Ohm's law: V–I graph for a resistor ओम का नियम: प्रतिरोधक का V–I ग्राफ़
Aim: To show that the current through a resistor is directly proportional to the potential difference across it, by plotting a V–I graph, and to find the resistance from the slope of the graph.
Apparatus उपकरण
- Battery of four 1.5 V cells (6 V)
- Resistor (unknown value)
- Ammeter (0–1 A, least count 0.02 A)
- Voltmeter (0–5 V, least count 0.1 V)
- Rheostat
- Plug key
- Connecting wires and sand paper
Theory सिद्धांत
Ohm's law says that, at constant temperature, the current I through a metallic conductor is directly proportional to the potential difference V across its ends: V ∝ I, or V = IR. The constant R is the resistance, measured in ohm (Ω). So a graph of V (y-axis) against I (x-axis) is a straight line through the origin and its slope ΔV/ΔI equals R. The ammeter is joined in series with the resistor and the voltmeter in parallel across it; the rheostat changes the current.
Procedure विधि
- Clean the ends of the wires with sand paper and connect the circuit: battery, plug key, ammeter, resistor and rheostat in series; voltmeter across the resistor.
- Check that the + terminals of the ammeter and voltmeter are joined towards the + terminal of the battery.
- Note the least count and zero error of both meters.
- Insert the key and set the rheostat so a small current flows. Read the ammeter and voltmeter.
- Move the rheostat slider to increase the current in steps and take at least five readings.
- Remove the key between readings so the resistor does not heat up.
- Plot V on the y-axis against I on the x-axis and find the slope of the straight line.
Simulated experiment अनुकरण प्रयोग
Your resistor is marked 4.7 Ω ± 5%. Close the key, set the rheostat, read both meters and record the reading. Every student gets slightly different values.
Observation table प्रेक्षण तालिका
Type what each meter shows. A ✓ means your reading matches the meter (within half a least count). Take at least four readings.
| S. No. | Setting | V (volt) | I (ampere) | R = V/I (Ω) | Remove |
|---|---|---|---|---|---|
| Insert the key and record a reading. | |||||
V–I graph
Enter at least two readings to draw the graph.
Precautions सावधानियाँ
- Use clean, tight connections; loose contacts change the current.
- Connect the ammeter in series and the voltmeter in parallel, with correct polarity.
- Pass current only while taking a reading, so the resistor does not heat up and change its resistance.
- Read the pointers looking straight down at the scale to avoid parallax.
- Choose meters whose range is larger than the largest expected reading.
Sources of error त्रुटि के स्रोत
- Heating of the resistor increases its resistance slightly.
- Zero error or parallax while reading the meters.
- The voltmeter draws a tiny current, so the ammeter reads a little more than the current in the resistor.
- Loose connections or a weak battery cause the readings to drift.
Viva voce questions मौखिक प्रश्न
Q1. State Ohm's law.
At constant temperature, the current through a conductor is directly proportional to the potential difference across its ends (V = IR).
Q2. What is the shape of the V–I graph for a resistor and what does its slope give?
A straight line through the origin. The slope V/I is the resistance R.
Q3. Why is the ammeter connected in series and the voltmeter in parallel?
An ammeter has very low resistance and must carry the whole current; a voltmeter has very high resistance and measures the potential difference between two points without drawing much current.
Q4. What is the SI unit of resistance? Define it.
Ohm (Ω). A conductor has a resistance of 1 Ω if a current of 1 A flows through it when the potential difference across it is 1 V.
Q5. What is the purpose of a rheostat?
It is a variable resistance used to change the current in the circuit without changing the battery.