Electricity

• Electric current is the flow of electrical charge.
• Electric charge will only flow round a complete circuit if there is a potential difference.
• An Ampere (A) is the unit of current.
• In a single, closed loop, the current has the same value at all times.
• Potential difference (voltage) is the driving force that pushes the charge around.
• Voltage is measured in Volts (V)
• Resistance is anything that slows down the flow of electrons through a circuit.
• Resistance is measured in Ohms (Ω)
• The current flowing through a component depends on the potential difference across it and the resistance of the component.
• The greater resistance across a component, the small the current flows.
• Switch (Open)

  
• Switch (Closed)

  
• Cell

  
• Battery

  
• Diode

  
• Resistor

  
• Variable Resistor

  
• LED

  
• Lamp

  
• Fuse

  
• Voltmeter

  
• Ammeter

  
• Thermistor

  
• LDR (Light dependent Resistor)

  
• Potential Difference (V) = Current (A) x Resistance (Ω)
• Required Practical - investigate the factors that affect resistance


  1. Connect the circuit as shown in the diagram above

  2. Connect the crocodile clips to the resistance wire, 100 centimetres (cm) apart

  3. Record the reading on the ammeter and on the voltmeter

  4. Move one of the crocodile clips closer until they are 90 cm apart

  5. Record the new readings on the ammeter and the voltmeter

  6. Repeat the previous steps reducing the length of the wire by 10 cm each time down to a minimum length of 10 cm

  7. Use the results to calculate the resistance of each length of wire by using R = V/I, where R is resistance, V is voltage and I is current

  8. Plot a graph of resistance against length for the resistance wire
• Required Practical - investigate the factors that affect resistance - Risks

  Heating of the resistance wire - Burns - Do not touch wire
• The ammeter measures the current in amps (A) and is connected in series with the component being measured.
• The voltmeter measures the voltage (P.D) across the wire (in Volts). Must always be placed parallel to the object you are investigating
• Ohmic conductors have a constant resistance.
• Resistance of Ohmic conductors is directly proportional to the potential difference across it. At a constant temperature.
• The resistance of some resistors and components does change however such as a diode or filament lamp.
• When electricity flows through the lamp, it transfers some energy to the thermal energy store of the lamp, which heats up. Resistance increases with temperature so as the current increases, so does the resistance
• Ohmic Conductor IV Characteristics

  
• Filament Lamp IV Characteristics

  
• Diode IV Characteristics

  
• LDR is a resistor that is dependent on the intensity of light. In bright light, resistance falls and vice versa.
• LDR IV Characteristics

  
• Thermistor is a temperature dependent resistor. In hot conditions the resistance drops and vice versa. Make very useful temperature sensors
• Thermistors graph