Saturday, 30 March 2013

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

The current in a series circuit is the same through out all parts of the circuit. It is worked out using the equation I= V/R. So its the total of the voltages received by the components divided by the total of all the components resistances.

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

In a series circuit everything is connected on one line. This means that the voltage is shared out between every component: this makes it useful for supplying low power things like fairy lights.

In a parallel circuit different components are connected separately to the supply. This means that of one component breaks the others can continue being powered as the whole circuit is still functioning, this makes it practical to use. It is also good for charging higher power things as the potential difference is equal all over a parallel circuit so each component receives the full voltage.

2.7 understand the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery

Direct current flows in one direction only. It is supplied by cells and batteries. It comes out as a straight line on an oscilloscope.

Alternating current changes from one direction to another rapidly. Mains electricity is alternating (interestingly this is because the electricity has to go through transformers on the national grid which only work on ac current, although that's not relevant here!)

2.6 use the relationship between energy transferred, current, voltage and time

energy transferred = current × voltage × time
E = I × V × t

n.b this is the same thing as saying power x time

2.5 know and use the relationship: power = current × voltage

power = current × voltage
P = I × V

2.4 understand that a current in a resistor results in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts

As a resistor slows down the movement of electrons, the kinetic energy that was moving them is converted into heat energy. This can be used, for example, in hair dryers or heaters.

2.3 understand the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances

Insulation is covering a live wire with a material that won't conduct the electricity.
Double insulation is a precaution that makes sure the live wire cannot touch the casing (so no shock can be conducted) usually by putting extra insulation round that wire. Double insulation can also mean that the casing of an object is plastic so even if the wire touches it, it wont conduct.
An earth wire is touching the case so that if a current is in the case, it will be directed through the earth wire, this will then take the current to the earth. Additionally the surge of electricity in the wire may break the fuse.
Fuses are sections of wire in the circuit that melt if too high a current goes through them. They come with different maximum currents.
Circuit breakers have an electromagnet that is activated if the current goes above a certain limit. the electromagnet pulls an iron switch towards it, this opens the switch and breaks the circuit.

3.26 understand that sound waves are longitudinal waves and how they can be reflected, refracted and diffracted

Sound waves are longitudinal waves (the one that looks like a bar code.) They can be reflected, refracted and diffracted much like light can. For example an echo is a reflection of sound.

2.2 understand and identify the hazards of electricity including frayed cables, long cables, damaged plugs, water around sockets, and pushing metal objects into sockets

In frayed cabling the insulation has worn down exposing live wires, electricity can be conducted from these.
Longer cables are at a higher risk of being damaged and there is more resistance with longer wires making them more at risk of over heating.
Damaged plugs create a risk that some of the safety features may be broken.
Water conducts electricity and can cause energy from the circuit to flow trough it creating a fire and electrocution risk. Metal objects in sockets have the same dangers.

Sunday, 3 March 2013

3.24 describe the advantages of using digital signals rather than analogue signals

The term noise means the random signals picked up by waves. Radios may crackle or internet may looses connection. This effects analogue signals badly as each time it is amplified the noise also gets amplified, this alters the signal making it hard or impossible to identify as the original signal.

In digital signals any noise picked up is likely to be of a smaller amplitude than that if the on state, this means something receiving it will ignore the noise as it is neither on nor off, this makes them less likely to be distorted.

3.23 understand the difference between analogue and digital signals

The amplitude and/or frequency constantly vary.

the signal is a wavy line that goes up and down in an uneven pattern

Consists of pulses with two states: on; off
the signal is a continuous line that goes up, down and across in straight lines with no curves

3.22 know and use the relationship between critical angle and refractive index

sin(critical angle)= 1/ refractive index

sin(c)= 1/n

3.21 explain the meaning of critical angle c

When light travels from one medium to another it is refracted; it changes angle due to change in density.
Past a certain angle the light will simply be refracted back into the medium it is in, this angle is the critical angle.

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

Beyond the critical angle, light will be reflected back into the medium they came from at the same angle. In this way they are trapped in the medium.
By reflecting light past its critical angle you can make it travel through a medium to send information: this is done in optical fibres.
Diagram showing how light reflects inside a glass fibre - the light "zig-zags" from one side of the fibre to the other

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

Shine a ray of light through a glass block, measure the angle of incidence and the angle of refraction.
Do sin(i) divided by sin(r) and you will have the refractive index of glass.

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

Refractive index= sin (angle of incidence)/ sin (angle of refraction)

n= sin(i)/ sin(r)

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

Place a block of glass on a piece of paper, drawing an outline.
At one point, draw the normal line.
Draw a line at 30 degrees to the normal line, shine a ray of light down this line.
Draw a line where the light comes out the other side. Connect the two lines, drawing the refracted ray.
Measure the angle of the emergent ray.
Repeat for different shaped glass.

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

The mirror- a straight line with hatchings to show the side with the reflective coating on it.
Incident ray- line with arrows pointing towards the mirror.
Reflection ray- line with arrows pointing away from the mirror.
The image- (where the reflection appears to be behind the mirror) dashed line.

The angle of incidence should equal the angle of reflection.
A perpendicular line from the object to the mirror, if repeated the other side of the mirror, shows where the image appears to be in the mirror. Draw a line from the image to the eye, where this passes the mirror is where the angle of incidence should also meet the mirror.


3.15 use the law of reflection (the angle of incidence equals the angle of reflection)

The angle of incidence is the angle that light hits a mirror; it is taken between 90 degrees from the mirror and the incidence wave (the wave that hits the mirror.)

The angle of reflection is the angle that light leaves the mirror; it is taken between 90 degrees from the mirror and the angle of reflection.

The angle of incidence is always the same as the angle of reflection.

