Energy and voltage in circuits

• Which way do electrons flow in a circuit?

  From negative terminal to positive terminal
• Define electric current

  Rate of flow of electrons carrying a negative charge

  • Unit = amp (A)
• Why can current flow through metals?

  • Metals have delocalised electrons
  • Can move freely throughout metal
• Why can current not flow through plastics?

  • No delocalised electrons
  • Charge cannot be carried
  • Plastics are insulators
• Give the equation linking charge, current and time

  • Charge = current x time
  • Q = I x t
  • Unit for charge = Coulomb (C)

  
• Which device is used to measure the size of a current?

  Ammeter
• How are ammeters connected in a circuit?

  In series
• Define voltage (also known as potential difference)

  Amount of energy transferred per coulomb of charge passed

  • Unit = volt (V)
  • 1 volt = 1 joule per coulomb
• Give the equation linking voltage, current and resistance

  • Voltage = current x resistance
  • V = I x R

  
• Which device is used to measure voltage?

  Voltmeter
• How are voltmeters connected in a circuit?

  In parallel- Around component being measured
• Give the equation linking energy transferred, charge and voltage

  Energy transferred = charge x voltage

  • E = Q x V
  • Unit for energy = joules

  
• Define resistance

  • Ratio of voltage and current
  • Unit = ohm (Ω)
• i) Calculate the energy transferred to an electron when it passes through a voltage of 125kV

  [charge of electron = 1.6 × 10–19C]

  Step 1: Convert 125kV to V 125kV = 125 000 V

  Step 2: Substitute into equation E = Q × V

  E = 1.6 × 10–19 × 125 000

  E = 2.0 × 10–14 J

  A) 1.6 × 10–19C]

  B) Step 1: Convert kV to V

  C) Step 2: Substitute into equation
• i) Calculate the energy transferred to an electron when it passes through a voltage of 125kV. [charge of electron = 1.6 × 10–19C]

  i)Step 1: Convert 125kV to V 125kV = 125 000 V

  Step 2: Substitute into equation E = Q × V

  E = 1.6 × 10–19 × 125 000

  E = 2.0 × 10–14 J
• ii) Worked Example 3: Using Electrical Formulae (1)


  Step 1: Convert 0.68ms into s             0.68 ms = 0.68 × 10⁻³s

  Step 2: Substitute into equation        Q = I × t

                                                                   2.9 × 10⁻⁸ = I × 0.68 × 10⁻³

                                                                   I = 2.9 × 10⁻⁸ / 0.68 × 10⁻³

                                                                   I = 4.3 × 10⁻⁵ A

  
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• A student investigates how the resistance of a thermistor varies with temperature. They use a voltmeter and ammeter to find the resistance at each temperature.

  Temp °C = 70

  Voltmeter reading (V) = 12.5

  Ammeter reading (mA)= 2.10

  -

  A) mA to A

  B) Ω

  
• What is this circut symbol?

  Closed switch

  
• What is this circut symbol?

  open switch

  
• What is this circut symbol?

  Cell

  
• What is this circut symbol?

  Battery

  
• What is this circut symbol?

  Diode

  
• What is this circut symbol?

  Resistor

  
• What is this circut symbol?

  Variable resistor

  
• What is this circut symbol?

  Light emitting diode (LED)

  
• What is this circut symbol?

  Lamp

  
• What is this circut symbol?

  Fuse

  
• What is this circut symbol?

  Voltmeter

  
• What is this circut symbol?

  Ammeter

  
• What is this circut symbol?

  Thermistor

  
• What is this circut symbol?

  Light dependent resistor (LDR)

  
• Describe a series circuit

  • All components connected in a single loop
  • Only one path for current to follow
• Describe a parallel circuit

  • Circuit has branches
  • Multiple paths for current to follow
• Give the advantages and disadvantages of connecting light bulbs in series

  Advantages:

  -One switch can turn all bulbs off/on

  -Uses fewer wires

  Disadvantages:

  -If one bulb breaks, the circuit breaks. All bulbs stop working

  -Voltage shared between all bulbs. More bulbs = less bright
• Give an example of when a series circuit should be used

  • Decorative lights (e.g. fairy lights)
  • Each bulb requires low voltage
  • Can all be controlled by one switch
• Explain why lights in a house are connected in parallel

  • Lights can be switched on and off separately
  • Brightness of each light does not change if another is switched on
  • All receive mains voltage (230V)
  • If bulb breaks/is removed, other lights keep working
• Series & Parallel Circuits

  REMEMBER: The arrows on a circuit diagram actually point in the OPPOSITE direction to the flow of electrons. This is called the ‘conventional current’, and is due to electrons being discovered after electricity!

  A) REMEMBER: The arrows on a circuit diagram actually poin

  
• Why are lamps and LEDs frequently incorporated into circuits?

  • Indicate presence of a current in the circuit
  • Lamp/LED on = current flowing
• What are the current and voltage rules in a series circuit?

  • Current is the same everywhere
  • If additional component added, current reduces through all components
  • Total voltage is the sum of all the individual components’ voltage
  • If voltage doubled, current also doubles