Ohm’s Law MCQs Quiz | Class 10

Understanding Ohm’s Law: A Foundation of Electricity

Ohm’s Law is a fundamental principle in electricity that describes the relationship between voltage, current, and resistance in an electrical circuit. Discovered by German physicist Georg Simon Ohm, this law is crucial for understanding how electrical components behave and for designing electronic circuits.

Statement of Ohm’s Law

Ohm’s Law states that at a constant temperature, the current flowing through a conductor is directly proportional to the potential difference (voltage) applied across its ends. This means if you increase the voltage across a resistor, the current flowing through it will increase proportionally, provided the resistance remains constant.

• Potential Difference (V): Measured in Volts (V), it is the work done per unit charge in moving a charge between two points.
• Current (I): Measured in Amperes (A), it is the rate of flow of electric charge.
• Resistance (R): Measured in Ohms (Ω), it is the opposition offered by the conductor to the flow of electric current.

The V–I Relation (V = IR)

Mathematically, Ohm’s Law is expressed as:

V = I × R

Where:

• V = Potential Difference (Voltage)
• I = Current
• R = Resistance

This equation can be rearranged to find any of the three quantities if the other two are known:

• To find Current: I = V / R
• To find Resistance: R = V / I

This linear relationship is the cornerstone of many electrical calculations.

Graph-Based Analysis: V-I Graph

When the potential difference (V) is plotted against the current (I) for an ohmic conductor at a constant temperature, the resulting graph is a straight line passing through the origin. This V-I graph provides a visual representation of Ohm’s Law.

• Straight Line: The linear nature of the graph confirms that V is directly proportional to I.
• Passing Through Origin: When there is no potential difference (V=0), there is no current (I=0).
• Slope of the Graph: The slope of the V-I graph (V/I) gives the resistance (R) of the conductor. A steeper slope indicates higher resistance, while a gentler slope indicates lower resistance.

Conductors that obey Ohm’s law, exhibiting a linear V-I relationship, are called ohmic conductors (e.g., most metallic conductors). Those that do not (e.g., semiconductor devices like diodes) are called non-ohmic conductors and have non-linear V-I graphs.

Factors Affecting Resistance

While Ohm’s law emphasizes constant temperature, the resistance of a conductor itself depends on several factors:

1. Length of the conductor (L): Resistance is directly proportional to length (R ∝ L).
2. Area of cross-section (A): Resistance is inversely proportional to the area of cross-section (R ∝ 1/A).
3. Nature of the material: Different materials have different inherent resistivities (ρ).
4. Temperature: For most metallic conductors, resistance increases with temperature.

Combining these, the resistance R = ρL/A, where ρ is resistivity, a material constant.

Quick Revision Points

• Ohm’s Law: V = IR
• Valid at constant temperature.
• V (Volts), I (Amperes), R (Ohms).
• V-I graph for ohmic conductors is a straight line through the origin.
• Slope of V-I graph gives resistance.
• Resistance depends on length, area, material, and temperature.

Extra Practice Questions

1. A resistor of 20 Ohm is connected to a 6V battery. Calculate the current flowing through the resistor.
2. Explain why the temperature of a conductor is kept constant while verifying Ohm’s Law.
3. Draw a V-I graph for a semiconductor diode and comment on its linearity.
4. If the length of a copper wire is doubled and its cross-sectional area is halved, how does its resistance change?
5. A wire of resistance R is cut into three equal parts. What is the resistance of each part? If these three parts are connected in parallel, what is the equivalent resistance?