Mainly Mains - nothingnerdy igcse physics

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  • Welcome to nothingnerdy’s presentation on Everything Electric. Part 3: Mainly Mains\n
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  • The dangers of mains electricity are fire and electric shock. The following are hazards for one or both of these reasons: Damaged plug; overloaded socket; frayed cable; metal objects in the wrong place; water around sockets.\n
  • Different devices are used to make us safe from electrical hazards. Wires are insulated and cables double insulated as in the photo above. A fuse is a weak point in a live wire which will melt if the current is too high. A circuit breaker also breaks the connection when the current is too high. An earth wire connects the metal case of a device to the ground so that if the case goes accidentally live the current flows to earth and blows the fuse of circuit breaker. See the earth symbol.\n
  • Different devices are used to make us safe from electrical hazards. Wires are insulated and cables double insulated as in the photo above. A fuse is a weak point in a live wire which will melt if the current is too high. A circuit breaker also breaks the connection when the current is too high. An earth wire connects the metal case of a device to the ground so that if the case goes accidentally live the current flows to earth and blows the fuse of circuit breaker. See the earth symbol.\n
  • If a loose live wire in a device touches the metal case, you can get a nasty shock if you touch the metal. This is why many devices have an earth wire connecting the case to the earth terminal in the house. Now if there is a fault, the current runs straight to the earth. This can cause a high current which blows the fuse preventing shock or fire.\n
  • Different devices are used to make us safe from electrical hazards. Wires are insulated and cables double insulated as in the photo above. A fuse is a weak point in a live wire which will melt if the current is too high. A circuit breaker also breaks the connection when the current is too high. An earth wire connects the metal case of a device to the ground so that if the case goes accidentally live the current flows to earth and blows the fuse of circuit breaker. See the earth symbol.\n
  • When a current flows in a resistor, the electrical energy converts to heat and the resistor becomes hot. We can use this process in heating devices. A kettle warms liquid with an electric element; a bar heater radiates heat into the room; a fan heater circulates hot air in the room.\n
  • Power in watts measures how much energy in joules is converted per second. The formula is P=VI. Many devices have a label showing their operating voltage and current. If you multiply these two values, you can calculate the electrical power.\n
  • We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  • We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  • We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  • We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  • \n
  • Mainly Mains - nothingnerdy igcse physics

    1. 1. presentsa production Everything Electric Episode 3 Mainly Mains
    2. 2. Mainly Mains The Mains is the electric power supply to a house or workplace. bestofyoutube.comIt is alternating current (AC) which means the electricityrepeatedly changes direction in the circuit (50 times persecond). Batteries provide direct current (DC); always inthe same direction.
    3. 3. The hazards of mains electricity Overloaded socket Fire Shock Images: Health and Safety signs Image: Citizencorps.gov Water near socketsImages: OCAL @ clker.com Image: Childsafetyaustralia Frayed cables Metals in appliances Image: Safework SA
    4. 4. Electrical safety Image: aka @ wikipedia Doubleinsulation Fuse Image: Jared C Bennett @ wikipedia Earth wireCircuitbreaker Image: wn.com Image: Aussiedingo @ flickr.com
    5. 5. Fuses and circuit breakers When the current is too high, they melt or break the circuit. Image: wn.comQ: A heater has a maximumcurrent of 10 A; what is theappropriate circuit breaker orfuse for safety?Choose from 8, 16, 20 or 32 A HINT: the ‘rating’ must be greater than the max.8 A - too low current, but not by too much.16 A - this one20 A, 32 A - too high; maybe good for an electric oven.
    6. 6. Earth wire Normally there is no current in the earth wire. A fault can cause the metal case to be live. The earth conducts thecurrent away from the case. The current in the earthwire is often high enough to melt the fuse. Images: OUPChina
    7. 7. Double insulation Image: Jared C Bennett @ wikipediaMany appliances have an insulating cover (eg plastic); this is called double insulation since the wires insideare also insulated. So a fault could not give the user a shock from the case. No earth wire is necessary.
    8. 8. Electrical heatingComponents transfer some or all of the electrical energy to heat energy which then warms the surroundings. Energy transfer by radiation Image: opencage.infoImage: Fireflyco.comEnergy transfer Image: Pokayoke @ wikispaces by conduction Energy transfer(and convection by convection in the water)
    9. 9. Calculating electrical powerPower is the rate of transfer of energy to other forms P I xV Power, P is measured in watts (W) Voltage,V is measured in volts (V) Current, I is measured in amps (A)To calculate power in watts, multiply volts times amps. Look at the label on a computer or kettle etc.
    10. 10. Calculating electrical energy Energy, E is measured in joules (J) Voltage,V is measured in volts (V) Current, I is measured in amps (A) Time, t is measured in seconds (s) Power, P is measured in watts (W) Reminder: Power = Energy/ Time P E E I xV Pxt VxIxt
    11. 11. Power calculations A movie projector converts 4.32 MJ showing a 3 hour movie. What is its power? A kettle rated at 1.5 kW boils water for 2 minutes. How much energy is converted? What current passes through a 12 V bulb which is rated at 3.0 W? How much energy is converted in 30 min by a laptop whose supply voltage is 12 V and which takes a current of 1.0A?ANSWERS ON NEXT BUT ONE PAGE
    12. 12. Extended Power calculations More than IGCSE demands: just for fun A van de Graaf generator develops a PD of 200 kV. It sparks with a current of 2.0 mA in 2.0 ms. A: How much energy is converted in joules?B: How much electric charge is discharged in coulombs? C: The charge on one electron is 1.6 x 10-19 C. How many electrons are discharged?D: One electron-volt (eV) is a unit of energy. An electronaccelerated through 200 kV converts 200 keV. The Large Hadron Collider gives protons energies of 4 TeV (2012). How many times more energetic is this than the vdGG? ANSWERS ON NEXT PAGE
    13. 13. Extended Power calculations 400 W 180 kJ 0.25 A 21.6 kJ 0.8 J 4.0 µC 2.5 x 10 13 2 x 10 7
    14. 14. a production MUCH MORE AThttp://nothingnerdy.wikispaces.comhttp://nothingnerdy.wikispaces.com/MAINS+ELECTRICITY

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