Electricity and waves Flashcards
Define current
The flow of electrons round a circuit
Define voltage
The driving force that pushes the circuit around
Define resistance
Anything that slows the flow down
Which way do electrons flow?
Opposite the conventional current
- to +
What colours are the wires in a three pin plug?
Brown- live
Green/yellow- earth
Blue- neutral
What does a fuse do in a three pin plug?
When the current exceeds the fuse’s max current, it will melt, leading to a break in the circuit so the plug can’t set on fire
What does the earth wire do?
If the heating element wire breaks and touches the metal case, current will flow to earth as it offers the least resistance
What is direct current (D.C)
Current that only flows in one direction (+ to -)
What is alternating current? (A.C)
Current flows one way, then back- constantly changing direction
What is frequency?
The number of waves/cycles in a second
What is the formula for power, current and resistance?
P = i*2 x R
What is the formula for energy transferred, power and time?
E = P x t
Why are some earth pins plastic on three pin plugs?
For double insulation, so that there’s no chance of touching live or neutral wires due to everything encased in plastic
Name three safety features in a three pin plug
Fuse, as it melts and breaks the circuit if the current is too high
Earth wire, as it directs the current into the earth if a wire breaks and connects with the metal covering
Cable grip, as it keeps all the wires in place, so there’s no chance you could accidentally touch one or it could move out of its socket
Why is an RCB different to a fuse?
A Residual Circuit Breaker is different as a fuse will only melt if the current exceeds the fuse rating. The RCB will detect a difference in the Live and Neutral wires and flip a switch, breaking the circuit as a difference in the two wires would make the current be able to flow through the user, to the earth
If a current is at 4A, what fuse should be used?
The next fuse available after 4A, but NOT 4A
What should the difference in voltage be between the Live and Neutral wire for it to be safe?
0 volts, they should be the same voltage
What is the voltage of mains electricity?
230 volts
What is the frequency of mains electricity?
50 Hertz
What does the ammeter measure?
Current in Amps
What does the voltmeter measure?
Voltage in Volts
How is the ammeter connected to a circuit?
In series
How is a voltmeter connected to a circuit?
In parallel
What does a resistor look like on a current/voltage graph?
A straight line
What does a filament lamp look like on a current/voltage graph and why?
A curve, as the hotter the bulb gets, the more resistance it has
What is an LDR?
A light dependent resistor (the resistance is highest in darkness)
This is used for automatic lights (when it gets dark, the light comes on)
What does a thermistor do?
They have a high resistance in cold conditions and the resistance falls as it gets hotter
This can be used for central heating thermostats
What is electromagnetic induction?
The creation of a voltage in a wire which is experiencing a change in magnetic field
What is the ‘dynamo effect’?
Using electromagnetic induction to transform kinetic energy into electrical energy
E.g a turbine
What two situations can you get an electromagnetic induction?
An electrical conductor moves through the magnetic field
The magnetic field through an electrical conductor changes
Why do generators produce an alternating current?
Because as a coil rotates in a magnetic field, every half a turn the current swaps direction as it is constantly switching between the north and south end
If the generator is going at 50Hz, how many times does it rotate a second?
50 times a second
What is the national grid?
A network that covers the whole country and takes electricity from power stations to homes and industries
How does a power station generate electricity?
Fuel is burned, creating steam that turns the turbine
The steam is condensed and passed back into the boiler as the turbine is connected to the generator
The turbine spins so the generator spins, creating electricity
What are transformers used for in the national grid?
Providing more electricity requires a higher voltage or current
Current will burn the wires so voltage is used to generate more electricity
The voltage is transformed to 400,000V by a step up transformer
The electricity goes through the pylons which are well insulated
It is then transformed by a step down transformer to 230V
Define ‘electrical power’
The rate of transfer of electrical energy
What is a kilowatt-hour?
The amount of electrical energy converted by a 1kW left on for an hour
What is ‘amplitude’ in a wave?
The displacement (gap) from the rest to the crest
What is the wavelength in a wave?
The length of one wave (crest to crest)
What is frequency in a wave?
The number of complete waves passing a certain point per second
Measured in hertz
What are transverse waves?
The vibrations are at 90 degrees to the direction of travel
Example : light, other EM waves
What are longitudinal waves?
The vibrations are along the same direction as the wave is travelling
Examples: sound, shock waves
What are compressions and rarefactions in longitudinal waves?
Compressions is when the vibrations are concentrated, compressing a point along the wave
Rarefactions are when the wave is stretched and there vibrations are less concentrated
What is the order of EM waves?
