Ohm’s Law Steve MacBride, Bunny Neil, Victor Roche, Nadmi Mustapha Physics 2 Lab Dr. Roman Kezerashvili
Theory THE ELECTRIC CURRENT (I) is defined as the amount of electric charge (q) that flows through a cross section of wire per unit of time (t). We represent this as : I = q/t
Voltage is Proportional to the Current => I ∝ V By adding a constant of proportionality, 1/R, the expression becomes: I = V/R Where R is the Resistance of the Conductor…
I = V/R THECURRENT THROUGH A RESISTOR IS DIRECTLY PROPORTIONAL TO THE APPLIED VOLTAGE V AND INVERSELY PROPORTIONAL TO THE RESISTANCE R OF THE RESISTOR This expression is known as Ohm’s Law
The relation : I=V/R Is known as Ohm’s law and is named after the scientist Georg Simon Ohm who discovered it in 1827.
Procedure We set up an electric current in a wire with a Resistor… A V R Ammeter ε Voltmeter Variable Power Supply The circuit for measuring the current dependence on the voltage and resistance
Verification of Ohm’s Law We will investigate the variation of the current with potential difference when the resistance is constant. First, we vary the output voltage in 1V increments, Then we raise the Resistance to 1200 Ohms and repeat. A V R
We plotted graphs of Current (I) Versus Voltage (V)… The Inverse of the slope should have been equal to the Resistance… Our percent error was 0% for 1000 Ohm Resistance Our percent error was 7.7% for 1200 Ohm Resistance FOR MORE INFO... List location or contact for competitive analysis (or other related documents) here
We then varied the current and kept the voltage constant… We set the output voltage to 12 Volts We varied the Resistance from 700 Ohms to 1200 Ohms, in increments of 100 Ohms.
We graphed the results We compared the Current I to the inverse of the Resistance. The slope should theoretically have been equal to the applied Voltage Our slope was 11.655 The Voltage was 12 Volts Our results showed Our results showed a 2.9% difference
PART II Combinations of Resistors in Series and in Parallel
Finding Equivalent Resistance Resistors in Series V A ε RARA RBRB RCRC Currents through all resistors in series are the same. The Potentials and, thus, the Resistances add: I = I A = I B = I C, V = V A + V B + V C R = R A + R B + R C
3 Resistors set up in Parallel… In Parallel, the Potentials are constant and the Charges add. Therefore… The equivalent resistance is equal to the inverse of the sum of the inverses of the individual resistances Resistors in Parallel RARARARA RBRB RCRC A V ε 1/R = 1/R A + 1/R B + 1/R C
Resistors in Series We set the Output Voltage to 12 Volts… We set the ammeter to the 200 milliAmp range… We measured the current through each resistor... Adding all resistances together, our experimental value was 4920 Ohms The actual value was 4930 Ohms, a.2 % difference
We then measured the Voltages We added the voltages that we measured at each resistor… Our total was 11.6 Volts, for a percent difference of 3.38%
Resistors in Parallel We set the output voltage to 6 volts We set the ammeter to a range of 200 milliAmps We measured the current and resistance across each resistor
Our findings The actual charge was.065 Amps. Our experimental value was.065 Amps. Our percent difference was 0%. The actual resistance was 180Ω Our experimental value was 186.47Ω Our Percent difference was 3.53%
Conclusions Our resistance followed Ohm’s law… In each of 4 sets of trials, resistance was equal to the voltage divided by the current. The total current measured in the parallel circuit was nearly equal to the sum of the individual currents.
Conclusions, continued In our series circuit, the total Voltage input (12 Volts) was nearly identical to the sum of the Voltage measured at each individual resistor. In the series circuit, the current remained the same at each individual resistor as it was on the total series network.
Conclusions, Continued In our parallel circuit, the voltage at each individual resistor was exactly the same as the input voltage (12.5 volts) The sum of currents measured at each individual resistor was nearly equal to the total current (.065 Amp) input.