Monday, 30 April 2012

Energy transfer

Section 4 part a) and b)

a) Units
4.1 use the following units: kilogran (kg), joule (J), metre (m), metre/second (m/s), metre/second(m/s2), newton (N), second (s), watt (W).

b) Energy transfer
4.2 describe energy transfers involving the following forms of energy: thermal (heat), light, electrical, sound, kinetic, chemical, nuclear and potential (elastic and gravitational)
some examples we had in class:
  • battery connected to a light bulb: chemical-kinetic-light & heat
  • candle and a box of matches: kinetic-light & heat
  • elastic band and a 'paper pellet': elastic-kinetic
  • bowl of breakfast cereal eaten: solar-chemical-kinetic + heat
  • plant in a pot by the window: light (solar) - chemical
Piece of equipment
Input energy
Output energy
Mains light bulb
Electric fire

4.3 understand that energy is conserved
Energy is not created nor destroyed, it is just transferred from one form to another. 
Thermal energy always flows from a region of higher temperature to a region of lower temperature. 

4.4 recall and use the relationship:
efficiency= useful energy output/total energy input 

If you use the following equation, remember to add a percentage sign (%):
efficiency= useful energy output/total energy input  x 100 

4.5 describe a variety of everyday and scientific devices and situations, explaining the fate of the input energy in terms of the above relationship, including their representation by Sankey diagrams

Sankey diagrams shows what happens to the input energy and in what proportion it is transferred to various forms. For example: 
This is the Sankey diagram for a typical filament lamp.
total electrical energy is 100 j, 90 j is transferred as heat energy and 10 j transferred as light energy
Sankey diagram for a filament lamp
Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat energy instead of light energy. 

The efficiency of the filament lamp is (10 ÷ 100) × 100 = 10%.
This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy).

total electrical energy is 100 j. 25 j is transferred as heat energy and 75 j transferred as light energy
Sankey diagram for a typical energy-saving lamp

From the diagram, you can see that much less electrical energy is transferred, or 'wasted', as heat energy.
The efficiency of the energy-saving lamp is (75 ÷ 100) × 100 = 75%. This means that 75% of the electrical energy supplied is transferred as light energy (25% is transferred as heat energy).
Note that the efficiency of a device will always be less than 100%.

Relating to specification point 4.3, remember that energy is conserved. It is neither created nor destroyed, just transferred from one form to another or moved. So here energy may be 'wasted', as it isn't the form we want-which is light. But the heat energy from the lamps don't just disappear, they are transferred to the surroundings so sometimes you feel hot sitting under the lamps. And the heat energy spreads out until it becomes difficult to do anything useful with it. 


4.6 recall that energy transfer may take place by conduction, convection and radiation

Conduction: is the process of thermal energy transfer without any flow of the material medium. It is from particle to particle (as they move faster, they push on neighbouring particles so they speed up too). (Occurs mainly in solids.)
**This has apparently confused some people, about what 'flow of the material medium' actually means. Sorry about that, I think I took the definitions from a textbook somewhere and didn't read it properly to see if it was easy to interpret. Hm, well, in conduction heat is transferred:
-when atoms vibrate and heat is transferred from one atom to another
-or when free electrons move from the hot end to the cold end (hence metals are good conductors of heat-they have free electrons)
So the material isn't moving, the metal wire (or whatever medium) isn't going anywhere when it conducts heat. Whereas in convection, the water/air molecules are moving and their movement creates convection currents to transfer heat. Remember conduction mainly occurs in solids, they can't move around like fluids (liquids/gases) can.
Hope that makes sense now. :)
Convection: Occurs in fluids (liquids and gases). It is the transfer of thermal energy by means of currents in a fluid (liquids or gases). E.g. If you were to heat a beaker of water in one spot, the water above the flame expands and thus becomes less dense. So it rises upwards as cooler, denser water around it sinks and displaces (pushed it out of the way) it. So this creates a circulating stream called a convection current. So as the water circulates it transfers the energy to other parts of the beaker. 