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

Light hitting a reflective surface will 'bounce' back from the surface (at the same angle they hit the surface.)

Light waves change speed when they pass through objects of different densities, this causes them to change direction. When they return to the original density they will continue in the original direction.

When light meats a barrier, it will carry on through the gap and spread out in the area beyond.

3.13 understand the detrimental effects of excessive exposure of the human body to electromagnetic waves and describe simple protective measures against the risks.

  • microwaves: internal heating of body tissue
    • this can damage cells if they overheat
  • infra-red: skin burns
    • skin cells are damaged by overexposiure
  • ultraviolet: damage to surface cells and blindness
    • can damage receptor cells in the retna
  • gamma rays: cancer, mutation
    • can cause cells to change their arrangement causing cancer

People tend to avoid long term low level exposure or short term high level exposure. Sun creames can protect against UV as can sun glasses.

3.12 explain some of the uses of electromagnetic radiations

  • radio waves: broadcasting and communications
    • Vibrations carry sound
  • microwaves: cooking and satellite transmissions
    • Vibrations create heat.
  • infra-red: heaters and night vision equipment
    • Vibrations create heat, cameras can detect where it is high and low to see by heat.
  • visible light: optical fibres and photography
    • Light reflected down tube to send signals, or onto film to take photos.
  • ultraviolet: fluorescent lamps
    • A coating inside the bulb will absorb UV light and re-emit it as visable light.
  • x-rays: observing the internal structure of objects and materials and medical applications
    • They pass through skin and soft tissue but reflect hard structures like bone.
  • gamma rays: sterilising food and medical equipment

3.11 identify the order of the electromagnetic spectrum in terms of decreasing wavelength and increasing frequency, including the colours of the visible spectrum

As you go up the electromagnetic spectrum wavelength decreases and frequency increases.
The same is true for visible light; with red being the longest wavelength and lowest frequency and violet being the shortest wavelength and highest frequency of all visible light.

3.10 understand that light is part of a continuous electromagnetic spectrum which includes radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray radiations and that all these waves travel at the same speed in free space

The electromagnetic spectrum is a range of different frequency waves, one section of the spectrum is visible light (light we can see.) All of the waves in the electromagnetic spectrum travel at the same speed when they are in a vacuum.

3.9 understand that waves can be diffracted through gaps, and that the extent of diffraction depends on the wavelength and the physical dimension of the gap.

Diffraction can happen through a gap, when waves go through a narrow space, on continuing they spread out again. The smaller the gap, in comparison to the wave length, the larger the diffraction.

3.8 understand that waves can be diffracted when they pass an edge

As this diagram shows, when a wave hits an edge, as it carries on it spreads out into the space beyond the edge. This happens with radio waves and hills, and water and islands.

3.7 use the above relationships in different contexts including sound waves and electromagnetic waves
Wave speed= frequency x wave length
Frequency= 1/time period

You should be able to manipulate these equations to answer questions that may ask in a different order; use the triangle method.

3.6 use the relationship between frequency and time period

Frequency= 1/ time period

3.5 know and use the relationship between the speed, frequency and wavelength of a wave

wave speed = frequency × wavelength
v = f × λ

3.4 understand that waves transfer energy and information without transferring matter

A wave is a transfers energy through a space or object, but it does not move the particles in it.
If you stand on one side of a door and say 'Hi' a person on the other side will be able to hear you saying 'Hi'. This is because the vibrations that you made have travelled through the door to the other side, the energy moving from one particle of the door to another: but the door its self has not move, none of its particles have changed position.

3.3 define amplitude, frequency, wavelength and period of a wave

Period of a wave
Time taken for the source to produce one complete wave.

As a wave vibrates to either side of the direction of travel, the amplitude is the distance between the line of the direction of travel and the furthest point the it vibrates away from the line:
The number of waves per second, it is measured in Hertz (Hz). You can think of it as how quickly the waves are travelling.

The distance between one point on a wave and the same point on the next wave; usually the point from the top/bottom of one wave (peak/trough) to the top/bottom of the next.

3.2 understand the difference between longitudinal and transverse waves and describe experiments to show longitudinal and transverse waves in, for example, ropes, springs and water

Vibrations (osculations) go up and down along the line of travel,
Light and electromagnetic waves travel in this way,
If you drop something in water the waves move up and down as they travel outwards,
If you lie a piece of string on a table and move one end up and down, the movement will pass through the object to the other end.

The vibrations are in the same direction as the line of travel,
Sound waves travel in this way,
Compressions are where vibrations are close together, rarefactions are where they are more spread out,
If you push one end of a stretched spring the compression will move down the spring.

Watch these animations to see how the examples work:

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

degree (°)- distance
hertz (Hz)- cycles per second
metre (m)- distance
metre/second (m/s)- speed
second (s)- time

2.1 use the following units: ampere (A), coulomb (C), joule (J), ohm (), second (s), volt (V), watt (W)

Ampere (A)- current
Coulomb (C)- charge
Joule (J)- energy
Ohm (Ω)- resistance
Second (s)- time
Volt (V)- potential/energy
Watt (W)- power

1.35 use the relationship between orbital speed, orbital radius and time period

The first half of the equation works out the circumference of the circle, this is the distance, which is then divided by the time.
Orbital speed = 2× π ×orbital radius/ time period 
v = 2× π × r/ T

1.33 explain that gravitational force:  causes moons to orbit planets  causes the planets to orbit the sun  causes artificial satellites to orbit the Earth  causes comets to orbit the sun

If an object is within the field of another objects gravitational force then it will travel around it in a path known as an orbit. In this way:

moons to orbit planets
the planets to orbit the sun
artificial satellites to orbit the Earth
comets to orbit the sun