# Circuits and Resistance

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## Resistors and Other Components

• Potential difference-current graphs (V-I graphs) are used to show the relationship between the potential difference (voltage) and current for any component.
• A straight line through the orign indicates that the voltage and current are directly proportional.
• A steep gradient indicates low resistance, as a large current will flow for a low potential difference.
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## Resistors and Other Components

• A shallow gradient indicates high resistance, as a large potential difference is needed to produce a small current.
• For some resistors the value of R is not constant but changes as the current changes, this results in a non linear graph.
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## Required Practical

Sample Method:

• Set up the standard test circuit.
• Use a variable resistor to adjust the potential difference across the test component.
• Measure the voltage and the current for a range of voltage values.
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## Required Practical

Sample Method:

• Repeat the experiment at least three times to be able to calculate a mean.
• Repeat for the other components to be tested.
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## Required Practical

Variables:

• The independent variable is the potential difference across the component (set by the variable resistor).
• The dependent variable is the current through the component, measuredby the ammeter.
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## Required Practical

Considerations, Mistakes and Errors:

• Before taking measurements, check the voltage and the current with the supply turnd off. This will allow zero errors to be identified.
• A common error is simply reading the supply voltsge as the voltage across the component. At low component resistances, the wires will take sizeable share of this voltage, resulting in a lower voltage across the component.
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## Required Practical

Hazards and Risks:

• 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.
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## 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
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## 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.
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## Diodes

• The current through a diode will only flow in one direction.
• The diode has a very high resistance in the reverse dirction.
• This is indicated by a horizontal line along the x-axis, which shows that no current flows.
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## Thermistors

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