Physics Laboratory Experiments: Ohm’s Law, Diodes, and Magnetism
Experiments in Electricity and Magnetism
Experiment 1: Verifying Ohm’s Law and Determining Resistance
To observe the variation of potential difference (V) across a given resistance with varying current (I), draw the I-V curve, and find the value of the given resistance from the graph.
Apparatus and Materials
- Battery
- Ammeter
- Voltmeter
- Resistance (unknown)
- Rheostat
- Key
- Connecting wires
- Sandpaper
Theory
Ohm’s law states that, provided the temperature remains constant, the current (I) flowing through a conductor is directly proportional to the voltage (V) across it.
Thus, $$I \propto V$$ or $$V = IR$$
Where $R$ is the resistance of the conductor. If $I$ is plotted along the X-axis and $V$ along the Y-axis, the graph should be a straight line passing through the origin. The slope of the graph measures the resistance ($R$) of the conductor.
Procedure
- Draw the circuit diagram as shown in Figure (assumed to be present).
- Note the least count and zero errors of the ammeter and the voltmeter.
Experimental Data
1) Least count of:
- Ammeter scale: $0.5 / 10 = 0.05\text{ A}$
- Voltmeter scale: $0.05\text{ V}$
2) Zero error of:
- Ammeter: $0\text{ A}$
- Voltmeter: [Value missing]
| No. of Obs. | Ammeter Reading (Amp) | Voltmeter Reading (Volt) | Value of R from Graph |
|---|---|---|---|
| 1 | $1 \times 0.05 = 0.05$ | $5 \times 0.05 = 0.25$ | |
| 2 | $2 \times 0.05 = 0.10$ | $14 \times 0.05 = 0.70$ | $5.33$ |
| 3 | $3 \times 0.05 = 0.15$ | $16 \times 0.05 = 0.80$ | |
| 4 | $4 \times 0.05 = 0.20$ | $20 \times 0.05 = 1.00$ |
3) Table for variation of potential drop with current:
Plotting of Graph
A graph is plotted by taking 10 divisions along the X-axis $= 0.05\text{ Amp}$ and divisions along the Y-axis $= 0.10\text{ Volt}$. A straight line passing through the origin is obtained. A point P is taken on the graph. Its coordinates are $(0.15, 0.80)$.
Result
- The I-V curve is a straight line. This implies that the current ($I$) is directly proportional to the potential difference ($V$) across the conductor, thus establishing Ohm’s law.
- The resistance of the conductor: $R = \frac{V_{OQ}}{I_{OQ}} \Omega$
Precautions
- Connections should be tight.
- The ammeter should be connected in series, and the voltmeter in parallel in the circuit.
Sources of Error
- The resistance may change due to the passage of current through it (heating effect).
- The unknown resistance may be too low.
Experiment 2: Tracing Forward Bias Characteristics of a P-N Junction Diode
To draw the I-V characteristics curve of a p-n junction in forward bias.
Apparatus and Materials
- A p-n junction diode
- Two variable power supplies or batteries ($0-3\text{V}$, $0-15\text{V}$)
- Two voltmeters (ranges $0-3\text{V}$ and $0-15\text{V}$)
- A milliammeter ($0-50\text{mA}$)
- A microammeter ($0-500 \mu\text{A}$)
- One-way key
- Connecting wires
Theory
A p-n junction is said to be forward biased when its p-region is connected to the positive terminal and the n-region is connected to the negative terminal of the battery. The forward current increases slowly in the beginning and then rapidly.
Observation
Least count of:
- Voltmeter ($0-3\text{V}$): $0.05\text{V}$
- Milliammeter: $0.02\text{mA}$
Zero error of:
- Voltmeter: $0\text{V}$
- Milliammeter: $0\text{mA}$
Table for Forward Bias Characteristics
| No. of Obs. | Forward Bias Voltage $V_f (\text{V})$ | Forward Current $I_f (\text{mA})$ |
|---|---|---|
| 1 | $0.05 \times 0 = 0$ | $0.02 \times 0 = 0$ |
| 2 | $0.05 \times 2 = 0.10$ | “ |
| 3 | $0.05 \times 4 = 0.20$ | “ |
| 4 | $0.05 \times 6 = 0.30$ | $0.02 \times 2 = 0.04$ |
| 5 | $0.05 \times 8 = 0.40$ | $0.02 \times 6 = 0.12$ |
| 6 | $0.05 \times 10 = 0.50$ | $0.02 \times 10 = 0.20$ |
| 7 | $0.08 \times 12 = 0.96$ (Corrected from $0.60$) | $0.02 \times 14 = 0.28$ |
Calculation
(1) A graph between forward bias voltage $V_f$ and forward current $I_f$, taking $V_f$ along the X-axis and $I_f$ along the Y-axis, is plotted. This graph is called the forward bias characteristic of a p-n junction diode (refer to Figure 74, assumed present).
Precautions
- All connections should be neat, clean, and tight.
