Circuits and Resistance

• What do V-I graphs represent in electrical components?

  Relationship between potential difference and current
• What does a straight line through the origin on a V-I graph indicate?

  Voltage and current are directly proportional
• What does a steep gradient on a V-I graph indicate about resistance?

  Indicates low resistance
• What does a shallow gradient on a V-I graph indicate?

  Indicates high resistance
• How does the resistance value change for some resistors as current changes?

  Resistance value is not constant
• What type of graph results when the resistance value changes with current?

  Non-linear graph
• Key Point

  A V-I graph shows the relationship between voltage and current. It therefore can be used to determine the resistance.
• RQP Sample Method - Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature

  1. Set up the standard test circuit as shown.

  2. Use the variable resistor to adjust the potential difference across the test component.

  3. Measure the voltage and current for a range of voltage values.

  4. Repeat the experiment at least three times to be able to calculate a mean.

  5. Repeat for the other components to be tested.
• RQP Considerations, Mistakes and Errors - Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature

  . Before taking measurements, check the voltage and current with the supply turned off. This will allow zero errors to be identified.

  . A common error is simply reading the supply voltage as the voltage across the component. At low component resistance, the wires will take a sizeable share of this voltage, resulting in a lower voltage across the component. This is why a voltmeter is used to measure the voltage across the component.
• RQP Variables - Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature

  . The independent variable is the potential difference across the component (set by the variable resistor.)

  . The dependent variable is the current through the component, measured by the ammeter.
• RQP Hazards and Risks - Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature

  . The main risk is that the filament lamp will get hotter as the current increases and could cause burns. If it overheats, the bulb will 'blow' and must be allowed to cool down before attempting to unscrew and replace it.
• Resistors

  . An ohmic conductor is a resistor in which the current is directly proportional to the potential difference at a constant temperature.

  . This means that the resistance remains constant as the current changes.

  . It is indicated by a linear (straight line) graph.
• Filament Lamps

  . As the current through a filament lamp increases, its temperature increases.

  . This causes the resistance to increase as the current increases.

  . It is indicated by a curved graph.
• Diodes

  . The current through a diode will only flow in one direction.

  . The diode has a very high resistance in the reverse direction.

  . This is indicated by a horizontal line along the x-axis, which shows that no current flows.
• Thermistors

  . The resistance of a thermistor decreases as the temperature increases.

  . This makes them useful in circuits where temperature control or response is required.

  . For example, a thermistor could be used in a circuit for a thermostat that turns a heater off at a particular temperature or an indicator light that turns on when a system is overheating.
• Light Dependent Resistors (LDRs)

  . The resistance of an LDR decreases as light intensity increases.

  . This makes them useful where automatic light control or detection is needed, e.g. in dusk till dawn garden lights/street lights and in cameras/phones to determine if a flash is needed.