius (°C), Fahrenheit (°F) or kelvin (K, the SI unit). The conversions are: F = (9/5) × C + 32 and K = C + 273. Water freezes at 0°C (32°F, 273 K) and boils at 100°C (212°F, 373 K) at normal pressure, and the normal human body temperature is 37°C (98.6°F). A clinical thermometer uses mercury and has a kink to hold the reading; it measures from 35°C to 42°C.

Modes of Heat Transfer

Metals are good conductors, while wood, plastic, air and wool are poor conductors (insulators). Dark and rough surfaces absorb and emit heat better than light and shiny surfaces; therefore, we wear light-coloured clothes in summer. Sea breeze blows from the sea to the land by day, because the land heats up faster than water.

Specific Heat, Latent Heat and Expansion

The specific heat capacity is the heat needed to raise the temperature of 1 kg of a substance by 1°C. The heat required is Q = m × c × ΔT. Water has a very high specific heat capacity, about 4,200 J/kg°C, which is why it heats up and cools down slowly. This is why coastal areas have a moderate climate, and why water is used in hot-water bottles and as a coolant. Example: to heat 2 kg of water from 10°C to 30°C needs Q = 2 × 4,200 × 20 = 168,000 J. Latent heat is the heat absorbed or released during a change of state without any change in temperature. The latent heat of fusion of ice is about 334 kJ/kg, and the latent heat of vaporisation of water is about 2,260 kJ/kg. Steam at 100°C causes more severe burns than boiling water at 100°C because of its extra latent heat. Evaporation causes cooling, as when sweat evaporates from the skin. Most substances expand on heating, but water shows anomalous expansion: it contracts when heated from 0°C to 4°C and has its maximum density at 4°C; this is why lakes freeze from the top while fish survive below the ice. The boiling point falls at higher altitudes because of lower pressure, and a pressure cooker raises the boiling point by increasing the pressure.

Sound

Sound is produced by vibrations and needs a material medium (solid, liquid or gas) to travel; it cannot travel in a vacuum. Sound travels as longitudinal waves, which are made of compressions and rarefactions. The speed of sound is highest in solids, less in liquids and least in gases: about 343 m/s in air at 20°C, about 1,480 m/s in water and about 5,000 m/s in steel. It increases with temperature. The relation of speed, frequency and wavelength is v = f × λ. For example, a sound of frequency 500 Hz and wavelength 0.68 m has a speed of 500 × 0.68 = 340 m/s. Frequency is the number of vibrations per second, in hertz (Hz), and decides the pitch (how shrill or deep a sound is). Amplitude decides the loudness, measured in decibels (dB).

Audible Range, Ultrasound and Infrasound

The human ear can hear frequencies from about 20 Hz to 20,000 Hz. Sounds below 20 Hz are called infrasound; elephants and whales use them, and earthquakes produce them. Sounds above 20,000 Hz are ultrasound; bats and dolphins use it. Ultrasound is used in medical imaging (ultrasonography), cleaning delicate parts, detecting flaws in metals and by SONAR (Sound Navigation and Ranging) to find the depth of the sea and locate submarines. If a SONAR signal returns after 2 seconds, and the speed of sound in water is 1,500 m/s, the depth is (1,500 × 2)/2 = 1,500 m.

Echo and Reverberation

An echo is the repetition of sound caused by its reflection from a hard surface. Our ears retain a sound for about 0.1 second, so to hear a distinct echo, the reflecting surface must be at least about 17 metres away (343 × 0.1/2 ≈ 17 m).

Electricity: Basic Terms

Electric charge is measured in coulombs (C). Electric current is the rate of flow of charge: I = Q/t, in amperes (A), and conventionally flows from the positive terminal to the negative terminal, while electrons actually move in the opposite direction. Potential difference (voltage) is the work done per unit charge, measured in volts (V). Resistance is the opposition to current, measured in ohms (Ω). Ohm's law states that the current through a conductor is proportional to the potential difference across it at constant temperature: V = IR. Example: if a potential difference of 12 V is applied across a resistance of 4 Ω, the current is 12/4 = 3 A. An ammeter is connected in series in a circuit to measure current, and a voltmeter is connected in parallel to measure voltage.

Series and Parallel Circuits

In a series circuit, resistors are connected end to end, the current is the same through all, and the total resistance is R = R₁ + R₂ + R₃. If one bulb fuses, the whole circuit stops. In a parallel circuit, resistors are connected across the same two points, the voltage is the same across each, and 1/R = 1/R₁ + 1/R₂ + 1/R₃. The total resistance is smaller than the smallest individual resistance. Example: 6 Ω and 3 Ω in parallel give 1/R = 1/6 + 1/3 = 3/6, so R = 2 Ω. Household appliances are connected in parallel so that each gets the full voltage and works independently.

