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Physics 8th Grade: Transmission of Heat
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Physics 8th grade Practice makes perfect Physics 8th grade Transmission of Heat • Heat can be transmitted in three main ways: Conduction Convection Radiation Practice makes perfect Physics 8th grade 1. Transmission of Heat by Conduction: Page 42 The metal door knob has a higher rate of thermal conductivity with respect to wood. Therefore, it is able to take heat away from you faster than the wooden door. That is why it feels colder, even if they are at the same temperature. Practice makes perfect Physics 8th grade • Conduction is the transfer of heat through direct contact. Therefore, heat is transferred atom by atom. • Conduction is the best way to transfer heat in solids. Practice makes perfect Physics 8th grade Note: In conduction More heat is transferred every second if: • The temperature difference (∆T) across the ends of the object is increased. • The cross-sectional area (A) of the object is increased. • The length (d) of the object is reduced. • Different object with higher thermal conductivity coefficient (k) is used. Practice makes perfect Physics 8th grade • The rate of heat transferred per second can be found by Fourier’s the thermal Law: conductivity (𝑊/𝑚 ∙ 𝐾) the rate of heat transferred (W) the cross-sectional area (𝑚2 ) Q k A T =− t d the length (m) indicates that heat flows from the higher temperature to the lower temperature Practice makes perfect the temperature difference (°C, K) Physics 8th grade Note: Thermal conductivity is a measure of how well a material can conduct heat. The higher the thermal conductivity, the better it can transfer heat. Practice makes perfect Physics 8th grade Page 42 Practice makes perfect Physics 8th grade Page 43 Practice makes perfect Physics 8th grade Page 43 Practice makes perfect Physics 8th grade Conductors and Insulators 1. Conductors are materials that allow heat to flow through them easily. • Example: Metal (like copper, aluminum). 2. Insulators are materials that do not allow heat to flow easily. • Example: Wood, rubber, plastic Practice makes perfect Physics 8th grade People can walk on hot coals without injury because coal is a poor conductor of heat, and they walk quickly, so their feet don't stay on the hot surface long enough to burn. Yes, if a person stands still on hot coals, they will likely get burned because the heat has more time to transfer to the skin. Practice makes perfect Physics 8th grade Transmission of Heat by Convection • Convection transfers heat when warm air or liquid rises and cool air or liquid sinks, creating a cycle. • In fluids convection is the best way to transfer heat. • A convection current is the circular movement of warm air or liquid rising and cool air or liquid sinking. Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Note: A solar chimney generates power using hot air rising through a chimney. Solar energy heats the air in a glass collector, creating an upward airflow. This moving air drives turbines inside the chimney to produce electricity. Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Transmission of Heat by Radiation On clear nights, heat escapes into space, so it gets cooler. On cloudy nights, clouds trap heat, keeping it warmer. Practice makes perfect Physics 8th grade • Radiation is the transfer of heat without any medium, through waves • Example: The Sun heats the Earth through radiation Practice makes perfect Physics 8th grade • • • Camper A – Gets heat by radiation. Camper B – Gets heat by convection Camper C – Gets heat by conduction Page 46 Practice makes perfect Physics 8th grade Melting time is inversely proportional with thermal conductivity. Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Page 47 Your HW Practice makes perfect Physics 8th grade Thermal Equilibrium • When two objects of different temperatures touch, heat moves from the hotter to the colder one until both reach the same temperature. This is called thermal equilibrium. • If there's no phase change, the final temperature is between the hot and cold object's temperatures. Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Conservation of Energy • Until equilibrium, heat gained by the cold object equals heat lost by the hot one. • In an isolated system, energy isn't created or destroyed—just changed into heat, work, or both. Practice makes perfect Physics 8th grade Page 47 Practice makes perfect Physics 8th grade Page 48 Practice makes perfect Physics 8th grade Page 48 Practice makes perfect Physics 8th grade Usage Of Heat 1. Role of Heat in Water Cycle: • Heat from the sun powers the water cycle. It causes water to evaporate, forming clouds through condensation. When the clouds get heavy, water falls as precipitation like rain or snow. Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade 2. Role of Heat in Enzyme Reactions: • Enzymes are bio-catalysts that speed up biochemical reactions. • They work best at a specific temperature. • At low temperatures, they are inactive, and at high temperatures, they are destroyed Practice makes perfect Physics 8th grade 3. Role of Heat in Chemical Reactions: • Heat causes atoms and electrons to vibrate, making it easier to form new bonds. This helps in synthesis (building molecules) and breakdown (breaking molecules) of substances. Practice makes perfect Physics 8th grade 4. Role of Heat in Cooking: • Cooking helps food digest easily and kills harmful microorganisms with heat. Practice makes perfect Physics 8th grade 5. Role of Heat in Electricity Generation: • Electricity can be made using dams, solar panels, wind turbines, or power plants. • In thermal plants, coal is burned to make steam, which turns a turbine to produce electricity. • In nuclear plants, nuclear energy (from uranium) makes the heat instead of coal. Practice makes perfect Physics 8th grade 6. Role of Heat in Melting and Molding Metals: • Metals are hard substances used in buildings, vehicles, and tools. • Cannot be easily shaped by force or pressure. • Can be molded easily when heated. Practice makes perfect Physics 8th grade 7. Role of Heat in Thermal Machines: • • • • • Thermodynamics helped improve engines. Steam engine