Exam practice

Thick mud walls (and roofs) are poor conductors of heat, and they hold a lot of heat. In summer, the outside heat takes a very long time to be conducted through them, so the rooms stay cool through the hot afternoon. In winter, the warmth inside escapes through them only slowly, so the rooms stay warm.

Porcelain is a poor conductor (an insulator) of heat. So the heat of the tea doesn’t reach your fingers quickly, and the cup is comfortable to hold, and the tea loses heat more slowly and stays hot longer.

Light-colored clothes reflect most of the Sun’s radiation and absorb little of it. So less heat reaches the body, and we feel cooler. Dark clothes absorb more radiation and get hot.

Warm, used air in a room (from our breath, bodies, stoves and lamps) is less dense, so it rises to the top of the room by convection. A ventilator high up lets this warm air out, while fresh, cooler air comes in through doors and windows lower down. This keeps a convection current of fresh air moving through the room.

Transparent plastic lets the Sun’s short-wave radiation (light) pass through into the greenhouse, where the soil and plants absorb it and warm up. The warm soil and plants give out long-wave radiation, which the plastic lets out only in part, and the plastic also stops the warm air from blowing away. So heat is trapped inside, and the plants get the light they need too.

The flow of heat energy from one object or place to another, always from the hotter one (higher temperature) to the colder one (lower temperature), is called the transmission of heat. It happens in three ways: conduction, convection and radiation.

Conduction is the transfer of heat through a solid from particle to particle, without the particles leaving their places. The heated particles vibrate harder and pass this vibration (heat) on to their neighbors. Example: the handle of a metal spoon in hot tea gets hot.

Wind blows because of convection in the air. During the day, the Sun heats the ground unevenly, and the ground heats the air above it. Where the air is heated, it expands, becomes less dense and rises, leaving an area of low pressure. Cooler, denser air from nearby (at higher pressure) flows in to take its place. This movement of air from high pressure to low pressure is the wind. A sea breeze is an example: by day, cool air blows from the sea towards the hotter land.

Convection is the transfer of heat in a liquid or a gas by the actual movement of its particles: the heated part becomes less dense and rises, and the cooler, denser part sinks to take its place, setting up a convection current.
Uses:

1. Water and other liquids in a pot are heated by convection.
2. Room heaters, fans and air conditioners warm or cool a whole room by moving air.
3. In many refrigerators the freezer is at the top, so that the air it cools sinks and warmer air rises to it.
4. Ventilators near the ceiling let warm, stale air out of a room.
5. Hot-air balloons rise because hot air is less dense than the air around.
6. In nature, convection causes winds, sea and land breezes, clouds and rain.

Radiation is the transfer of heat as electromagnetic waves (mainly infrared), without any material medium. It can happen even through a vacuum, at the speed of light.
Uses:

1. The Earth gets heat and light from the Sun by radiation.
2. We warm ourselves near a fire or heater by radiation.
3. Clothes, grain and fish are dried in the sun.
4. Solar water heaters and solar cookers, painted black inside, absorb the Sun’s radiation.
5. The black condenser at the back of a refrigerator gives out heat partly by radiation.
6. Light-colored clothes are worn in summer because they absorb less radiation.

An electromagnetic wave is a wave that does not need any material medium to travel; it can travel through a vacuum, at the speed of light (3 × 10⁸ m/s). It is made of vibrating electric and magnetic fields.
Examples (any three): light, infrared rays, radio waves, microwaves, ultraviolet rays, X-rays, gamma rays.

Draw a cross-section like Figure 7.10 and label: cap (cup), insulating stopper, double glass walls, vacuum, silvered surfaces, hot liquid, outer case, insulating support.

The greenhouse effect is the warming of the Earth’s surface because greenhouse gases in the air (carbon dioxide, water vapor, methane, nitrous oxide, CFCs) absorb the long-wave heat radiation given out by the Earth and send part of it back down, instead of letting it all escape into space.
Importance:

1. It keeps the Earth’s average temperature at about 15 °C; without it, the Earth would be about −18 °C and frozen, and life would hardly be possible.
2. In artificial greenhouses, farmers grow off-season vegetables and earn a good income.
3. Plants that can’t survive the cold can be protected in greenhouses.
4. Plants of hot places can be grown even in cold countries.

(Too strong a greenhouse effect, caused by extra greenhouse gases, leads to global warming.)

The imaginary straight line passing through the pole (P) and the center of curvature (C) of a spherical mirror.

The principal focus (F) is the point on the principal axis where rays parallel to the principal axis meet after reflection from a concave mirror, or from which they appear to come after reflection from a convex mirror. It lies halfway between the pole and the center of curvature.

The center of the hollow sphere of which the spherical mirror is a part. It is written C.

The distance between the pole (P) and the principal focus (F) of a spherical mirror. It is written f, its SI unit is the meter, and it is half the radius of curvature ($f = R/2$).

When rays of light parallel to its principal axis fall on a concave mirror, they are reflected and meet (converge) at one point, the principal focus. Because it brings rays together, it is called a converging mirror.

