Unit 6 - Physics 2: Heat, light and sound

Question 1

Describe evaporation

Answer 1

The escape of more-energetic molecules from the surface of a liquid.

Question 2

State the distinguishing properties of solids, liquids and gases

Answer 2

Solids:

• Retains a fixed volume and shape (rigid particles locked into place)
• not easily compressible (little free space between particles)
• does not flow easily (rigid particles can’t move/slide past one another)
• definite size/volume and shape
• dense

Liquids

• assumes shape of container (particles move/slide past one another)
• not easily compressible (little free space between particles)
• flows easily
• definite volume
• most liquids expand with an increase of temperature and constant air pressure
• dense

Gas

• assumes shape and volume of its container (particles have enough energy to overcome attractive forces)
• the volume of a quantity of gas is dependent on its temperature and the surrounding pressure
• compressible (lots of free space between particles)
• flows easily (particles can move past one another)
• have no definite shape or volume
• if unconstrained gases will spread out indefinitely
• each of the particles are well separated resulting in a very low density

Question 3

Describe qualitatively the molecular structure of solids, liquids and gases

Answer 3

Solids

• particles are close together, held together by strong molecular forces
• particles vibrate around a fixed point, so has little freedom of movement.

Liquids

• particles are quite close together, forces not as strong as of solids but still strong
• takes shape of container

Gas

• particles far apart, move randomly (diffusion)
• bonds between particles are weak

Question 4

Relate the properties of solids, liquids and gases to the forces and distances between molecules and to the motion of the molecules.

Answer 4

Solid
Distance between particles are very close together

Liquid
The distance here is not as close as that of the solids, but the particles are still quite close together, and the forces between the particles are still quite strong.

A liquid, because the particles can move and slide over each other, liquids fill the shape of its container.

Gas
Particles are very far apart here. Again, this can be seen through the diagram.

Because particles are not in place, gases can flow like a liquid.

Question 5

Interpret the temperature of a gas in terms of the motion of its molecules.

Answer 5

The temperature of a substance is proportional to the kinetic energy of the particles. So if the temperature of the gas increases, it will have more kinetic energy, and will move around faster and have more energy

Question 6

Relate evaporation to the consequent cooling

Answer 6

Evaporation is one of the four principal ways that heat can be transferred (the other three are radiation, convection, and conduction). Evaporation of sweat from the skin is an endothermic reaction: the reaction essentially “consumes” heat, which cools the body in the process.

For a liquid to turn into gas, either by boiling or evaporation, energy is required. This is known as the latent heat of vapourisation. In the case of boiling this is provided by external heat sources but in the case of evaporation it has to come from within the liquid itself, thus cooling it.

The energy is required to break the strong bonds between the molecules in the liquid and allow them to break free in gaseous form.
Evaporation is an endothermic (heat-absorbing) process because molecules must be supplied with energy to overcome the intermolecular forces.

Evaporation occurs at the surface of a liquid, and energy is required to release the molecules from the liquid into the gas. The use of this energy, known as latent heat, causes the temperature of the liquid to fall.

Evaporation cools the surface of the thing that the liquid is evaporating from. Reason being evaporation is the departure of the warmest and most active (energetic) particles because they are the ones with enough energy to ‘escape’ into the air. Which leaves behind the cooler, less energetic particles, which brings down the average temperature of what’s left behind.

Question 7

State the meaning of melting point and boiling point.

Answer 7

Melting point:
The temperature at which a substance melts

Boiling point:
The temperature at which a liquid boils and turns to vapour.

Question 8

Describe qualitatively the thermal expansion of solids, liquids and gases

Answer 8

Solids:

When thermal/heat energy is applied to a solid, the molecules of the solid gain kinetic energy and begin to vibrate more vigorously.

As a result, the solid expands slightly in all directions.

Liquids:

When a liquid is heated, the volume of the liquid increases as a result of the molecules having more kinetic energy. The liquid expands to take more of the volume of its container.

Gas:

If the gas is kept in a container of a constant volume, like a canister, and is then heated up, the gas does not expand. This is because gases take up the entire volume of its container. Instead, the pressure inside the container increases since the molecules have more kinetic energy and therefore collide with the walls of the container more often.

On the other hand, if the gas is kept at a constant pressure inside its container, as the gas expands when heat is applied, the volume of the container will increase proportionally to the change in temperature.

