P4 Flashcards
How can an insulating material become charged? (2 points)
- Rubbed with another insulating material
- Electrons transfer from material to other
(one material left positively charged)
(one material left negatively charged)
How can you generate static electricity? (2 points)
- Rubbing balloon, comb or strip of plastic against a jumper
- Electrically charged object can attract small objects
What are 2 items that can become charged? (2 points)
- Dusting brushes (can be charged so that they attract dust when it passes over brush)
- Synthetic clothing (can become charged from friction between body and clothing when put on, when removed static sparks are sometimes produced)
How can an object become discharged? (3 points)
- By earthing it
- Electrons are transferred from the object to the earth
- If you become charged then earthed, you may receive an electrostatic shock
What are 2 nuisances of static electricity? (2 points)
- Dirt and dust can be attracted to insulating materials (TV screens/computer monitors)
- Some materials can statically cling to your skin
What is repulsion and give an example? (2 points)
- 2 insulating materials with the same charge repel
- E.G. 2 positively charged Perspex rods will repel
What is attraction and give an example? (2 points)
- 2 insulating materials with different charges will attract
- E.G. Negatively charged ebonite rod will be attracted to a positively charged Perspex rod
What are 3 ways of reducing the chance of receiving an electric shock? (3 points)
- Making sure appliances are correctly earthed
- Using insulation mats effectively
- Shoes with insulating soles
Why do certain lorries need to be earthed before unloading? (4 points)
- Contain inflammable gases, liquids or powders
- Friction causes a build up of charge
- Charge leads to spark
- Spark could ignite flammable substance
How do anti-static sprays, liquids and cloths help to reduce the problems of static electricity? (2 points)
- Prevent the transfer of charge from one insulator to another
- So, if there’s no build up of charge, there can’t be any discharge
What are atoms and molecules called that have become charged, and what are the 2 types of gained charge? (3 points)
- Ions
- Gain of electrons = negative ion
- Loss of electrons = positive ion
How does electric/static charge build up, and what are the 2 types of build up? (3 points)
- Electrons rubbed off one material to another
- Material receives electrons = negatively charged (excess of electrons)
- Material gives electrons = positively charged (loss of electrons)
How is static electricity used in spray painting, and what is another example of how electrostatics are used in this way? (6 points)
1) Paint particles gain electrons when passed through the nozzle of positive spray gun, gives a negative charge
2) These paint particles repel = forms a fine spray, even coat
3) Car panel has lost electrons so has a positive charge, which attracts the paint
4) As the paint sticks, the charges cancel, has neutral charge
5) A neutral charge means that no more paint will be attracted, so less paint is wasted, and all the sides (even shadows) are covered
6) In a similar way, electrostatics are used in crop-spraying
How is static electricity used in defibrillators? (6 points)
- Electricity is used to stabilise irregular heartbeat
- They work by discharging electric charge
1) The 2 paddles are charged from a high voltage supply
2) Gel is used to make good electrical contact with the chest
3) Care is taken not to shock operator: - The paddles have insulating handles
- The operator and any one nearby stand clear
4) The charge is passed through the patient = heart contracts
How is static electricity used in smoke precipitators? (5 points)
- Can remove smoke particles from chimneys
1) The metal grids in the electrostatic precipitator are given a high voltage
2) Depending on the design, the grids may be positively charged or negatively charged: - dust particles lose electrons if grids are positively charged
- dust particles gain electrons if the are negatively charged
3) Smoke particles pick up certain (above) charge
4) The charged dust particles then induce a charge on the earthed/oppositely charged metal collecting plates and the dust particles are attracted to the plates
5) The collecting plates are knocked to remove the smoke particles, which then fall into a collector
What is a circuit? (1 point)
A complete loop that allows a current to flow
How does a circuit work? (1 point)
Electrons flow around the circuit from the negative electrode of power source to positive electrode
Why is a circuit shown as positive —> negative? (1 point)
The theory has only been discovered recently
What are 2 important things for a circuit to work? (2 points)
- There must be a complete circuit
- There must be no short circuits
What is resistance? (1 point)
Measure of how hard it is to get a current through a component in a circuit at a particular voltage
What is resistance measured in? (1 point)
Ohms Ω
What are the 2 types of resistor? (2 points)
