2 - Electricity

Cards (29)

    • 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 93 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