Gavin Howe
Ohm's law states that the voltage across a resistor is directly proportional to the current flowing through the resistance. It was first verified experimentally by German physicist Georg Ohm. Ohm's law can be easily verified experimentally by creating a circuit with a nichrome wire, an ammeter, a voltmeter, and three cells. The key is closed to allow the current to flow, and the rheostat is adjusted to get the minimum reading on the ammeter and voltmeter. The current in the circuit is then gradually increased, and the corresponding values of voltage and current are recorded. This process is repeated, and for each set of values, the ratio of voltage to current is calculated. Plotting these values on a graph produces a straight line, indicating that the current is proportional to the voltage, thus verifying Ohm's law.
| Characteristics | Values |
|---|---|
| Ohm's Law | The law that for any circuit, the electric current is directly proportional to the voltage and is inversely proportional to the resistance |
| Relationship | V = IR, where V = potential difference, I = current, and R = constant of proportionality (resistance) |
| Verification Experiment | Using a nichrome wire, an ammeter, a voltmeter, and three cells, create a circuit as shown in the diagram |
| Procedure | 1. Close the key to start the current flow in the circuit. 2. Note the readings for potential difference (V) and current (I). 3. Increase the current in the circuit gradually. 4. Repeat steps 2 and 3 multiple times. 5. Plot a graph of V against I, which should result in a straight line, verifying Ohm's Law. |
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What You'll Learn
Ohm's Law and the relationship between voltage and current
Ohm's Law, named after German physicist Georg Ohm, establishes the relationship between voltage and current in a circuit. The law states that the voltage across a resistor or conductor is directly proportional to the current flowing through it, provided that temperature and other physical factors are kept constant.
Mathematically, this relationship can be expressed as:
V = IR
Where:
- V = Voltage (potential difference)
- I = Current
- R = Resistance (the constant of proportionality)
Ohm's Law can be experimentally verified through the following procedure:
- Set up a circuit with a nichrome wire, an ammeter, a voltmeter, and one or more cells as the voltage source.
- Close the key to allow current to flow in the circuit.
- Note the initial readings of the ammeter (current, I) and voltmeter (voltage, V) for the given circuit configuration.
- Gradually increase the current in the circuit by adjusting the rheostat or using additional cells.
- For each current value, record the corresponding voltage across the wire.
- Calculate the ratio of V/I for each set of values. You should observe that this ratio remains nearly constant, indicating that V/I = R.
- Plot a graph with voltage on the y-axis and current on the x-axis. The resulting graph should be a straight line passing through the origin, confirming the direct proportionality between voltage and current.
It is important to note that Ohm's Law does not hold true for all materials or under all conditions. It applies primarily to metallic conductors when physical conditions, such as temperature, remain constant. Non-ohmic conductors like semiconductors, junction diodes, and thermistors do not obey Ohm's Law. Additionally, components that exhibit a temperature increase with an increase in current, such as lightbulb filaments, violate Ohm's Law.
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The effect of temperature on resistance
Ohm's Law, a fundamental principle in electrical circuits, describes the relationship between voltage, current, and resistance. It was first experimentally verified by German physicist Georg Simon Ohm.
To experimentally verify Ohm's Law, a specific procedure is followed. This procedure involves creating a circuit with a nichrome wire, an ammeter, a voltmeter, and three cells of 1V each. Initially, only one cell is used as the source in the circuit, and readings of the ammeter and voltmeter are noted. The current in the circuit is then gradually increased, and corresponding values of potential difference and current are recorded. This process yields different sets of voltage and current values, and calculating the ratio of voltage to current for each set reveals that this ratio remains nearly constant. Plotting a graph of current against potential difference further illustrates a straight line, confirming that the current is directly proportional to the potential difference, thus verifying Ohm's Law.
Now, onto the effect of temperature on resistance. It is essential to recognize that resistance is influenced by factors such as the geometry of a conductor and the materials used in its construction. However, temperature also plays a significant role. As the temperature rises, the atoms and molecules within the conductor exhibit increased vibrations and movement, impeding the flow of electrons more effectively. Consequently, resistance tends to increase with higher temperatures. This relationship between resistance and temperature is described by the temperature coefficient of resistance, which quantifies the fractional increase in resistivity per unit rise in temperature.
However, it is worth noting that this relationship is not universally consistent. Some materials, like silicon, exhibit a negative temperature coefficient of resistivity, resulting in decreasing resistance with increasing temperature. Additionally, certain metallic alloys, such as nichrome, manganin, and constantan, possess relatively large resistivities and low temperature coefficients. These materials are utilized in devices like the platinum resistance thermometer, which operates effectively under conditions where other thermometers may not be suitable.
The impact of temperature on resistance can also be observed in semiconductors. As temperature rises, more electrons are freed from their valence duties, resulting in an increase in the number of conduction electrons. Consequently, the conductivity of the semiconductor improves with higher temperatures.
Furthermore, it is possible to design a resistor that exhibits minimal dependence on temperature. This is achieved by connecting two resistors in series, one with a positive temperature coefficient and the other with a negative temperature coefficient. By carefully selecting the resistance values, the increase in resistance in one resistor due to temperature change can be counterbalanced by a corresponding decrease in resistance in the other resistor.