- Fixed resistor = constant resistance, if resistance is increased, current decreases (and vice-versa)
- Variable resistor (rheostat) = changeable resistance, can be used to vary the amount of current in a circuit
How can the resistance of the rheostat be changed? (2 points)
Changing the length of wire between the contacts
- Long wire = high resistance = low current
- Short wire = low resistance = large current
- Thin wire = not much space = high resistance
- Thick wire = more space = less resistance
What is the formula of current, voltage and resistance, and what are they measured in? (1 point)
Resistance = voltage ÷ current
- Resistance is measured in ohms, Ω
- Voltage (potential difference) is measured in volts, V
- Current is measured in amperes (amps), A
For a given resistor, how does the current change as the voltage changes? (1 point)
For a particular resistor, if the voltage (potential difference) is increased the current increases (and vice-versa)
For a fixed voltage, how does the current change as the resistance changes? (1 point)
For a particular voltage (potential difference), if the resistance is increased the current decreases (and vice-versa)
What is the function of the live wire (brown)? (1 point)
Carried current to the appliance at high voltage
What is the function of the neutral wire (blue)? (1 point)
Completes the circuit and carries current away from appliance
What is the function of the earth wire (green + yellow)? (1 point)
Safety wire to stop the appliance becoming live
Why are mains electricity cables made of copper? (1 point)
Copper is a good conductor of electricity
Why are the outer layers of a mains electricity cable flexible plastic? (1 point)
Plastic is a good electrical insulator
Where does each wire of a plug go? (3 points)
- BLue goes Left
- BRown goes Right
- STriped goes to the Top
Explain why some appliances have double insulation (6 points)
- All appliances with metal outer cases (conductors) have earth wire and are earthed
- An earthed conductor cannot become live
- Appliances with insulator cases don’t have an earth wire (they do have a fuse), because they have plastic casings, or they have been designed so that the live wire can not touch the casing
- As a result, the casing cannot give an electric shock, even if the wires inside become loose
- These appliances have double insulation
- Double insulated = can’t become live
Explain how the earth wire and fuse work together? (5 points)
1) Fault in appliance = casing becomes live
2) Circuit short-circuits (swaps path of flow), as earth wire offers less resistance
3) Fuse wire melts
4) Circuit broken
5) Appliance and user are protected
What are fuses and circuit breakers? (1 point)
Safety devices designed to break a circuit if a fault occurs, prevents fires, injuries and deaths
What is a fuse, what is it for, what must it be, and how does a circuit breaker differ? (4 points)
- Short, thin piece of wire with low melting point
- Prevents cables/appliances from overheating
- Current rating of fuse must be just above normal current that flows through appliance
- On the other hand, a circuit breaker can be easily reset rather than replaced
What are the 4 stages of how a fuse works? (4 points)
1) Fault causes current in appliance to exceed the current rating of the fuse
2) Fuse gets hot and therefore melts/breaks
3) Circuit is therefore broken - current is unable to flow
4) Appliance and user are protected
What is the power rating of a device? (1 point)
How quickly electrical energy is being transferred into another form within
What is the formula for power, and what are they measured in? (4 points)
Power = voltage × current
- Power is measured in watts, W
- Voltage (potential difference) is measured in volts, V
- Current is measured in amperes (amps), A
Give an example of a fuse in action? (3 points)
1) If the current of appliance is below the current rating of the fuse, it works properly
2) If a fault occurs, live wire comes into contact with neutral wire, the current flowing is then higher than current rating of fuse, due to lower resistance
3) Cases fuse to get hotter until it breaks circuit. Current is unable to flow, so there is no danger of flex overheating (causes fire), damage to appliance and injuries prevented
What is ultrasound, and how does it travel? (2 points)
- Sound waves with frequencies of above upper limit of human hearing range (20,000Hz)
- Travels in longitudinal wave
What are the 5 key features of waves? (5 points)
1) Rarefaction - a region of lower pressure where particles are spread out
2) Compression - a region of higher pressure where particles are squashed together
3) Wavelength - the distance between 2 corresponding points on 2 successive disturbances
4) Frequency - no. of waves produced in 1 second, measured in hertz (Hz), a higher pitched sound has a higher frequency than a lower pitched sound
5) Amplitude - the maximum disturbance caused by a wave, the louder a sound is, the more energy it carries and the bigger its amplitude