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a                      production


    Everything Electric
          Episode 3
        Mainly Mains
Mainly Mains
           The Mains is the electric power
           supply to a house or workplace.
       bestofyoutube.com




It is alternating current (AC) which means the electricity
repeatedly changes direction in the circuit (50 times per
second). Batteries provide direct current (DC); always in
the same direction.
The hazards of mains electricity

             Overloaded socket                              Fire             Shock




                                                          Images: Health and Safety signs
               Image: Citizencorps.gov
                                                        Water near sockets
Images: OCAL @ clker.com




                                  Image: Childsafetyaustralia
    Frayed cables               Metals in appliances                Image: Safework SA
Electrical safety

                                                   Image: aka @ wikipedia
 Double
insulation

  Fuse
              Image: Jared C Bennett @ wikipedia

  Earth
  wire

Circuit
breaker
             Image: wn.com                            Image: Aussiedingo @ flickr.com
Fuses and circuit breakers
              When the current is too high,
              they melt or break the circuit.
                                          Image: wn.com
Q: A heater has a maximum
current of 10 A; what is the
appropriate circuit breaker or
fuse 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 one
20 A, 32 A - too high; maybe good for an electric oven.
Earth wire


                                Normally there is no
                              current in the earth wire.
                                A fault can cause the
                                metal case to be live.

  The earth conducts the
current away from the case.
 The current in the earth
wire is often high enough
    to melt the fuse.
                                           Images: OUPChina
Double insulation

                                   Image: Jared C Bennett @ wikipedia




Many appliances have an insulating cover (eg plastic);
 this is called double insulation since the wires inside
are also insulated. So a fault could not give the user a
   shock from the case. No earth wire is necessary.
Electrical heating
Components transfer some or all of the electrical energy
  to heat energy which then warms the surroundings.


                          Energy transfer
                           by radiation
                        Image: opencage.info




Image: Fireflyco.com

Energy transfer                                Image: Pokayoke @ wikispaces
 by conduction                                 Energy transfer
(and convection                                 by convection
  in the water)
Calculating electrical power
Power 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.
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
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
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 electron
accelerated 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
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
a                                      production



         MUCH MORE AT
http://nothingnerdy.wikispaces.com
http://nothingnerdy.wikispaces.com/MAINS+ELECTRICITY

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Mainly Mains - nothingnerdy igcse physics

  • 1. presents a production Everything Electric Episode 3 Mainly Mains
  • 2. Mainly Mains The Mains is the electric power supply to a house or workplace. bestofyoutube.com It is alternating current (AC) which means the electricity repeatedly changes direction in the circuit (50 times per second). Batteries provide direct current (DC); always in the same direction.
  • 3. The hazards of mains electricity Overloaded socket Fire Shock Images: Health and Safety signs Image: Citizencorps.gov Water near sockets Images: OCAL @ clker.com Image: Childsafetyaustralia Frayed cables Metals in appliances Image: Safework SA
  • 4. Electrical safety Image: aka @ wikipedia Double insulation Fuse Image: Jared C Bennett @ wikipedia Earth wire Circuit breaker Image: wn.com Image: Aussiedingo @ flickr.com
  • 5. Fuses and circuit breakers When the current is too high, they melt or break the circuit. Image: wn.com Q: A heater has a maximum current of 10 A; what is the appropriate circuit breaker or fuse 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 one 20 A, 32 A - too high; maybe good for an electric oven.
  • 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 the current away from the case. The current in the earth wire is often high enough to melt the fuse. Images: OUPChina
  • 7. Double insulation Image: Jared C Bennett @ wikipedia Many appliances have an insulating cover (eg plastic); this is called double insulation since the wires inside are also insulated. So a fault could not give the user a shock from the case. No earth wire is necessary.
  • 8. Electrical heating Components transfer some or all of the electrical energy to heat energy which then warms the surroundings. Energy transfer by radiation Image: opencage.info Image: Fireflyco.com Energy transfer Image: Pokayoke @ wikispaces by conduction Energy transfer (and convection by convection in the water)
  • 9. Calculating electrical power Power 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. 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. 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. 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 electron accelerated 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. 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. a production MUCH MORE AT http://nothingnerdy.wikispaces.com http://nothingnerdy.wikispaces.com/MAINS+ELECTRICITY

Notes de l'éditeur

  1. Welcome to nothingnerdy’s presentation on Everything Electric. Part 3: Mainly Mains\n
  2. \n
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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
  9. 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
  10. We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  11. We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  12. We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  13. We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
  14. \n