Racist Muppets Insult Very Ugly Xenophobic Geese
Radio waves Microwaves Infra-red Visible light Ultraviolet X-ray Gamma
What is the order of wave length in colours?
Red Orange Yellow Green Blue Indigo Violet
How are gamma rays formed?
They are formed by the decay of a father nucleus
How are x-rays formed?
Fast moving electrons that hit a metal surface
How do radio waves transfer through an Ariel
There is an alternating radio wave when it hits, the alternating wave goes to the speaker
What happens to different EM waves in a vacuum?
They all have the same speed
Do longer wavelengths or shorter wavelengths have the most energy?
Shorter
What is interference?
When two or more waves of a similar frequency come into contact and combine, creating a combined amplitude
What happens when a wave is reflected?
The waves rebound off of the material
What happens when a wave is refracted?
They go through a material and change direction
What happens when a wave is diffracted?
The wave bends round an obstacle, causing the waves to spread out
How does diffraction work?
Waves spread out at the edges when they pass through a gap or past an object
This depends on the size of the gap relative to the wavelength
What 5 things can an object do to a wave?
Diffract it Absorb it Transmit it Reflect it Refract it
What happens if a wave goes into a material that’s denser?
It goes slower (except for sound waves that travel faster)
What happens if a wave hits a dense material head on?
It carries in the same direction but slower
What happens if a wave hits a denser material at an angle?
It gets refracted
The side that hits first slows down whilst the other side continues at the same pace
This makes the wave turn until both sides are in the material
In refraction, what are the rays called?
Incident ray - the one entering the material
Refracted ray - ray refracted through the material
emergent ray - ray coming out of the material
In refraction, what is the normal?
90 degrees to the material
In refraction, what happens if the angle of incidence is less than the critical angle?
Most of the light passes out but some is internally reflected
In refraction, what happens when the angle of incident is equal to the critical angle?
The emerging Ray comes out along the surface of the material and there’s quite a bit of internal reflection
In refraction, what happens when the angle of incident is bigger than the critical angle?
No light comes out, it’s all internally reflected
How do optical fibres work?
They bounce visible or infrared light waves off the sides of a thin inner core of glass or plastic using total internal reflection
The wave enters one end of the fibre and is reflected repeatedly until it emerges at the other end
Why can optical wires not be bent sharply?
They use total internal reflection to bounce light rays across the wire
If it’s bent, the angle of incidence might fall below the critical angle
Why are higher frequency waves more dangerous?
The energy of EM waves is proportional to its frequency
So higher frequency waves have more energy and energy does damage
Give examples of potential health risks with waves?
Infrared could burn you Visible light could blind you UV light causes cancer Microwave could potentially be absorbed by water molecules in cells Gamma rays can destroy cells
How does UV light give you cancer?
They damage the dna in your cells
Dark skin protects against UV rays as it absorbs them before they get to sensitive cells
How do radio waves get around the earth?
Long radio waves could diffract, bend around the earth - different countries
Short radio waves could break through the ionosphere, reach a satellite and be redirected back down - cell phone
Long radio waves can’t break through the ionosphere so is instead bounced off it - tv and radio
What is the ionosphere?
An electrically charged later in the earth’s upper atmosphere
How do microwave ovens work?
Microwaves are go in a few cm and are absorbed by the water molecules, the energy is then connected to other parts
How do X-rays work?
X rays pass through flesh but can’t pass through denser materials like bone
So the graph shows the places the x rays aren’t absorbed
How does infrared vision work?
It measures the IR radiation coming off of something (heat)
How does an ultrasound scan work?
Ultrasound is made up of high frequency sound waves
As these hit a surface, some are partially reflected, this creates an image on the computer
How does iris scanning work?
Each iris has a unique pattern
Light is reflected off your iris to make a picture, this shows whether it’s your identity or not
How does a CD work?
The surface has shallow bits and high bits
A laser is reflected into it and the two bits shine differently, this difference is picked up and converted into an electrical signal
What is the difference between analogue signals and digital signals?
Analogue takes any value in a certain range
Digital is either on or off
Why do signals need to be amplified?
They get weaker the further they go and also pick up interference
Why is noise less of an issue with digital signals?
Digital signals are much higher quality
Digital signals are also easier to work with computers
What are seismic waves caused by?
Earthquakes
What are the two different seismic waves?
P-waves - longitudinal and travels through solids and liquids (is faster than S-waves)
S-waves - transverse and only travels through solids (slower than P-waves)
How do S-waves tell us the outer core of the earth is liquid?
They get through the mantle because it’s solid but can’t get through the outer core so it must be liquid
(They can only go through solids)
How do P-waves show is that the inner core is solid?
They travel slightly faster through the inner core
