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relationship between voltage and current

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Introduction

Physics Coursework

Aim:

  • To find out the relationship between voltage and current in a circuit across a filament lamp.
  • To investigate the connection between temperature and alternating voltage also resistance and temperature.
  • To find out whether a filament bulb acts as a black body.

Ohm’s Law

The relationship between current, voltage, and resistance is given by Ohm’s law. This law states that the amount of current passing through a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor at a given constant temperature. Ohm’s law can be expressed as an equation, V = IR, where V is the difference in volts between two locations (called the potential difference), I is the amount of current in amperes that is flowing between these two points, and R is the resistance in ohms of the conductor between the two locations of interest. V = IR can also be written R = V/I and I = V/R. If any two of the quantities are known, the third can be calculated. For example, if a potential difference of 110 volts sends a 10-amp current through a conductor, then the resistance of the conductor is R = V/I = 110/10 = 11 ohms. If V = 110 and R = 11, then I = V/R = 110/11 = 10 amp.

The unit of resistance is the ohm, which is equal to one volt per ampere, or one volt second per coulomb.

Under normal conditions, resistance is constant in conductors made of metal. If the voltage is raised to 220 in the example above, then R is still 11. The current I

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Middle

image18.png

 P = Power radiated  in W (J/s) 
image17.png = Stefan's Constant 5.67 x 10-8 W m-2 K-4 
 A = Surface area of body (m²)
 T = Temperature of body (K)

That is, the power per unit area is directly proportional to the fourth power of the thermodynamic temperature.

The value of the Stefan-Boltzmann constant is approximately 5.67 x 10-8 watt per meter squared per Kelvin to the fourth (W · m-2 · K-4).

Prediction

I predict that since the temperature of the filament lamp will prove to be some what difficult to keep constant, the resistance of the filament lamp will increase as temperature increases, therefore I will not be able to prove ohms law. Also as the voltage is increased I believe the current is increased although I do not think they are proportional to one another. Additionally I believe that the filament lamp works as a black body.

Circuit

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Safety

To ensure that all students come to no harm and injuries, the basic safety procedures should be considered, such as tucking in all chairs, not in eating or drinking in class and being careful when handling equipment. The need for dry hand is somewhat important to reduce the risk of electrocution but should not touch circuit when DC supply is on, because after all we have ions in us which conducts electricity, although most material we will use is protected by plastic covering. Additionally the filament lamp will get extremely hot, and could cause burns. Large current as well as burning can also spoil sensitive equipment.

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Conclusion

I could not really get vast amounts of data, because of the limited voltage that the power supply went to (14V) and also due to the limited time we had which kind of pressurised me which may have caused some errors but I accept full responsibility.

Improvements I would make to the experiment:

  • I would use a power supply that would exceed 14 Volts.
  • I would either have a couple of filament lamps so, after every experiment I could change it so it cools down and stays at the constant of room temperature, this would mean may get even closer to the straight line graph that we expect, and prove ohms law, or could use a optical thermometer to accurately have each filament lamp at a certain temp, although we are talking about an expensive equipment and highly advanced for us.
  • I could either use an exceptional voltmeter and ammeter to reduce fluctuations or an ohmic meter to see resistance and see the contrast in results we get, which would really determine if my formula is as accurate as it seems to be.
  • I could use the equation V=IR for non-ohmic devices, but it then ceases to represent Ohm's Law. In non-ohmic cases, R depends on V and is no longer a constant of proportionality but a variable called differential resistance. To check whether a given device is ohmic or not, one plots V versus I and checks that the curve is a straight line

My success was quiet high due the fact that my percentage error in the end was only 2.39% which was determined using my power law.

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