Electric Power and Energy

Electric power is P = V × I = I²R = V²/R, in watts (W). Electrical energy is power multiplied by time. The commercial unit of energy is the kilowatt-hour (kWh), commonly called one "unit", which equals 3.6 million joules. Example: a 1,000 W (1 kW) heater used for 2 hours consumes 2 kWh. If the rate is 7 rupees per unit, the cost is 14 rupees. The filament of an incandescent bulb is made of tungsten, which has a very high melting point (about 3,400°C), while the heating element of an iron or heater is nichrome, an alloy with high resistivity. Joule's law of heating says that heat produced is H = I²Rt.

Domestic Electricity and Safety

In India, the household supply is alternating current (AC) at about 220 to 240 volts and 50 Hz. A three-pin plug has the live, neutral and earth wires. The earth wire protects users from electric shocks by giving a safe path for leaking current to the ground. A fuse is a wire of low melting point (a tin-lead alloy) that melts and breaks the circuit when too much current flows, protecting appliances; a Miniature Circuit Breaker (MCB) does the same job and can be reset. Short circuit occurs when the live and neutral wires touch directly, and overloading occurs when too many appliances run on the same circuit.

Magnetism and Electromagnetic Induction

A current-carrying wire produces a magnetic field (Oersted, 1820). A coil with an iron core becomes an electromagnet, used in cranes, electric bells and relays. An electric motor converts electrical energy into mechanical energy, and its direction of force follows Fleming's left-hand rule. An electric generator converts mechanical energy into electrical energy using electromagnetic induction (discovered by Michael Faraday), and the direction of the induced current follows Fleming's right-hand rule. A transformer changes the voltage of AC: a step-up transformer increases voltage, and a step-down transformer decreases it. It works on mutual induction and does not work on DC.

Quick Revision

• F = 9C/5 + 32; K = C + 273; water has maximum density at 4°C; specific heat of water is 4,200 J/kg°C.
• Audible range 20 Hz to 20 kHz; v = fλ; echo needs a distance of at least about 17 m; sound cannot travel in a vacuum.
• V = IR; P = VI; 1 kWh = 3.6 million joules; ammeter in series, voltmeter in parallel.
• Fuse melts on overload; earth wire prevents shock; motor uses Fleming's left-hand rule.

Heat, sound and electricity are best learned by linking them to home appliances and daily experiences. Revise the formulas, solve a few numericals daily, and observe how each principle works in your surroundings.

Related Notes to Read Next

Continue your preparation with these related notes from the same subject:

• Cell Biology, Genetics and Plant Science Notes for Competitive
• Atomic Structure, Carbon Compounds and Fuels Notes
• SI Units and Measurement Notes for Competitive Exams
• All General Science notes

Frequently Asked Questions (FAQ)

What is the difference between heat and temperature?

Heat is the energy that flows due to a temperature difference, measured in joules, while temperature measures how hot or cold a body is, measured in degrees Celsius or kelvin.

How do I convert Celsius to Fahrenheit?

Use F = (9/5) × C + 32. For example, 37°C equals 98.6°F.

What are the three modes of heat transfer?

The three modes are conduction, convection and radiation.

At what temperature does water have maximum density?

Water has its maximum density at 4°C.

Why does sound need a medium?

Sound is a mechanical wave produced by vibrations of particles, so it needs a material medium and cannot travel through a vacuum.

What is the audible range of the human ear?

The human ear can hear sounds from about 20 Hz to 20,000 Hz.

What is the minimum distance for hearing an echo?

The reflecting surface should be at least about 17 metres away for a distinct echo to be heard.

What does Ohm’s law say?

Ohm’s law states that the current through a conductor is directly proportional to the potential difference across it at constant temperature, V = IR.

Why are household appliances connected in parallel?

In a parallel connection each appliance gets the full voltage and works independently, so if one fails, the others continue to work.

What is a unit of electricity?

One unit of electricity is one kilowatt-hour (kWh), the energy used by a 1,000-watt appliance in one hour, equal to 3.6 million joules.

What is the function of a fuse?

A fuse is a safety device with a low-melting wire that melts and breaks the circuit when the current becomes too high.

Which rule gives the direction of force in an electric motor?

Fleming’s left-hand rule gives the direction of force on a current-carrying conductor in a magnetic field, as in an electric motor.