Radiation: This type of energy transfer doesn't require a medium. The energy is transferred by electromagnetic radiation travelling at the speed of light-it is actually a specific part of this family of electromagnetic waves we are talking about called infrared waves. (Electromagnetic radiation is covered in another post.) So radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium. 
  • Dull, dark surfaces absorb infrared radiation faster compared to shiny, white surfaces but they are also better emitters of the radiation, so they actually cool down faster. 
  • Shiny, bright surfaces reflect the radiation and so absorb less of it. 

4.7 describe the role of convection in everyday phenomena

e.g. convection in air-gases--warm air rises as it is displaced by cooler, denser air sinking around it. 
Heated by the sun, warm air rises above the equator as it's displaced by cooler, denser air sinking to the North and South. 
Result: huge convection currents in the Earth's atmosphere. This causes winds across all oceans/continents. 

During the day, land heats faster than water, so the air over the land becomes warmer and less dense. It rises and is replaced by cooler, denser air flowing in from over the water. This causes an onshore wind, called a sea breeze. Conversely, at night land cools faster than water, as does the corresponding air.
In this case, the warmer air over the water rises and is replaced by the cooler, denser air from the land, creating an offshore wind called a land breeze. 

4.8 describe how insulation is used to reduce energy transfers from buildings and the human body
air is a bad conductor of heat so many insulators contain tiny pockets of trapped air to stop heat being conducted away

House, energy can be lost through + insulation:
  • roof: loft and roof insulation
  • walls: wall cavity filled with insulator such as polystyrene foam
  • draughts: draught excluder on doors and windows
  • floor: carpet 
  • windows: double-glazed windows (has gap where air is trapped between the two panes of glass, which reduce thermal energy transfer through the windows), curtains also help insulate
For the human body, we wear woolen clothes to keep ourselves warm, wool feels warm because it traps a lot of air. 

Air is a relatively bad conductor of heat because the particles are much further apart than in a solid. So collisions between particles are much less frequent, thus transfer of kinetic energy to neighbouring particles is much slower. 
Whereas in solids, the particles are packed tightly together so if one end is heated the particles vibrate vigorously and collide with neighbouring particles making them vibrate as well. Thus the kinetic energy of the vibrating particles at the hot end is transferred to the neighbouring particles quickly. 

Hope this helped. :) 

Sunday, 22 April 2012

Light and sound

3.14 recall that light waves are transverse waves which can be reflected, refracted and diffracted

Light waves are transverse waves, they travel in straight lines so it is 'rectilinear'. It can be reflected, if not we wouldn't be able to see, because light is constantly reflecting off objects into our eyes. 
It can also be refracted, which people like to think of as the light 'bending', as it is changing direction. But keep in mind the light ray is still straight. 
Diffraction is for single science, but it's basically about the waves spreading out after passing through a gap. For light, you can only see it diffracting if it passes through a very narrow slit as it has such a small wavelength. 

3.15 recall that the angle of incidence equals the angle of reflection

This is the Law of Reflection

When a light ray strikes a plane mirror, it reflects off it. 
And the angle of incidence=the angle of reflection. 
We draw a 'normal', which is an imaginary line perpendicular (at right angles to) to the plane mirror. The angle between the light ray and the normal is the angle of incidence, and the angle between the reflected ray and the normal is the angle of reflection. If it is a plane mirror these angles would be the same.
If it's not a flat surface like a plane mirror, the law does not apply. 

3.16 construct ray diagrams to illustrate the formation of a virtual image in a plane mirror


  1. First you draw your object, a short straight line will do to keep things simple. And you draw a plane mirror in the centre. 
  2. You draw light rays as straight lines from the 2 ends of your line, 2-3 will do. 
  3. Reflect this off the plane mirror in the centre of your page. (Remember the law of reflection, the angle of incidence=the angle of reflection.) 
  4. Then you extend these reflected light rays beyond the plane mirror. Like the above diagram. See how the dotted lines are extended 'into' the mirror? 
  5. Remember you drew lines from both ends of your object? Well where the dotted lines meet behind the mirror will form the 2 ends of the virtual image of your object. Try it!
  6. Notice that the virtual image is the same size as the object, but is laterally inverted.
Lateral inversion is basically reversing your image from left to right. If you look in a mirror and part your hair to your left, when people see you they actually see the hair parting as to the right..

