Internal Resistance of a Dry Cell

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Physics Experiment – Internal Resistance of a Dry Cell May 16, 2009 Written by Daniel Chiuchiarelli 1. Contents Contents Background Aim Equipment Method Results Discussion Conclusion 2. Background Ohm’s law, a law that applies to electrical circuits, states that if the temperature or resistance of a device is constant, and an electric current is sent through it, then the current flowing through will be directly proportional to the electric potential across it. In mathematical terms, I ∝ V. From this relation, the formula V=IR was derived, where V is the electric potential, I is the current, and R is the resistance. Every component of an electrical circuit offers some amount of resistance when an electrical charge passes through it. Components such as light bulbs, resistors and motors all restrict the flow of current, and components such as wires and batteries, which are considered to have a theoretical resistance of 0, in reality, have a very small level of resistance. By calculating the potential different and current across a component, we are able to calculate the resistance due to Ohm’s law. 3. Aim To measure the internal resistance of a fresh and a flat dry-cell battery. 4. Equipment Power pack Voltmeter Ammeter Switch Connecting wires Rheostat Dry-cell batteries 5. Method An electrical circuit was set up with one battery as the power source, including an ammeter, switch and variable resistor (rheostat) in series. A voltmeter was set up in parallel with the battery. The variable resistor was adjusted until the ammeter gave a reading of approximately 1.5 A. The switch was closed, and the values on the ammeter and voltmeter were recorded as fast as possible, in order to avoid the resistor heating up. The variable resistor was adjusted through 4 or 5 different resistors, and 4 or 5

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