A concave mirror is a converging mirror. The Sun’s rays arrive almost parallel, and the mirror reflects them all to one point, its focus. The heat of a large area of sunlight is gathered onto the small area of the pot placed there, so it gets hot enough to cook food.

A convex mirror always forms an erect and diminished image. Because the image is small, a wide field of view, a long stretch of road and many vehicles behind, fits in the small mirror, and because it is erect, the driver sees the traffic the right way up.

A real image forms only where reflected rays actually meet. A convex mirror spreads (diverges) the rays, and a plane mirror reflects them so that they keep spreading, just as they came from the object. In both, the reflected rays never meet in front of the mirror; they only appear to meet behind it, where no light goes. So both always form virtual images, which can’t be caught on a screen.

The bouncing back of light into the same medium after it strikes a surface is called the reflection of light. A smooth, polished surface like a mirror reflects light regularly and forms an image; a rough surface scatters it in all directions.

A mirror whose reflecting surface is a part of a hollow sphere is called a spherical mirror. There are two kinds: a concave mirror, which reflects from the inner (hollow) side, and a convex mirror, which reflects from the outer (bulging) side.

It depends on where the object is: the object’s distance from the mirror, compared with the mirror’s focal length (whether the object is at infinity, beyond C, at C, between C and F, at F, or between F and P). This decides where the image forms, whether it is real or virtual, erect or inverted, and how big it is compared with the object.

A concave mirror. When the face is held close to it, nearer than its focus (between F and P), it forms a virtual, erect and magnified image, so the face looks bigger and the right way up, and small details are easy to see.

A convex mirror is a diverging mirror. Placed behind the bulb, it spreads the light reflected from it over a wide area of the road, instead of sending it in a narrow beam.

(i) Beyond C: the image forms between F and C; it is real, inverted and diminished.

(ii) At infinity: the parallel rays meet at F; the image is real, inverted and highly diminished (a point).

(iii) Between C and F: the image forms beyond C; it is real, inverted and magnified.

In each, draw at least two of the three rays: parallel then through F; through C and back; through F then parallel.

The speed of sound depends only on the medium it travels through (and its temperature), not on the sound itself. In a given medium, $v = f \lambda$ is fixed. So if the frequency increases, the wavelength decreases in the same proportion (and the other way round), and their product, the speed, stays the same.

From about 20 Hz to 20,000 Hz (20 kHz). This is audible sound.

Speed = frequency × wavelength, $v = f \lambda$, where v is in m/s, f in Hz and λ in m. In one second, f waves pass, each one λ long, so the wave travels f λ meters every second.

Draw the compressions and rarefactions of the air, and below them the wave as a graph, labeling the wavelength (between two crests, or two compressions), the amplitude, a crest and a trough, as in Figure 7.23.

A sound wave with a frequency above 20 kHz, too high for humans to hear, is an ultrasonic wave (ultrasound).
Animals: bats and dolphins (also rats and mice, and some insects).

1. In ultrasonography (USG), to see a baby growing in the womb and to find stones, tumors and diseases of the internal organs.
2. In SONAR, to find the depth of the sea and to locate shoals of fish, wrecks and submarines.
3. To clean medical instruments and jewelry and kill bacteria, and to break up kidney stones and destroy some tumors without cutting.

The intensity of sound decreases with distance. As the sound travels outwards, its energy spreads over a bigger and bigger area, so less of it passes through each square meter. Also, on the way, some of the energy is absorbed by the air and some is reflected. So less sound energy reaches the ear of a listener farther away, and the sound is fainter.

Effects: (any two) it weakens hearing and can cause deafness; it raises blood pressure; it causes headaches, stress and lack of sleep; it makes it hard to concentrate.

Ways to reduce it: (any two) plant trees along roads and around factories; fit silencers on vehicles and avoid unnecessary horns; keep the volume of TVs, radios and loudspeakers low; run awareness programs.

v = 332 m/s, f = 10 Hz, λ = ?
$\lambda = \frac{v}{f} = \frac{332 \text{ m/s}}{10 \text{ Hz}} = 33.2 \text{ m}$
The wavelength is 33.2 m. (A 10 Hz sound is infrasound: no one can hear it.)

λ = 22 m, f = 15 Hz, v = ?
$v = f \lambda = 15 \text{ Hz} \times 22 \text{ m} = 330 \text{ m/s}$
The speed is 330 m/s.

Humans hear from 20 Hz to 20,000 Hz. The highest frequency has the shortest wavelength.

Shortest wavelength, from the highest frequency, 20,000 Hz:
$\lambda = \frac{v}{f} = \frac{332}{20000} = 0.0166 \text{ m}$ (1.66 cm)
Longest wavelength, from the lowest frequency, 20 Hz:
$\lambda = \frac{332}{20} = 16.6 \text{ m}$
So humans hear sound waves from 0.0166 m to 16.6 m long.

I = 2 × 10⁻⁵ W/m², A = 6 m²
From $I = \frac{P}{A}$, $P = I \times A = 2 \times 10^{-5} \times 6 = 1.2 \times 10^{-4} \text{ W}$
Power is the energy per second, so in 1 second the sound carries 1.2 × 10⁻⁴ J of energy through the area.