Electricity

• Alternating current: potential difference alternating quickly between positive and negative. Flow of electric charge constantly reverses direction.
• Direct current: a potential difference that is always constant and always positive. Flow of electric charge is in only one direction.
• A battery: two or more cells working together
• Difference between AC & DC : DC is the constant flow of electric charge in one direction whereas AC is the flow of charge constantly changing directions. AC: p.d. alternated between negative & positive whereas in a DC the charge flows from positive to negative.

  
• Units:

  (V) Volts -> voltage/potential difference

  (W) Watts -> power

  (A) Amps -> current

  (Q) Coloumbs -> electric charge

  (J) Joules -> energy

  (Ω) Ohms -> resistence
• Hazards of electricity: electrocution, fire and water, overloading, overheating...

  • electric shock and burns from contact with live parts
  • injury from exposure to arcing (when electricity jumps from one circuit to another)
  • fire from faulty electrical equipment or installations
  • explosion caused by unsuitable electrical apparatus
  • static electricity igniting flammable vapours or dusts eg. a spray-paint booth
• • Power stations get their power for burning fossil fuels like coal, oil and gas or nuclear fission which makes thermal energy
  • Power station convert thermal energy into electrical energy

  
• What does a step up transformer do?

  It increases the voltage which is used to transmit electricity to great distances
• Transformers alter voltage.

  Step up transformers: Increase the voltage: Lowering the current reduces the amount of power that is lost due to resistance of the power lines

  Step down transformers: Decrease the voltage before it enters the town: to conserve electrical power and energy, and to make it safer and more useable.
• Power = Voltage x Current
• In the UK, alternating current oscillates 50 times every second, meaning the frequency is 50 Hertz.
• Conductor: has little or no resistance to the flow of electrons: materials or substances which allow electricity to flow through them
• Insulator: a material that does not allow electricity to flow easily through it: rubber or plastic make good insulators
• When there is a current through a resistor, there is a transfer from electrical to thermal energy : Electrical Heating
• Fuses are used for electrical safety, they prevent fires.
• Earth pin comes first into contact with the electrical circuit to protect it from electric shock. It provides safety. Colour of the earth wire : Green
• Live pin (on the left): supplies electrical current to the device. Colour of live pin: Brown
• Neutral pin (next to the fuse): carries the current from the device/appliance to the power source. Colour of neutral pin : Blue
• An device is connected to the mains electricity supply, the p.d is 230 V.

  Calculate the energy transferred when 13 C of charge flows through the device.

  Energy transferred = charge flow x potential difference

  Answer is : 13 x 230

  Unit for energy transferred : Joules
• What is the equation that links charge flow, current and time?

  Q = I * t
• The fuse wire melts when 1.52 coloumbs of charge flows through the fuse in 0.40 seconds.

  Calculate the current at which the fuse wire melts.

  Q = I*T

  Rearranging: I = Q/T

  I = 1.52/0.4 = 3.8

  Unit: Amps
• Most electric kettles use the AC mains electricity supply.

  The AC mains supply has a potential difference that continuously changes polarity.
• Control variable: A variable that is kept constant throughout the experiment to ensure that it does not affect the results.
• Independent variable: The variable that is altered during the experiment.

  eg. testing different fertilisers on the soil

  Independent variable would be the fertilisers
• Dependent variable: The variable that is measured
• A student investigated how the power output of a kettle affected the time taken to boil a fixed volume of water.

  The water in all 3 kettles had an initial temperature of 25°C.

  What were the variables?

  Control: Initial temperature of the water

  Dependent: Time taken

  Independent: Power of the kettle
• Which kettle will boil water in the shortest time?

  The one with the highest power
• How to calculate power output of the kettle from a graph?

  Rise over run = Power output (W)
• Electric current is the rate of flow of charge.
• State the colour of the covering of the neutral wire?

  Blue
• Which two parts of the plug protect the wiring of a circuit?

  Earth wire and fuse
• What does 'double insulated' mean?

  It's a method of extra protection against shock. It's an outside layer directly over the live parts of a plug that insulates the casing.
• Which wire is not required?

  Earth wire
• What must be fitted to the cable before it can be connected to the mains electricity supply?

  Plug
• What type of current does a battery supply?

  direct current only
• A fuse or a circuit breaker is connected to the live wire in a circuit.
• What happens to cause a fuse or circuit breaker to disconnect a circuit?

  overheating, overloading or too much current blows the fuse
• Two advantages to using a circuit breaker to disconnect a circuit.

  1. Faster, more efficient
  2. Doesn't need replacing
• The increased amount of resistors in a series circuit, the lower the current.