Electrical power

• In a single, closed loop the current has the same value everywhere in the circuit
• The size of the current is the rate of flow of charge
• A current can only flow if there's a source of potential difference
• The unit of potential difference is the volt, V
• The unit of resistance is the ohm, Ω
• The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component)
• Electric current is a flow of electrical charge
• Resistance is anything that slows the flow down
• Potential difference (or voltage) is the driving force that pushes the charge round
• Total charge through a circuit depends on current and time
• Electrical charge will only flow round a complete (closed) circuit if there is a potential difference
• The unit of current is the ampere, A
• The current flowing through a component depends on the potential difference across it and the resistance of the component
• More charge passes around the circuit when a larger current flows
• The formula linking pd and current is very useful (and pretty common)
• Potential difference

  Current (A) x Resistance (Ω)
• Resistance and V = IR
• Ammeter


  Measures the current (in amps) flowing through the test wire
• The ammeter must always be placed in series with whatever you're investigating
• Voltmeter


  Measures the potential difference (or pd) across the test wire (in volts)
• The voltmeter should never be placed around any other bit of the circuit, e.g. the battery
• The voltmeter must always be placed in parallel around whatever you're investigating
• The resistance of some resistors and components DOES change, e.g. a diode or a filament lamp
• The resistance of ohmic conductors (e.g. a wire or a resistor) doesn't change with the current
• Resistance change in diodes

  Depends on the direction of the current, high resistance if it is reversed
• Resistance change in a filament lamp

  As the current increases, the filament lamp heats up more and the resistance increases
• Ohmic Conductors


  • Have a constant resistance
• Experiment to find a component's I-V characteristic

  Set up the test circuit, vary the variable resistor, take readings from the ammeter and voltmeter, swap over the wires to reverse the current, plot a graph of current against voltage for the component
• At a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it
• Linear components

  • Have an I-V characteristic that's a straight line (e.g. a fixed resistor)
• Non-linear components


  • Have a curved I-V characteristic (e.g. a filament lamp or a diode)
• The term '-V characteristic' refers to a graph showing how the current flowing through a component changes as the potential difference across it is increased