2 - Electricity

• • Electric current flows from the negative end to to the the positive end of the cell (electrons are negative)
  •  Electrons carry energy from the cell and pass it to the components in the circuit such as the lamp (electrical energy is transferred to light and thermal energy)
  • When electrons return to the positive side of the cell, they are carrying less energy 

  A) cell

  B) lamp

  C) open switch

  D) closed switch

  
• Series current: has no branches, the current can only flow in one path 

  • Current = flow of electric charge unit- is ampere (A)/amp 
  • Measure current by using an ammeter 
  • Current is never used up in a series circuit, it is the same all the way around
  • Conventional current: drawing in the opposite direction

  
• Parallel circuits contain branches: some current passes through both of the branches 

  • The current in the branches adds up to the total current leaving the cell 

  
• Potential difference (voltage)

  • Potential difference of 1 volt tells us that 1 joule of energy is transferred for each coulomb of charge that is moving through the circuit
• • The total p.d across both lamps (in series circuit) is the same as the p.d across the whole cell- the higher the p.d the brighter the lamp  will be as it’s transferring more energy 

  if there are two lamps then the p.d will be /2 because it's being shared

  
• • For components connected in parallel, the potential difference

   across each component (branch) is the same  

  
• Batteries: two or more cells together

  • Must be connected in the same direction 
  • The volt would multiple by the amount of ‘cells’ present

   eg; 3 cells, volt is 9 → 3 x 9 = 18 volts 

  
• Electrical charge is measured in coulombs (C)

  • Current of 1 ampere = 1 coulomb of charge flowing
  •  the size of electric current is the rate of flow of electrical charge 

  Charge flow (coulombs) = current (amperes) x time (seconds)

  Q = I X T

  A) charge flow

  B) current

  C) time

  
• energy transferred (J) = charge flow (c) x potential difference (v)

  E = Q x V

  
• As electrons move through a conductor, they collide with atoms in the metal, electrical energy is transferred into other forms (eg: thermal) 

  Cell contains chemical energy → transferred to electrical energy → carried by current around circuit 

  • Resistance : the potential difference required to drive a current through a component (ohms/Ω) high p.d, high resistance 

  resistance = p.d / current

  R = V/I

  
• Resistors: circuit components that provide a given resistance to a circuit

  • current through resistor is directly proportional to the p.d
  • resistance is constant; an ohmic conductor

   (will only stay constant if the temperature is constant! )

  
• Filament Lamp: very fine wire (filament) the tightly coiled wire gets hot when a current passes through- this gives off the light 

  • Current through a filament lamp is not directional proportional to to p.d 
  • That's because the filament gets hot, which causes the resistance to increase- at high temp the atoms vibrate more, the electrons in the current collide more with the atoms so more energy is needed to push current through 
  • As p.d increases the current no longer increases as much 

  
• • Current in diode flows in one direction: it has very high resistance in the reverse direction 
  • Extremely useful for controlling the flow of current in circuits 

  Light-emitting diode (LED)

  • Gives off light when current flows through
  • Extremes energy-efficient source of light 

  
• Signs

  A) LED

  B) diode

  
• Resistors in series add together; the current has to pass through each resistor in turn. It cannot bypass any resistor

  • Equivalent resistance: you can instead replace multiple resistors with one (eg: 150 resistor instead of 100 and 50)

  
  

  Current = potential difference / resistance

  I = V/R

  
• LDR = Light dependant resistor

  In dark conditions the LDR has high resistance, but in light it is low

  • In light = resistance is low, it takes little energy for current to pass through LDR, the p.d is low
  •  P.d is shared between components so the p.d across the lamp is large and the lamp would let light out

  If the phone is help to a person ear, the LDR is in darkness, so the resistance rises sharply; great deal of  energy for current to pass through, so p.d across LDR is very high

  • Less electrical energy for the lamp- p.d is now very low and lamp is dim, screen turns off
• Thermistors: resistance decreases when temperature increases 