you could also try more complicated objects, but the result would be the same:

image formation in a plane mirror

3.17 describe experiments to investigate the refraction of light, using rectangular blocks,
semicircular blocks and triangular prisms

Place a glass block on a sheet of white paper and draw a line all around it. Using a ray/light box, shine a ray of light from the air into the glass. Mark the incident ray with crosses and where it enters the block, and do the same for the refracted ray. Draw your lines and you would note that the ray of light didn't travel straight through the glass. It was bent, or refracted where it entered the glass and where it left the glass. 
Rays of light travelling from air into glass are refracted towards the normal. (less dense to denser material)
Rays of light travelling from glass into air are refracted away from the normal. (denser to less dense material)

The emergent or refracted ray is parallel to the incident ray because the change in angle of the light ray is the same as it enters and leaves the glass.

Triangular prisms refract white light into its spectrum, violet refracts the most as it has the shortest wavelength. 

Experiment using semicircular block:

About refraction of light, super good to help you understand, it's slow and easy to follow:

3.18 recall and use the relationship between refractive index, angle of incidence and angle
of refraction:

n=sin i/sin r
where n=refractive index
i=angle of incidence
r=angle of refraction

This is about how much the light would be refracted going from air into another substance. The refractive index of most kinds of glass is about 1.5, but for water it is less, 1.33, which means that the light is not bent as much when it enters water-because it is not slowed as much. (Water is denser than air, so when the light enters water it slows down.)

Refractive Index
Speed of light (m/s)
300 000 000
225 000 000
200 000 000
About 1.5
200 000 000
120 000 000

When light hits a denser material you know that it refracts towards the normal, and it also slows down. Though its velocity decreases, its frequency does not. A constant frequency means that the same number of light waves must pass by in the same amount of time. If velocity decreases, the wavelength must decrease as well to maintain the same frequency. 
I'm not so sure how to explain why frequency doesn't change but to change it you'll have to change it from its source. Frequency is a function of the energy level of the photons in light, and changing the medium doesn't change that energy level. The energy level of the photon is determined at the time of its emission and stays constant thereafter.. This is way beyond IGCSE btw so don't bother, it's do with Faraday and Maxwell's equations or smth.. 

3.19 describe an experiment to determine the refractive index of glass, using a glass block

Shine a ray of light into the glass box and draw the incident ray and the refracted ray. Draw the 2 normals like the above diagram, and measure the angle of incidence and the angle of refraction. Then use n=sin i/sin r to find out the refractive index of glass. 

3.20 describe the role of total internal reflection in transmitting information along optical
fibres and in prisms

To understand total internal reflection you must understand what the critical angle is first. This is the angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90°. As seen in the above diagram, the red ray of light is refracted along the edge of the water, if you draw a normal to it, it would be perpendicular, so the angle of refraction is 90°. 

Total internal reflection is when all the light is reflected inside the more dense substance. 
Total internal reflection only takes place when:
  1. the rays are travelling in a dense medium towards a less-dense medium
  2. the angle of incidence in the optically denser medium is greater than the critical angle
Optical fibres are very thin and flexible. The outer cladding has a lower refractive index than the core so the light rays inside the core can be totally internally reflected. 
The light rays entering the optical fibre will come out the other end as it experiences total internal reflection (if its angle of incidence is greater than the critical angle of course). So optical fibres can transmit information easily and are now commonly used in telecommunications. 
this may help you understand the optical fibre stuff better, see how if the light ray enters at an angle greater than the critical angle it reflects instead of refracts out? 

Prisms in a periscope: 
As the ray of light goes straight through the prism it meets the second surface at an angle of 45°, this is greater than the critical angle of glass which is 42°. So total internal reflection takes place. So the light ray is turned through 90°. This happens again in the second prism and the light ray eventually enters your eye and you can see the object the light ray came from. 

3.21 recall the meaning of critical angle c
I explained critical angle earlier to help you understand total internal reflection, but anways, just remember: 
This is the angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90°.