Question 9

Identify and explain some of the everyday applications and consequences of thermal expansion

Answer 9

Thermal expansion could be used to fit metal axles onto wheels. The metal axle is first cooled so that it contracts. It is then placed through the hole of wheel so that when it warms up and expands, it forms a tight grip on the wheel.
Train tracks are built with gaps between each section of the track so that when it expands under hot weather, the train tracks won’t warp as a result of the pressure of being squished together.

Bridges are also built with gaps (teeth).

Question 10

Describe experiment to demonstrate the properties of good and bad conductors of heat

Answer 10

A simple experiment can be conducted to find out whether something is a good or bad conductor of heat!

Prepare a few rods made from different materials. Use wax to attach small pins to their ends and then heat the other end of the rods. The thermal energy will be transferred by conduction, from one end to the other. Eventually the wax will melt (due to the heat from the rod) and the pin falls off. The best conducting rod will have its pin dropped off fastest because it transfers the thermal energy the fastest!

or use marbles as in class. Upside down.

Question 11

Explain heat transfer in solids in terms of molecular motion

Answer 11

Particles gain heat energy. Their average KE increases. They vibrate around their average position more. Gaps between the particles get larger. Particles do not increase in size. In conduction the particles do not change position - they increase their motion about their fixed position. Metals contain freely-moving electrons, which transfer heat energy easily through the metal. At higher temperatures these electrons have more kinetic energy and move faster. Vibrating atoms in one part of a material pass on their vibrations to atoms close to them. This is how heat is conducted.

Question 12

Recognise convection as the main method of heat transfer in liquids and gases.

Answer 12

Water gets hot and expands. This lowers its density so it rises. Water at the top is pushed aside by more rising water. Water over here is further away from the heat therefore it cols, contracts and increases the density so it sinks. Water at the bottom is pushed along by falling water. In convection the particles move, completely changing their positions.

Question 13

Describe experiments to illustrate convection in liquids and gases

Answer 13

Manganese dioxide. Glass tank thing as in class.

Question 14

Recognise radiation as the method of heat transfer that does not require a medium to travel through

Answer 14

Heat energy waves or rays are known as infrared radiation. Infrared radiation waves will travel out from a hot source in all directions. Heat energy from the sun travels to earth by radiation. Since the space between the sun and the earth is a vacuum, the heat cannot travel by conduction or convection.

Question 15

Describe experiments to show the properties of good and bad emitters and good and bad absorbers of infra-red radiation

Answer 15

Dark or black surfaces tend to be good emitters and absorbers of heat

White or light-coloured surfaces tend to be poor emitters and absorbers of heat.
Reflective surfaces will reflect the radiation and heat.

An easy experiment can be set up to see if a material is a good or poor emitter/absorber.

Place different materials of different colour under the sun, or close to a source heat radiation.
After a period of time, measure their temperature. The dark surfaces should be warmer because they are better absorbers of infra-red radiation.

Painted cans experiment as in class.

-Matte surfaces cool faster than shiny surfaces
Dark surfaces cool faster than light surfaces
so dull black - best emitter and absorber
shiny white - worst emitter and absorber

Question 16

Identify infra-red radiation as the part of the electromagnetic spectrum often involved in heat transfer by radiation

Answer 16

Radiation is a type of thermal energy transfer due to Electro-magnetic waves.
All hot object emit radiation

• Infra-red radiation is the most common type of heat transfer by radiation. Infra-red waves are a type of electro-magnetic waves and they are part of the electro-magnetic spectrum.

Question 17

Identify and explain some of the every applications and consequences of conduction. convection and radiation.

Answer 17

Conduction, convection and radiation are constantly happening in our every-day environment. They have many applications and can be very useful, some examples include:

• When we heat up pots and pans, the thermal energy is transferred through conduction
• Air condition uses convectional currents to cool the indoor environment.
• When heating up water, the heat is applied from below so it creates a convectional current, which heats up the whole body water
• Sea breezes are caused by convectional currents. This is due to the difference between the temperature of the sea and land.
• Radiation enters greenhouses through the glass and the heat is absorbed and trapped to maintain the temperature
• Solar panels produce electricity by absorbing the heat from sunlight by radiation

Soup!

Question 18

Describe what is meant by wave motion as illustrated by vibration in ropes and springs and by experiments using water waves

Answer 18

A wave is a method of transferring energy without a net movement of the medium through which the wave is travelling. (the mean of the amplitude is he same as rest position)

Wave motion is the transfer of energy from one point to another.