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Ohm's Law and non-ohmic conductors
Ohm's Law, named after German physicist Georg Ohm, states that the voltage across a resistor is directly proportional to the current flowing through the resistance. In other words, there is a linear relationship between voltage and current. Ohm's Law can be verified experimentally by creating a circuit with a nichrome wire, an ammeter, a voltmeter, and three cells of 1V each. By noting the readings of the ammeter and voltmeter, one can observe that the current is proportional to the potential difference, thus verifying Ohm's Law.
Conductors and electronic components that follow Ohm's Law are called Ohmic conductors, while those that do not are called non-Ohmic conductors. Ohmic conductors exhibit a linear relationship between voltage and current, meaning that if the voltage across the conductor is doubled, the current will also double. Examples of Ohmic conductors include items made from metals, carbon, and many metal alloys.
Non-Ohmic conductors, on the other hand, do not follow Ohm's Law and exhibit a non-linear relationship between voltage and current. This means that doubling the voltage will not result in a doubling of the current. The relationship between voltage and current in non-Ohmic conductors can be represented by a curve on a graph rather than a straight line. Examples of non-Ohmic conductors include incandescent light bulbs, semiconductors, filament lamps, and thermistors.
The behaviour of non-Ohmic conductors can be influenced by factors such as the type of material, temperature, and the nature of the electric potential applied. For example, in filament lamps, as the current flowing through the lamp's filament increases, its temperature rises, leading to an increase in resistance and a non-linear relationship between voltage and current.
In summary, Ohm's Law can be verified experimentally by observing the linear relationship between voltage and current in Ohmic conductors. Non-Ohmic conductors deviate from Ohm's Law and exhibit a non-linear relationship between voltage and current due to various factors influencing their behaviour.
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Using a rheostat to increase current
Ohm's Law states that the electric current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor, provided the temperature and other physical conditions of the conductor remain the same.
To verify Ohm's law experimentally, you will need a conductor of resistance R, an ammeter, a voltmeter, a battery, a variable resistance (or rheostat used to change the current in the circuit), connecting wires, a key, and sandpaper.
Step 1: Set Up the Circuit
Connect the components as shown in the diagram provided in the source. Ensure that all connections are tight to avoid any issues with current flow.
Step 2: Start the Current Flow
Close the key to allow current to begin flowing in the circuit. This is a crucial step as it initiates the experiment and ensures that there is a current present in the circuit.
Step 3: Measure Initial Values
Note down the potential difference (V) across the conductor of resistance R using the voltmeter, and simultaneously record the corresponding current (I) shown on the ammeter. These initial values serve as a baseline for comparison in the next steps.
Step 4: Increase the Current Using the Rheostat
Now, move the knob on the rheostat to increase the current flowing through the circuit. The rheostat is a variable resistor that allows you to adjust the current by changing the resistance in the circuit. By turning the knob, you can introduce more or less resistance, thereby controlling the current according to Ohm's law, which states that current and voltage are directly proportional.
Step 5: Measure Values at Increased Current
After increasing the current, again note down the potential difference (V) across the conductor of resistance R using the voltmeter, and record the new corresponding current (I) shown on the ammeter. These values will be different from the initial readings due to the change in current.
Step 6: Repeat and Plot a Graph
Repeat steps 4 and 5 at least four more times, each time increasing the current by moving the knob of the rheostat. This will give you multiple data points for potential difference (V) and current (I).
Finally, plot a graph with potential difference (V) on the x-axis and current (I) on the y-axis. According to Ohm's law, if the law holds true, you should obtain a straight line passing through the origin. This graphical representation visually demonstrates the direct proportionality between voltage and current, thus verifying Ohm's law experimentally.
It is important to take certain precautions while conducting this experiment, such as ensuring that the current does not pass through the circuit continuously for an extended period to prevent overheating the conductor.
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The V-I graph and its implications
Ohm's law states that the voltage across a resistor or conductor is directly proportional to the current flowing through it, provided all physical conditions and temperatures remain constant.
To experimentally verify Ohm's law, a circuit is set up with a nichrome wire, an ammeter, a voltmeter, and cells of 1V each. The key is closed to allow current to flow, and the rheostat is adjusted to obtain the minimum reading on the ammeter and voltmeter. The rheostat's sliding terminal is then moved to gradually increase the current in the circuit, and corresponding values of voltage and current are recorded. This process is repeated multiple times, resulting in different sets of voltage and current values.
For each set of values, the ratio of voltage to current (V/I) is calculated. It is observed that this ratio remains almost the same for each set of values. Plotting these values on a graph of voltage against current (a V-I graph) results in a straight line passing through the origin. This indicates that the current is directly proportional to the voltage, confirming Ohm's law. The slope of the V-I graph represents the resistance of the circuit.
However, it is important to note that Ohm's law does not hold true under all conditions. It is applicable only to metallic conductors when physical conditions such as temperature remain constant. Non-ohmic conductors like semiconductors, junction diodes, and thermistors do not obey Ohm's law. Additionally, in certain components, increasing the current raises the temperature, which violates Ohm's law, such as in the filament of a light bulb.
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