3.22 recall and use the relationship between critical angle and refractive index:
sin c = 1/n

3.26 recall that sound waves are longitudinal waves which can be reflected, refracted and
all you have to do is 'recall'... so just know it. 

3.27 recall that the frequency range for human hearing is 20 Hz –– 20 000 Hz
just remember this. Above 20 000 Hz is ultrasound. 

3.28 describe how to measure the speed of sound in air

Here's one from my textbook:
Stand a measured 50 metres from a large wall. Clap and listen to the echo. Then try to clap in an even rhythm of clap-echo-clap-echo-clap... while a friend times 100 of your claps with a stopwatch.
During the time from one clap to the next clap, the sound would have time to go to the wall and back, twice-that is a distance of 200 metres.
In the time of 100 claps, the sound would travel 200 x 100 = 20 000 metres. 
speed=distance travelled/time taken=20 000 metres/time in seconds

I mean if they ever ask you a question about finding speed, just recall s=d/t. 

Monday, 16 April 2012

Properties of Waves

3.1 use the following units: degree (°), hertz (Hz), metre (m), metre/second (m/s), second (s)

What is a wave? 
A wave is made up of periodic motion. Periodic motion is motion repeated at regular intervals. 
For example, a pendulum moving from left to right and back again is said to be periodic. One such complete motion, from one position to the other and back is known as an oscillation or a vibration. These are key words in describing periodic motion. 
The source of any wave is a vibration or oscillation. 

3.2 describe longitudinal and transverse waves in ropes, springs and water where appropriate

Waves move up and down, but energy moves forward. 

Waves move back and forth but energy moves forward. 

Transverse waves are waves that travel in a direction perpendicular to the direction of vibration. 
To clarify this: The waves on ropes are transverse when you move the rope up and down. The direction of wave motion is forward, to the opposite end of the rope. But the wave crest and trough are moving up and down, which is the direction of vibration as you are moving the rope up and down, which is perpendicular to the direction of wave motion-which is forwards. (Perpendicular-at right angles to)
Eg. Water and light waves are transverse waves. 
Longitudinal waves are waves that travel in a direction parallel to the direction of vibration. 
If you were to push and pull a spring so that it compresses and expands, you would notice that when the coils move forward and backwards, the direction of the wave motion is parallel to the direction of vibration. It is moving along the spring, left and right, in the same direction that you are pushing/pulling it, not up and down like a transverse wave. 
E.g. sound waves are longitudinal waves. 
transverse wave

3.3 state the meaning of amplitude, frequency, wavelength and period of a wave

Amplitude (A): This is the maximum displacement from the rest/centre position. It is the height of the crest or depth of a trough measured from the rest position. It tells you the 'loudness' if it's a sound wave. The bigger the amplitude, the louder the sound. Its SI unit is the metre (m). 

Frequency (f): This is the number of complete wave produced per second. SI unit-hertz (Hz). Frequency relates to the pitch of a sound, the higher the frequency the higher the pitch. 

Period (T): This is the time taken for one point on the wave to complete one oscillation. Or you can think of it as the time taken to produce one complete wave. The SI unit is second (s). 

Wavelength (λ): The shortest distance between a point on one wave and the same point on the next wave. (in fancy words: the shortest distance between any two points in a wave that are in phase) such as two successive crests or troughs. 
For longitudinal waves, it is the distance between two successive compressions or rarefactions. Its SI unit is the metre (m). 

rarefaction of longitudinal waves


3.4 recall that waves transfer energy and information without transferring matter

A wave transfers energy and information from one place to another without transferring matter in the process. For example, when we drop a pebble into a pond, a few circular ripples move outward on the surface of the water. As the ripples spread outward, any object on the surface of the water (e.g. a leaf) would only bob up and down, not moved. This shows that waves transfer energy without transferring any matter! (The leaf bobs up and down because water waves are transverse.)
Another example: you can produce waves on a rope by fixing one end to a wall and moving the other end up and down. These up-and-down movements produce vibrations, or oscillations. Observe: the rope waves travel towards the wall, while the rope itself only moves up and down. The top is said to be the medium through which the waves move, or propogate. (This may also clarify a bit about transverse waves...) Waves transfer energy, not matter. The kinetic energy from the up-and-down movement is transferred by the wave from one end to the other. The rope itself, however, does not move from one end to the other. 