E.g.

Vibration in ropes: Particles in rope vibrate in a fixed position and energy in the particles are transferred from one end of the rope to the other end. Wave travels as a “sideway pulse”
Water ripples: An object that is floating experiences both “up and down” motions.

Question 19

Distinguish between transverse and longitudinal waves and give suitable examples.

Answer 19

Longitudinal: wave where particle motion is parallel to wave direction
Transverse: waves where particles move at 90° to the direction of wave

longitudinal - slinky pushed forward
transverse - slinky waved side to side

Question 20

State the meaning of and use the terms speed, frequency, wavelength and amplitude

Answer 20

Speed: Speed at which a wavefront passes through a medium, relative to the speed of light.

Frequency: the number of waves per second. The number of waves passing any given point each second, measured in Hertz (Hz).

Wavelength: distance between 2 identical points on adjacent waves.

Amplitude: maximum displacement from rest position. Since this is displacement, we usually give amplitude the same units as we would for distance/displacement. E.g. metre, kilometre

Question 21

Recall and use the equation v = f λ

Answer 21

v = f λ
where v = speed, f = frequency, λ = wavelength
remember v = displacement/time

v=ms-1
f=Hz
λ=m

kHz=10^3
mHz=10^6
GHz=10^9

Question 22

Identify how a wave can be reflected off a plane barrier and can change direction as its speed changes

Answer 22

Reflection: If a wave hits a mirror plane, and the plane is nice and smooth, the wave will be directly bounced off and reflected.

Refraction: If the surface of the mirror/medium has interference and is not completely smooth, the wave is partially reflected but most of the wave will be refracted instead. Refraction means that the wave passes through the interface, and in the process acquiring a different direction from the trajectory of the wave that first hit the interface/medium.

During refraction, because the wave travels through a medium, naturally, its speed will also decrease. Most electromagnetic waves travel through the medium at the speed of light, but when they are refracted, they travel at a slower speed.

Question 23

Describe the formation and give the characteristics of an optical image by a plane mirror

Answer 23

The distance of the object can be reflected upon the distance of the image in the mirror – they are equivalent.
Image is upright.
Image is virtual – No light focuses behind the image, as the light rays are reflected across the mirror, so it cannot be projected onto a screen – real images however, can be projected onto a screen
The size/height of the formed image in the mirror is exactly the same as the original object

• upright
• same size
• laterally inverted

Question 24

Perform simple constructions, measurements and calculations based on reflections in plane mirrors

Answer 24

For constructions, simply memorize the diagram above and try to understand how it works.

The process is very simple:

Light is reflected off the object and the rays strike into the mirror
The rays then are reflected back to our eyes.
Calculations are also quite simple – They might ask a question like what is the size of the object in the mirror? It’s obviously the same, or the distance between the mirror and the object is 5cm, what is the distance from the mirror of the image. It’s also 5cm.

Basically, these are all “simple” constructions.

Question 25

Use the law of reflection

Answer 25

The angle of reflection must equal the angle of incidence

Question 26

Describe an experimental demonstration of the refraction of light

Answer 26

In the above image, the image of the submerged pencil appears to be bent at an angle.

This is because when the light that allows us to see the pencil reaches the surface of the water, the angle of the light bends.

This is known as Refraction, and it is the bending of light waves as it passes from one medium to another medium with a different density.

http://www.acceleratedstudynotes.com/2012/05/12/igcse-coordinated-sciences-refraction-of-light/

Question 27

Identify and describe internal and total internal reflection using ray diagrams

Answer 27

When the Angle of Incidence (Ɵi) is less than the Critical Angle (Ɵc), both refraction and reflection is achieved. Only some of the light is reflected inside. As the angle of incidence gets closer to the critical angle, a greater of amount of light is reflected internally.

When the Angle of Incidence (Ɵi) is equal to or more than the Critical Angle (Ɵc), total internal reflection is achieved and the ray of light is completely internally reflected.

http://www.acceleratedstudynotes.com/2012/05/12/igcse-coordinated-sciences-refraction-of-light/

Critical angle -the minimal value of angle i which causes total internal reflection is called the CRITICAL ANGLE

If angle i is increased angle r also increases
If angle i exceeds a value that causes angle r to be greater than 90° then the ray of light is not refracted, it undergoes TOTAL INTERNAL REFRACTION.

