Jezmeen Seetha Muralitharan 11S
PHYSICS LAB REPORT OHM’S LAW Aim: To investigate the relationship between that exists between the current passing through different lengths of wire and the potential difference across the wires. Variables: Independent variable: Resistance of wire Dependent variable: Current, I Potential difference, V Constant variable: Temperature of metal wire Hypothesis: When the current that passes through a metal wire increases, the potential difference across the metal wire increases. As well as the value of resistance changes value of other variables in the circuit. According to Ohm’s law, voltage is directly proportional to the current (U=IR). Altering terminal voltage changes voltage and current in circuit. Data can be obtained by measuring value of constant resistance of unknown resistor via calculation (V=IR). Final value can be obtained from graph, where R is slope of the graph of linear trend-line which is also the gradient of the graph.
Materials: Power supply (0-12 V), ammeter (0-5 A), voltmeter (0-10 V), variable resistor, connecting wires with crocodile clips, on-off switch, 10.0 cm of nichrome wire and 10.0 cm of constantan wire Diagram:
Methods: 1. The battery, ammeter, voltmeter and rheostat are connected in a circuit as shown above. The 10 cm constantan wire is connected across terminal of two points. 2. The circuit is switched on and the rheostat is adjusted until the ammeter gives a reading of I= 0.05A. The potential difference, V across two points is recorded from the voltmeter. 3. Step 2 is repeated by adjusting the rheostat until ammeter reading I= 0.06, 0.07, 0.08 and 0.09A.The corresponding potential differences are also recorded. 4. Step 2 and 3 is repeated by replacing the constantan wire with nichrome wire. 5. All the values are recorded in a table. 6. A graph of V versus I is plotted. Data Collection: Resistance = Voltage (V)/Current(I) or R = V/I Current, I/ A 0.05
Potential Differences, V/V 5.95
Resistance, Ω 119
0.06 0.07 0.08 0.09
6.10 6.40 6.45 6.50
101.67 91.43 80.63 72.22
Data Processing and Presentation: Sample Calculation R= V/I When current is 0.05, voltage is 5.95, R= 5.95V/0.05A =119Ω
Conclusion and Evaluation: Electricity is conducted through a conductor, which is indicated in this investigation the wire, via of free electrons. The number of free electrons depends on the material and more free electrons means a better conductor, due to it has less resistance. Such as copper has more free electrons than iron resulting in it becoming a better conductor. The free electron are high in energy which moving and colliding with other free electrons that are adjacent to it. This occurs across the length of a wire and the electricity is conducted. Resistance is due to the energy lost as heat. It involves collisions between free electrons and the fixed particles of the metal. These collisions convert part of the energy which the free electrons are conducting into heat. The resistance of a length of wire is
calculated by measuring the current present in the circuit (series) and the voltage across the wire (in parallel). These measurements are formulated into V / I = R . (Where V= Voltage, I= Current, and R= Resistance). Ohm's Law states that the current through a metallic conductor at a constant temperature is proportional to the potential difference (voltage). Therefore V / I is constant. This is defined as that the resistance of a metallic conductor is constant providing that the temperature also remains constant. If the resistance of a metal increases then its temperature increases. This is because at higher temperatures, the particles of the conductor are moving around swiftly and increasing the number of collisions of free electrons. Limitations and weaknesses in this investigation were: - The methods are not clearly justified as I could not understand the main purpose of the experiment. The handout on the experiment stated 10m of constantan wire and 10m nichrome wire which is obviously is not 10m. - The usage of wires are not as suggested in handout. I have no idea whether it is constantan or nichrome wire. - The ammeter and voltmeter are not digital, which might comprises the results due to inaccuracy. - Resistance of a wires as well as internal resistance of a source (which actually caused inaccuracy of the final result) were neglected . - Human error in reading the data from ammeter and voltmeter . Modifications to improve this investigation: - A proper briefing and step-by-step instruction will facilitate the students better. - In future, we should use two resistors, from which only one will be unknown . - Use digital ammeter and voltmeter with greater preciseness - Measure internal resistance of source and estimate value of resistance of wires - Use various levels of schemes – resistance of resistor is still the same – and obtain different and comparable results.