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To investigate the relationship between the power consumed by a torch bulb and the resistance, by measuring the potential difference across the bulb and its current.

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Name: Fawwaaz Hosein

Group Members: Eric Mui and Idris Khan

Experiment 2: Electrical Power

Date: 26th February 2010

Aim: To investigate the relationship between the power consumed by a torch bulb and the resistance, by measuring the potential difference across the bulb and its current.


  • Four (1.5V) batteries
  • Two multi-meters
  • Connecting wires
  • Rheostat
  • Switch
  • Torch bulb


  • The equipment is set up as shown in the diagram (1.0) below.
  • One of multi-meters is used as an ammeter and is connected in series so as to measure the current (I) of the circuit and the other is used as a voltmeter connected in parallel with the bulb, to acquire the potential difference (V) across the bulb in question. The positive terminals of the multi-meters are connected so that they are facing the positive terminal of the batteries. The voltmeter is set to read volts and ammeter to read micro-amps.
  • The variable resistor (Rheostat) is adjusted until the current (I) observed on the ammeter is at its minimum value.
  • Readings are taken from each multi-meter. The ammeter gives the current (I) whilst the voltmeter gives the p.d across the bulb in question.
  • The variable resistor is adjusted so that the current (I) increases while the resistance (R) decreases. The readings of twelve (12) other points of equally spaced intervals are then taken so as to have thirteen (13) points in total. The values of V and I from the respective multi-meters are recorded and tabulated  
  • The relationship between the Power (P) and resistance(R), is determined using the equation stated below:



                             Where P = IV, R = image02.png and k and n are constants.

  • In order to obtain a straight line graph to graphically represent the relationship between P and R, the following equation is derived:
...read more.





























P% = V% + I% = ± 4%

R% = V% + I% = ± 4%

P (W)

P (W)

R (Ω)

R (Ω)

























...read more.



From the experiment we concluded the following:

  • P=1.04 x 10-7  × R3.3559
  • n = 3.3559  
  • k = 1.04 x 10-7  WΩ−3.3559

To reduce the uncertainties of power and resistance, the uncertainties of voltage and current must first be reduces as they are linked. To achieve this V should have been measured more than twice at each current level. The number of current and voltage values should also be increased so as to have more values to plot on the graph. The power law in particular the formula used applies to this experiment, as lnP increases, lnR increases linearly; P is increased by the product of k and R to the power of n. This is also shown by the strong positive correlation of the graph that further shows that the power law is applied as when lnR increases, lnP increases proportionally or linearly.

...read more.

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