3.5 recall and use the relationship between the speed, frequency and wavelength of a wave:
wave speed=frequency x wavelength
v= f x  λ 

3.6 use the relationship between frequency and time period:

frequency= 1/time period
f= 1/ T

So a higher frequency implies that more waves are produced in one second. This means that the period T will be shorter. 

3.7 use the above relationship in different contexts including sound waves and electromagnetic waves

Question: A wave is introduced into a thin wire held tight at each end. It has an amplitude of 3.8 cm, a frequency of 50 Hz and a distance from a crest to the neighbouring trough of 12.8 cm. Determine the period of such a wave.

Answer: f=1/T, so T=1/f
The question had a lot of useless information to throw you off, all you have to do is use T=1/f, so T=1/50

  1. Some ripples travel 55cm in 5 seconds. Find their speed in cm/s.
  2. The wavelength of these waves is found to be 2.2cm. What is their frequency?
  1. 55/5=11cm/s
  2. v= f x  λ  so f=v/ λ (use the equation triangle I included above) f=11/2.2=5Hz

Question: What is the wavelength of a sound wave of frequency 100 Hz. (speed of sound=340 m/s). 

v= f x  λ 
λ= v / f 
λ= 340 / 100 = 3.4m

Thursday, 12 April 2012

Electric charge

2.20 identify common materials which are electrical conductors or insulators, including metals and plastics

Any metal is a conductor of electricity. Any non-metal is an insulator, apart from graphite due to its unusual  structure. (Graphite has delocalised electrons between its layers, these electrons were not used up in covalent bonding and are free to move to carry charge, hence graphite has the unusual property of being able to conduct electricity.)

Copper wires are common, as copper is a very good electrical conductor. PVC is now often used to insulate wires-Polyvinyl chloride. In IGCSE Chemistry, you might know this as poly(chloroethene), a polymer. PVC is cheap to make and flexible, so it now replaces rubber in insulating wires. Because as rubber grew old, it would crack and it would be dangerous if you touched the live wire. Insulation can become unsafe if it is damaged or wet because water can conduct electricity.

SS 2.21 recall that insulating materials can be charged by friction

SS 2.22 explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons

SS 2.23 recall that there are forces of attraction between unlike charges and forces of repulsion between like charges

SS 2.24 explain electrostatic phenomena in terms of the movement of electrons

SS 2.25 recall the potential dangers of electrostatic charges, e.g when fuelling aircraft and tankers

SS 2.26 recall some uses of electrostatic charges, e.g. in photocopiers and inkjet printers

Wednesday, 11 April 2012

Energy and potential difference in circuits

Energy and potential difference in circuits
This would be Part C of Section 2: Electricity of the IGCSE Physics specification. 

-          The circuit symbols for a cell and a battery (2 or more cells connected)
-          An ammeter – you connect it in series to measure the current
-          A voltmeter – you connect it in parallel to the circuit component in question (e.g. a bulb) to measure the potential difference across it

2.9 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting

Parallel circuits are more appropriate for domestic lighting:
a) Bulbs are connected in parallel glow brighter because 2 bulbs in series have a higher resistance than a single bulb. (Current doesn't have to flow through combined resistance.)
b) If one light bulb blows, the others will continue to glow as there will still be a complete circuit through the other parallel branch. This also means that you can control each bulb independently, why would you want a circuit where if someone in your family switches off their light yours goes off too?
c) In a series circuit, the potential difference (p.d.) across each bulb in smaller. Each charge only gives up some of its energy in each bulb, relates to (a), so the bulbs are dimmer.

2.10 understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

P= I x V

If there are resistors then the current will be smaller, and having similar resistors connected in series in a circuit would mean the combined resistance is larger, hence the current will be even smaller.