- Voltmeter and milliammeter of appropriate least counts and ranges should be selected.
- Zero error of the instruments should be accounted for.
Experiment 3: Determining Unknown Resistance Using a Metre Bridge
To find the value of a given resistance by using a metre bridge.
Apparatus and Materials
- A metre bridge
- A Leclanché cell
- A galvanometer
- A resistance box
- A jockey
- A one-way key
- Unknown resistance
- Connecting wire and sandpaper
Theory
If the unknown resistance $X$ is connected to the right gap of a metre bridge while a known resistance $R$ is connected to its left gap, the working formula is:
$$X = \frac{(100 – l)}{l} \times R$$Where $l$ is the balancing length of the metre bridge (in cm).
Observation
Table for Unknown Resistance
| No of Obs | Resistance from The Resistance Box $R (\Omega)$ | Balancing Length $AB = l (\text{cm})$ | Length $BC = (100 – l) (\text{cm})$ | Unknown Resistance $X = \frac{(100 – l)}{l} \times R$ | Mean $\Omega$ |
|---|---|---|---|---|---|
| 1 | 2 | 57.6 | 42.4 | 1.47 | |
| 2 | 3 | 64.0 | 36.0 | 1.68 (Corrected from 1.67) | $1.49 \Omega$ |
| 3 | 4 | 73.7 | 26.3 | 1.42 | |
| 4 | 5 | 77.8 | 22.2 | 1.42 |
Mean Resistance
$X_{\text{mean}} = \frac{(1.47 + 1.68 + 1.42 + 1.42)}{4} = 1.49 \Omega$
Precautions
- The connections should be tight. Ends of the wires and terminals of accessories should be rubbed with sandpaper.
- The plugs should be kept close only when the reading is being taken; otherwise, unnecessary heating will change the resistance.
Sources of Error
- The bridge wire may not have a uniform cross-sectional area throughout its length.
- End Correction: The metal strips have some resistance which is neglected. Moreover, the end points of the wire and those of the scale may not coincide. Due to these factors, some errors are always present at the two ends of the bridge; these are called end errors, and the corresponding corrections are called end corrections.
Experiment 4: Tracing Magnetic Field Lines (N-Pole North)
To trace the lines of force on one side due to a bar magnet, placed in the magnetic meridian with its north pole pointing north, and mark the position of the neutral point.
Apparatus and Materials
- Bar magnet
- Compass needle
- Sheet of white paper
- Drawing board
- Brass pins
- Sharp pencil
- Piece of chalk
Theory
A magnetic line of force is a curve in the magnetic field such that a tangent to it at any point gives the direction of the magnetic field at that point.
The direction of the axis of a freely suspended magnetic needle gives the direction of the resultant field. If the successive positions of the needle are found out from one end of a magnet to the other and a line is drawn, it will represent a line of force.
While tracing the lines of force in a magnetic field due to a magnet, we come across points where the field due to the magnet and the horizontal intensity of Earth’s field are neutralized by each other. Such points are called neutral points. A compass needle placed at these points tends to remain in any direction in which it is kept.
For the N-pole of a magnet pointing the geographical north, neutral points lie on the perpendicular bisector to its length.
Result
The magnetic lines of force on one side of a bar magnet with its north pole pointing towards the geographical north are shown in Figure 34 (assumed present). Two neutral points (one is shown) are located on the perpendicular bisector to the length of the magnet on its two sides.
Precautions
- There should be no magnetic material in the vicinity of the magnet. Drawing pins should be made of brass.
- While plotting the lines of force, the position of the drawing board or that of the magnet should in no case be disturbed.
Experiment 5: Tracing Magnetic Field Lines (S-Pole North)
To trace the lines of force on one side of a given bar magnet, placed in the magnetic meridian with its south pole pointing north, and mark the position of the neutral point.
Apparatus and Materials
- Bar magnet
- Compass needle
- Sheet of white paper
- Drawing board
- Brass pins
- Sharp pencil
- Piece of chalk
Theory
A magnetic line of force is a curve in the magnetic field such that a tangent to it at any point gives the direction of the magnetic field at that point.
The direction of the axis of a freely suspended magnetic needle gives the direction of the resultant field. If the successive positions of the needle are found out from one end of a magnet to the other and a line is drawn, it will represent a line of force.
While tracing the lines of force in a magnetic field due to a magnet, we come across points where the field due to the magnet and the horizontal intensity of Earth’s field are neutralized by each other. Such points are called neutral points. A compass needle placed at these points tends to remain in any direction in which it is kept.
For the N-pole of a magnet pointing the geographical north, neutral points lie on the perpendicular bisector to its length. When the south pole of a magnet points towards the geographical north, the neutral points lie on the axial line.
Observation
The neutral point is found to lie along the extended axis of the magnet.
Precautions
- There should be no magnetic material in the vicinity of the magnet. Drawing pins should be made of brass.
- While plotting the lines of force, the position of the drawing board or that of the magnet should in no case be disturbed.