  • Under cool conditions; the resistance of the thermistor is high which means it takes a lot of energy for the current to pass through - p.d would be high 
  • If temperature is hot, resistance of thermistor falls, less energy for current to pass through thermistor p.d is now low 
•  Required Practical: Resistance

  • Attach a wire to a metre ruler and then connect the wire to the rest of circuit using crocodile clips- only the part of the wire between the clips is connected - move clips to change the length  

  Problems: 

  1. Zero error- subtract the zero error from the reading (due to crocodile clips not being on zero on ruler, resistance caused between contact of  clips and wire) 
  2. Heating effect: temperature of wire will increase then resistance will also increase- use a low p.d to keep current low, reducing any heating in the wire  & only turn on current when taking a reading 

  
•  Required Practical : Current / PD Characteristics

  
  

  Resistor

  1. Read voltmeter to find the p.d across the resistor, use ammeter to read current through the resistor 
  2. Then record the values in a table 
  3. Adjust the variable resistor and record the new reading on the  ammeter & voltmeter (do this multiple times to get a rage of readings)
  4. Switch the direction of the battery, this means the direction of the p.d should reverse- readings should be negative values 
  5. Record a range of readings

  
•  Required Practical : Current / PD Characteristics

  Filament Lamp

  1. Read voltmeter to find the p.d across the filament lamp, use ammeter to read current
  2. Then record the values in a table 
  3. Adjust the variable resistor and record the new reading on the  ammeter & voltmeter (do this multiple times to get a rage of readings)
  4. Switch the direction of the battery, this means the direction of the p.d should reverse- readings should be negative values 
  5. Record a range of readings

  
•  Required Practical : Current / PD Characteristics

  Diode: Extra resistor: diode are easily damaged by high current- keep current relatively low, because current is slow we need a sensitive ammeter (milliammeter

  1. Read voltmeter to find the p.d across the diode, milliammeter to read current
  2. Then record the values in a table 
  3. Adjust the variable resistor and record the new reading on the  ammeter & voltmeter (do multiple times to get a range of readings)
  4. Switch direction of the battery - direction of the p.d should reverse- readings should be negative values 
  5. Record a range of readings

  
• Energy Transfer by Appliances

  Blender and fan = Electrical energy → kinetic energy of electrical motors 

  Iron & Kettle  = Electrical energy → thermal energy 

  Hair Dryer & washing machine = electrical energy → kinetic energy of motors + thermal energy  

  Power = the rate at which energy is transferred 1 watt = 1 J per second 

  • Thermal energy appliance needs higher power 
• energy = power x time

  E = p x t

  
• power = p.d x current

  
• power = current^2 x resistance

  
• Current  from cell = direct current (electrons only flow in one reaction) Mains power supply = alternating current (constantly changing in direction) 

  • Benefit- easy to use, transformer to increase or decrease the p.d 

  In UK, Frequency = 50 Hertz (hz), p.d = 230 volts Oscilloscope: to look at AC and DC, Frequency = Number of cycles in one cycle

  
• Main Electricity

  • The wires are made of copped which is a good conductor of electricity: coating are made of plastic which does not conduct electricity 

  Brown Wire: live wire carries alternating p.d from power supply (230V)

  • Connected to a fuse in the plug
  • Dangerous, fatal if touched: has a high p.d - Current would flow through the person into the earth they'd be electrocuted 

  Blue wire: neutral, completes the circuit-  p.d = 0V compared to the live wire

  •  transfers electricity away from the appliance to avoid overloading 
• Green & yellow wire: earth wire, a safety wire 

  • Connected into the ground with a metal rod 
  • If the case becomes live, a huge current flows to the earth- the fuse melts and shuts of the current, prevents anyone from getting an eclectic shock from touching the case  

  Metal cases: can be dangerous 

  • If live wire comes loose and touches metal case, the case can become live: could give someone a fatal electric shock if they touch it. 
  • Acts as a safety wire to stop the appliance from becoming live

  
• National grid: system of transformers and cables

  1. Electricity passed through step-up transformer which increase the potential difference to several hundred thousand volts- less energy is lost in power cables when p.d is high 
  2. Step down transformer- reduces p.d to around 230V