used steam instead of people or animals. It was the first good machine to do big work. Heat is made outside the engine (external combustion). James Watt made it better in 1769. Practice makes perfect Physics 8th grade • Richard Trevithick (1814) made the first steam locomotive. • Heat moves from high to low temperature → does mechanical work. • Boiler (coal) heats water → steam moves pistons. • Helped create ships and railways. • Converts heat → kinetic energy → other energy forms Practice makes perfect Physics 8th grade Note (Internal combustion engine or explosion engine): • • • • The engine burns fuel (like petrol). The fuel creates small explosions. These explosions push parts called pistons. The pistons make the car’s wheels turn Practice makes perfect Physics 8th grade Page 53-57 Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Magnets • • • • • • A magnet can lift some things, like iron nails. This shows magnets have a special force. Magnetism was found around 800 BC. A natural magnet is called magnetite (Fe₃O₄). First found near a place called Magnesia in Turkey. The word magnet comes from Magnesia. Practice makes perfect Physics 8th grade Magnetic Properties of Materials • Iron, nickel, cobalt can attract metals – they are magnetic. • Some materials become magnetic when near a magnet. These are called magnetic materials. • Artificial magnets are made from magnetic materials like iron or steel. • Magnet shapes: bar, U-shape, horseshoe, ring, disk. Practice makes perfect Physics 8th grade • Non-magnetic materials (like copper, silver, lead) don’t react to magnets. • Magnet’s ends are strongest (where iron filings gather). • A magnet always has two poles: North (N) and South (S). • If broken, each piece still has both poles. • A magnet with one pole cannot exist. Practice makes perfect Physics 8th grade Magnetic Force • Magnetic force is the push or pull a magnet gives – it's a noncontact force. • Same poles of magnets repel (push away). • Opposite poles attract (pull together). • A magnet always attracts pins (makes part of the pin opposite). • Iron filings show magnetic force lines. • Filings form lines from one pole to another Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Page 61 Practice makes perfect Page 62 Physics 8th grade Practice makes perfect Page 62 Physics 8th grade Practice makes perfect Page 62 Physics 8th grade Practice makes perfect Physics 8th grade Classification of Materials with Respect to Their Magnetic Properties: • • • • • Ferromagnets (strongly attracted): Iron, nickel, cobalt. Paramagnets (weakly attracted): Air, aluminum, silicon. Diamagnets (not attracted): Water, copper, gold, silver. Ferromagnets are used in motors and electromagnets. Paramagnets are hard to notice with weak magnets. Practice makes perfect Physics 8th grade Magnets in Daily Life • • • • Used to close doors (fridges, cabinets). Found in kitchens, bags, wallets, phone covers. Credit cards have magnetic strips. Compasses help sailors find directions Practice makes perfect Physics 8th grade Electromagnets • Magnets have a force field. • Electromagnetism is when electricity and magnetism work together. • Electromagnets use electricity to work. • Turning off electricity makes the magnet stop. • Stronger electromagnets need more electricity and turns. • Electromagnets attract ferromagnetic materials. Practice makes perfect Physics 8th grade Electromagnets in Daily Life • Electromagnets are used in doorbells, telephones, motors, and cranes. • Cranes use electromagnets to lift iron objects. • Coils in electromagnets are in many modern devices Practice makes perfect Physics 8th grade Earth as a Magnet • Earth's core is made of iron and nickel, which act like a giant magnet. • Compass needles point to magnetic north because of Earth's magnetism. • Magnetic north is at geographic south, and magnetic south is at geographic north. • Auroras (Northern and Southern Lights) happen because Earth's magnetism pulls charged particles to the poles, creating bright lights. Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade 67-69 Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Electric Charges • Everything is made of atoms. • Atoms have 3 parts: • Electrons (tiny, negative charge, move around the center) • Protons (positive charge, in the center) • Neutrons (no charge, in the center) Practice makes perfect Physics 8th grade • Center of atom = Nucleus (has protons & neutrons) • Electrons are attracted to protons (because opposite charges attract). • Neutrons have no charge, so no attraction. • Charges: • Electron = -1 charge • Proton = +1 charge • Neutron = 0 charge (neutral) Practice makes perfect Physics 8th grade • Neutral atom: Same number of protons and electrons → total charge = 0 • Ion: Atom that gains or loses electrons → total charge ≠ 0 • Gains electrons → Negatively charged • Loses electrons → Positively charged • Unit of charge: Coulomb (C) • 1 C = charge of ~6.25 x 10¹⁸ electrons • Electron/proton charge = 1.6 x 10⁻¹⁹ C Practice makes perfect Physics 8th grade • Masses: • Proton = 1.67 x 10⁻²⁷ kg • Electron = 9.11 x 10⁻³¹ kg • Proton is ~2000 times heavier than electron Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Electrical Forces • Electric force (or electrostatic force) is the force between electric charges. • It is a non-contact force (works without touching). • It can be: • Attractive → pulls charges together • Repulsive → pushes charges apart • Like charges (both positive or both negative) → Repel • Unlike charges (one positive, one negative) → Attract Practice makes perfect Physics 8th grade Practice makes perfect Physics 8th grade Page 73 your HW Practice makes perfect Physics 8th grade Page 73 Practice makes perfect Physics 8th grade Page 73 Practice makes perfect Physics 8th grade Electroscope • An electroscope is a tool that helps us find out if something is electrically charged. It has two metal leaves that move when charged. • If the electroscope is negatively charged, the leaves push away from each other. • If it’s positively charged, the leaves also push away. • If it's neutral, the leaves stay close together. Practice makes perfect Physics 8th grade Practice makes perfect
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