2.11 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally

  1. A wire would have the graph of the resistor as it has a fixed resistance too-as long as the temperature doesn't change, the resistance will be constant as well. 
  2. For a filament bulb, as the p.d. (potential difference/voltage) across the lamp increases, the current does not increase proportionally. Its resistance increases at higher temperatures. 
  3. For a semi-conductor diode, as voltage increases the resistance decreases (resistance decreases at higher temperatures). A semiconductor diode is a device which allows current to flow in one direction only, called the forward direction. So the graph basically shows a relatively larger current flowing through when a p.d. is applied in the forward direction. Almost no current will be observed if the p.d. is in the reverse direction. 

2.12 describe the qualitative effect of changing resistance on the current in a circuit

The resistance R of a component is defined as the ratio of the potential difference across it to the current flowing through it. 
The unit for resistance is ohm Ω. 
[One ohm is the resistance of a material through which a current of one ampere flows when a potential difference of one volt is maintained across it. 1 ohm Ω = 1 volt (V) / 1 ampere (A) ]

From this definition of resistance, we can see that for a particular potential difference, the higher the resistance, the smaller the current passing through. You can see by putting numbers into the equation. 

2.13 describe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperature

LDR-as light intensity increases, resistance decreases hence current increases.
Thermistor-as temperature increases, resistance decreases hence current increases. This is commonly used in air conditioners to control the temperature.

You may be wondering why their resistance decreases, well, as there is more light energy and heat energy, more electrons are shaken free so it can conduct better and let more current through. I think that's the simplest explanation I can think of.

2.14 know that lamps and LEDs can be used to indicate the presence of a current in a circuit
If they light up there has to be a current in the circuit...

2.15 recall and use the relationship between voltage, current and resistance:

voltage= current x resistance
V= I x R

2.16 understand that current is the rate of flow of charge

  • Current is the rate of flow of charge.
  • Current is not used up, what flows into a component must flow out.
  • Current is measured in amps (amperes)A.
  • Current is measured with an ammeter, connected in series

Potential difference (voltage)
A potential difference/voltage across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.

Measuring potential difference
  • Potential difference is measured in volts, V
  • Potential difference across a component in a circuit is measured using a voltmeter
  • The voltmeter must be connected in parallel with the component.

Some of you may be confused about voltage and current, I am too sometimes. Well:  It is possible to have voltage without current, but current cannot flow without voltage.

Voltage but not current:
The circuit is broken and current cannot flow.

Voltage and current:
The circuit is complete so current can flow.

2.17 recall and use the relationship between charge, current and time: 

charge = current x time
Q= I x t

2.18 recall that electric current in solid metallic conductors is a flow of negatively charged 

Remember that the electrons carry a negative charge, and that they are repelled from the negative terminal and are attracted to the positive terminal in a circuit. So they flow from negative to positive.

But the conventional current goes the other way… (see Conventional current vs Electron flow notes)

SS 2.19 recall that:

  • voltage is the energy transferred per unit charge passed
  • the volt is a joule per coulomb
Sometimes voltage is called potential difference. So what is potential difference? E.g. when a dry cell is connected to a light bulb, the electrical energy provided by the dry cell is converted into light and thermal energy by the bulb. This amount of energy converted across the light bulb for each unit of charge is called the potential difference. So the potential difference (p.d.) between two points is one volt if one joule of electrical energy is converted into other forms of energy when one coulomb of positive charge flows through it. 

In symbols,
V=W/Q where V is the p.d., W is the electrical energy converted to other forms, and Q is the amount of charge. 
As stated above under 'Measuring potential difference', you must connect a voltmeter in parallel with the component, e.g a lamp, to record the potential difference across it. 

d) Electric Charge

2.20 identify common materials which are electrical conductors or insulators, including metals and plastics

Any metal is a conductor of electricity. Any non-metal is an insulator, apart from graphite due to its unusual  structure.
Copper wires are common, as copper is a very good electrical conductor. PVC is now often used to insulate wires-Polyvinyl chloride. In IGCSE Chemistry, you might know this as poly(chloroethene), a polymer. PVC is cheap to make and flexible, so it now replaces rubber in insulating wires. Because as rubber grew old, it would crack and it would be dangerous if you touched the live wire. Insulation can become unsafe if it is damaged or wet because water can conduct electricity.