Electrical Energy in the Home
Initially wood was used as a source of heat energy and domestic animals were used for mechanical energy. Over time our main sources of energy have changed to wind and water, coal (19th century steam engine) to coal gas for street lighting. Now, fuel burning power stations are the main source of energy (nuclear power stations in some places) and hydro-electric power stations still make use of water energy.
Because it was now possible to provide light as bright as daylight, activities once confined to daylight hours could now be carried out anytime. Factories started running 24hrs a day and shops stayed open longer. Sporting events started to take place at night where more could attend. Electric heaters became a “cleaner” source of heating and small electric motors brought refrigerators and washing machines into the home.
Transmission lines are used to quickly convey electricity to where it is needed and transformers modified voltage and current for households. Small generators powered by petrol or oil, solar panels or wind generators could be used in remote places to provide electricity.
Electrostatic charges: opposite attract, like repel and charged attract neutral objects.
Electric charge is measured in coulombs (C)
E = f/q E = magnitude of electric field (NC-1), q = charge (C), F = force (N)
Current is the rate at which charge flows (C\sec. or Amperes) under the influence of an electric field.
DC (direct current) charge moves in one direction, AC (alternating current) charge moves back and forth periodically.
Electric potential difference (Voltage) between two points is the change in potential energy per unit charge moving from one point to another (joules/coulomb or Volts).
Potential difference can vary at different points around a circuit for example there will be different voltage drops across various resistors, light globes and rheostats which would be different to the voltage rise across the power pack.
A conductor is a material that contains a large number of charge carriers, or charged particles which are free to move through the material. E.g. metals, salt solutions. An insulator is a material that contains no charge carriers. E.g. dry air, glass or plastic.
Ohm’s Law states “The potential drop across a resistor is proportional to the current passing through it” R = V/I
Resistance is proportional to length; the longer a conductor the greater the resistance. Resistance is inversely proportional to the area of cross-section; the larger the cross-section the lower the resistance. The material of a conductor influences resistance; copper is commonly used for household wiring, gold or silver used when minimal resistance required. Temperature is proportional to resistance, as temperature increases ions vibrate increasing resistance.
Series circuits have only one pathway for electricity to flow, whereas parallel circuits have more than one pathway which electricity can flow through.
In series, current remains constant and voltage varies (VT=V1+V2+V3). In parallel, voltage remains constant and current varies (IT=I1+I2+I3).
Ammeters measure current in a circuit, voltmeters measure potential difference.
Ammeters measure current in the position they are placed, therefore they need to be connected in series. However, voltmeters measure potential difference across two points therefore, they are required to be connected in parallel.
In a house there are separate circuits for lighting, heating and other appliances so that appliances that require large amounts of current can still function without over-loading the circuit. If everything was on one circuit there would be too much current used and the wires would become hot and potentially cause a fire.
Power is the rate at which energy is transformed from one form to another. If energy is transformed in t time then: P = energy/t
Power is the number of joules per second dissipated, for every I coulomb, V joules are dissipated. Therefore: P = VI
Energy = Pt (P=VI) therefore Energy =VIt, total amount of energy used depends on the time current flows.
Because of the large amount of energy household use, it is inappropriate to use joules. Instead the kilowatt-hour is used. This is the amount of energy used by a 1 kW device in an hour. Energy in kilowatt-hours = power (kW) * t (hrs)
Like magnetic poles repel, unlike attract.
The direction of a magnetic field at a point is the direction of force on a very small north magnetic pole when placed at that point.
Right hand grip rule: Grip wire with right hand, thumb pointing in direction of conventional current, fingers will curl around in the direction of the magnetic field.
A solenoid has a closed loop from north to south much like a bar magnet. At one end of the solenoid the fields go in (South Pole) at the other they come out (North Pole). The only difference is that in a solenoid the field continues through the middle as parallel lines.
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Circuit breakers shut off circuit when the current exceeds a safe level preventing electrical fires. Fuses are designed the same way so that a wire melts shutting off the circuit before the actual wires in the house would melt. Fuses need to be replaced, whereas a circuit breaker can just be reset. Circuit breakers usually come in two types; either bi-metallic strip, or ones that make use of an electromagnet. Earthing ensures any lose current goes to the ground instead of building up and causing potential problems. Double insulation protects from electrocution in the case the basic insulation fails, items that have been double insulated do not need to be earthed.