Current, Potential Difference and Resistance

• An electric current is the flow of electrical charge, usually electrons, around a circuit.
• The size of the electric current is the rate of flow of electrical charge.  In a series circuit (one with a single loop of wire) the current is the same at any point of the loop.
• •The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component.
• •The greater the resistance of the component, the smaller the current for a given potential difference (V) across the component.
• The resistance in a circuit will depend on the components used in the circuit as well as the length of wire used in the circuit. 
• The longer the wire, the greater the resistance.
• Some resistors have a fixed value that does not depend on the current flowing through the circuit.  These are ohmic conductors.
• Ohm’s Law states “the current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor”.
• Ohmic conductors will produce a straight line I – V graph that goes through the origin.
• The resistance of components such as lamps, diodes, thermistors and LDRs is not constant. It changes with the current through the component.
• As the current increases, the temperature of filament increases therefore the resistance of the filament lamp increases
• Thermistors are types of resistors where the resistance varies with temperature.

  The resistance of a thermistor decreases as temperature increases.
• Thermistors are used in thermostats to control temperature in the home.
• Light Dependent Resistors - LDRs are types of resistors where the resistance varies with light intensity.

  The resistance of a LDR decreases as light intensity increases.
• LDRs are used as  switches to turn on street lights when it gets dark.
• Measuring Resistance

  To measure the resistance of an electrical component the following circuit needs to be set up.

  
• By measuring the current, using the ammeter, and the potential difference, using a voltmeter, the resistance can be found from

  R=V/I 
• To get a range of potential differences and currents a variable resistor can be added into the circuit or the input potential difference changed.