Investigate the current - voltage relationship for a resistor and filament lamp. To determine whether each obeys ohms law.

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Task 1


To investigate the current – voltage relationship for a resistor and filament lamp.  To determine whether each obeys ohms law.

        Ohms law states that the current through a conductor is proportional to the potential difference across it provided that there is no change in temperature.  It is given by the formula V = IR


  • The circuit is set up as above, the power supply needs to be set at about 4V.  The first experiment will be carried out with the filament lamp.
  • Electrons in a circuit move from the negative terminal to the positive terminal, during which energy can be lost through components causing a difference in the electrical potential energy between the negative and positive terminals.  This is called the potential difference and is measured with a voltmeter.  The voltmeter is connected in parallel with the component, to measure the voltage going through it.
  • The ammeter measures the electrical current and is measured in amperes, or amps.  When the current is 1 amp, the flow of charge is 1 coulomb per second.

Care should be taken to make sure the voltmeter and ammeter are working correctly.

  • The variable resistor is moved along slightly and readings from the ammeter and voltmeter are recorded in an appropriate table.  Readings are continued to be recorded at intervals along the variable resistor until it reaches the end or till sufficient results are taken.  
  • The current is then directed backwards, for negative results to be taken. This is done to see if there is any change in the results, as sometimes it can make a difference. It will help prove if a component is an ohmic conductor.
  • Now the fixed resistor is put in place of the filament lamp.  
  • Again, readings are taken from the voltmeter and ammeter as the variable resistor is moved along.
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  • Readings are recorded in another table.  When the variable resistor has reached the end or there are sufficient results, the current can be directed backwards again for negative readings.

The results should then be presented graphically, as the outcome of the line will show the relationship between the current and voltage.  A straight line will show the

component does obey ohms law as the current is constant and proportional to the p.d. A  gradient can be calculated to determine the resistance.

A curved graph will show the alternative, that it is not an ohmic conductor.  The p.d and ...

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