Question 28

Describe, using ray diagrams, the passage of light through parallel sided transparent material, indicating the angle of incidence i and of refraction r

Answer 28

When light travels from a less dense material to a more dense material e.g. like from air to glass, the direction of the light bends towards the normal, which is 90o from the boundary.

Angle of incidence > Angle of Refraction when light travels from a less dense material to a more dense material.

Likewise, the opposite occurs when light travels from a dense medium to a less dense medium.

Question 29

Definition of critical angle

Answer 29

The minimal value of angle i which causes total internal reflection.

Question 30

Describe the action of optical fibres particularly in medicine and communications technology

Answer 30

Light is shone in. Whenever the ray hits the edge of the glass fibre at angle greater than the critical angle it will be internally reflected.

Optic fibre networks: thinner, lighter can carry more information
Endoscopes: observations inside the body - safer and easier than invasive surgery.

Question 31

Describe the main features of the electromagnetic spectrum

Answer 31

All the parts of the electromagnetic spectrum can be reflected, refracted and diffracted which shows that each part is a type of wave. In fact, the electromagnetic spectrum is a whole continuous family of waves. They are all transverse waves. They all travel at the same speed through space. This speed is 300,000,000 m/s (the speed of light) this can also be written as 300 thousand km/s.

Electromagnetic waves do not need a medium (matter) to travel in. They can travel through space. This is fortunate as we get most of our energy from the sun as electromagnetic waves.

All part of the electromagnetic spectrum can travel through a vacuum.
All travel at 3 x 10^8 ms-1 in a vacuum (speed of light)

Question 32

State that all electromagnetic waves travel with the same high speed in vacuo

Answer 32

Speed of light 3 x 10^8 ms-1

Question 33

Describe the role of electromagnetic waves

• radiowaves
• microwaves
• infrared
• x-rays

Answer 33

Radiowaves: radio and television communications.
The first type of wave observed by Heinrich Hertz, radio waves are often used for AM/FM Radio transmissions as well as TV broadcasting. Radio waves are produced as a result of accelerating electrons within a circuit.

Microwaves: satellite television and telephones. Cooking food. Microwaves are used extensively in communications as well. Radar allows ships and planes to detect remote objects using microwave radiation. Many forms of wireless also rely on microwaves to communicate between one and another, although not at a level which can cause thermal heating. Lastly, microwaves are used in the aptly named microwave oven to heat food.

Infrared: electrical appliances, remote controllers for televisions and intruder alarms. Infrared waves is commonly associated with thermal radiation. A vast majority of hot objects emit IR radiation, including ourselves, hot pieces of coal, heaters, and the sun. Hence, modern military and security often employ equipment capable of detecting IR radiation (Thermal goggles etc.) as a means to spot humans within a difficult to see environment.

X-rays: medicine and security. The largest source of x-rays on earth happens to be the natural environment and outer space. X-rays are also produced in x-ray tubes by firing electrons at a high velocity and having them rapidly decelerate as they collide with a metal anode (usually tungsten), giving off x-rays in the process.

X-rays are very penetrating and thus have important uses, especially in the field of medicine. First and foremost, x-rays are used in x-ray machines to provide images of our body’s internals without the need for surgery. In addition, they are used for radiotherapy and the management of cancer.

AND ALSO

VISIBLE
Occupying a small window in the electromagnetic spectrum is the all-important visible light. The human eye is capable of detecting this part of the spectrum and it gives us an image of our surroundings. Thankfully, light comes in seven main flavours, giving life as we know it colour.

UV
Ultraviolet waves are very energetic and can actually ionize air, thus forming a part of the upper atmosphere known as the ionosphere. Harmful UV radiation given off from the sun is largely absorbed by ozone in the atmosphere. Still, not all of it is blocked and prolonged exposure to UV radiation can lead to conditions such as skin cancer.

GAMMA
Gamma rays are the most energetic, thus penetrating, waves of the electromagnetic spectrum. On earth, they are produced as a result of radioactive decay of an atom’s nucleus. The remaining source of gamma radiation comes from astrophysical sources such as pulsars, magnetars and quasars in space, giving off gamma radiation that then travel to earth.

Question 34

Demonstrate an awareness of safety issues regarding the use of microwaves and x-rays

Answer 34

The higher the frequency of the wave, the more energy it carries as it travels.

When a wave hits an object, the energy it carries is transferred to the object as kinetic energy. This is why when we microwave water, it gets warmer. The kinetic energy transferred is making the particles of the water vibrate faster, hence its temperature increases. This is the same with humans. If we are exposed to microwaves for a long period of time, there is the danger that permanent damage will be caused to our internal organs as they will warm up when the waves travel through our body.

Since higher frequency waves carry more energy, x-rays are more penetrating than microwaves. They can easily penetrate through most material including our bodies. Brief expose to x-rays carry the increased risk of cancer, since x-rays are ionizing. There is a potential that x-rays may cause mutations in living cells when they pass through the body, hence it is a very good idea that you limit your exposure to x-rays.

Microwaves: heats up water and our body has a lot of water so heating body tissues
X-rays: too much exposure can damage the cells of your body and may result in cancer later.

Question 35

Describe the production of sound by vibrating sources

Answer 35

Sound travels through a wave: that is, a periodic disturbance in space and time. Mechanical vibrations (oscillation pf particles) cause a periodic disturbance in space and time, producing a wave.

Sound waves are longitudinal, however they are often represented as a transverse wave.

Question 36

Describe the transmission of sound in air in terms of compressions and rarefractions

Answer 36

-Vibrations from the source of the sound compress the air, giving it kinetic energy. The KE in the air particles cause them to move from its equilibrium position, exerting a force on adjacent air particles. -The vibrations propagate through a series of perfectly elastic collisions.
-This is why sound is a form of energy. (E=hf)
After KE is transfered, the original particles experience a resultant force (cf. Newton’s 3rd law) and move back to its equilibrium position. This creates a rarefraction in the wave.
-These Compressions and Rarefractions comprise of the sound wave. When it reaches the ear, the vibrations of the air particles are translated into sound.

Question 37

State the approximate human range of audible frequencies

Answer 37

20Hz to 20,000 Hz
Remember this as 20-20: 20 Hz to 20 kHz

Frequency relates to the pitch - higher pitch = higher frequency.

Question 38

Demonstrate understanding that a medium is needed to transmit sound waves

Answer 38

A sound wave is really a bunch of oscillating air particles (particles of any other medium). When there are no particles to vibrate, there is no sound wave.

Question 39

Describe an experiment to determine the speed of sound in ar

Answer 39

Stand a measured distance from a wall, x.
Make a short, loud burst of sound, start timing.
When you hear the echo, stop timing. let the value obtained be x.
The sound wave has traveled 2x.
Since v =ds/dt
substituting values
v = 2x/t = ~330 ms^-1

http://www.instructables.com/id/How-to-measure-the-speed-of-sound-with-two-lumps-o/

Question 40

Relate the loudness and pitch of sound waves to amplitude and frequency

Answer 40

Higher pitch = higher frequency
The louder the sound = larger the amplitude

Frequency = number of waves per second
Amplitude = maximum displacement from the particle's rest position

Question 41

Describe how the reflection of sound may produce an echo

Answer 41

When a sound wave is reflected, it has the same magnitude and different direction (sometimes moving back to the source). Since the wavelength and frequency is still the same, the observer perceives this as the same sound as the original. This is an echo.

Echoes are sound waves bouncing off surfaces. Sound waves obey the same first rule of reflection. (Remember: the angle of incidence is the same as the angle of reflection.)

The echo is usually quieter than the original noise as energy is lost as the wave travels along.

You can work out how far away something is using echo-sounds.

If it takes 20 seconds for the echo to be detected it must have taken 20 seconds for the sound to travel to the object and back. Using:

Distance = Speed x Time

The distance can be calculated. The speed of sound is 330 m/s so the calculation becomes:

Distance = 330 m/s x 20 s = 6600 m

This is the distance there and back, so the object is half that distance away, 3300 m.

Watch out. Many students forget to halve the distance.

Shiny hard surfaces reflect sound better than soft, surfaces. Bathrooms are good rooms to sing in as the sound bounces well off tiled walls. If you sing in the living room most of the sound energy is lost, because the energy is absorbed by the carpet, furniture and curtains.

Question 42

State the order of magnitude of the speed of sound in air, liquids and solids.

Answer 42

Solids – Fastest
Liquids – Middle
Gases – Slowest
Explaination: in terms of the atomic theory of matter, solids are the densest and gases are the least dense of the 3 states. Since sound travels by oscillating atoms (propagated through collisions), the denser the medium, the faster the spee