Ohm’s Law – CBSE Class 10 Science (Physics – Electricity)

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1. Statement of Ohm’s Law

• Ohm’s Law states that the current ($I$) flowing through a conductor is directly proportional to the potential difference ($V$) across its ends, when temperature and physical conditions remain constant.
• If you increase the voltage, the current also increases in the same ratio, as long as the conductor is not heated or changed in any way.
• This law is a fundamental rule for simple electric circuits.

Examples:

• If you have a copper wire and you double the voltage across it without heating it up, the current through the wire will also double.
• For a steel rod at room temperature, increasing the applied voltage from 2 V to 4 V will increase the current from 0.1 A to 0.2 A.
• In a simple torch with a metallic filament, inserting two cells (instead of one) doubles the voltage — which doubles the current, making the bulb burn brighter.

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2. Mathematical Expression of Ohm’s Law

• The law is mathematically expressed as:

  $$V = IR$$

  Where:

  • $V$ is the potential difference (in volts, V)
  • $I$ is the current (in amperes, A)
  • $R$ is the resistance (in ohms, $\Omega$)

• Rearranging the formula helps you solve various questions:

  • To find current: $I = \frac{V}{R}$
  • To find resistance: $R = \frac{V}{I}$

Examples:

• Suppose you have a resistor of $R = 20\; \Omega$, and a battery of $V = 10\; V$ is connected: $$I = \frac{V}{R} = \frac{10}{20} = 0.5\,A$$
• If a bulb draws $0.3$ A when a $6$ V battery is connected, $$R = \frac{6}{0.3} = 20\, \Omega$$
• An unknown wire draws $2$ A at $4$ V: $$R = \frac{4}{2} = 2\, \Omega$$

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3. Ohm’s Law – Activity (Activity-Based Understanding)

Let’s make this fun and easy!

Activity: Verifying Ohm’s Law

Objective:
To show that current is directly proportional to the applied voltage for a conductor at constant temperature.

Materials Required:

• Battery or power supply
• Connecting wires
• Ammeter (to measure current)
• Voltmeter (to measure voltage)
• Resistor (a simple wire or coil)
• Rheostat (to vary resistance, optional)
• Switch

Step By Step Instructions:

1. Arrange the Circuit:

   • Connect the battery, resistor, ammeter, and switch in series.
   • Connect a voltmeter in parallel across the resistor.

2. Close the Switch:

   • Let the current flow.

3. Record Readings:

   • Note the current shown by the ammeter ($I_1$) and the voltage across resistor ($V_1$).

4. Change Voltage:

   • Change the battery voltage or move the rheostat to increase the voltage.

5. Read Again:

   • Note new ammeter ($I_2$) and voltmeter ($V_2$) readings.

6. Repeat:

   • Take at least three sets of readings for different voltages.

Observations to Make:

• Prepare a table:

  Serial No.	Potential Difference (V)	Current (I) (A)	V/I
  1	2	0.1	20
  2	4	0.2	20
  3	6	0.3	20

• You will notice $V/I$ (the resistance) remains nearly constant.

• When you plot voltage against current, you get a straight line.

What’s Happening?

• This shows the proportionality described by Ohm’s law.
• Resistance of the wire remains constant as long as it doesn't heat up.

Fun Angle:

• You can use colored water for current and draw a "water circuit" with pipes and syringes, where more pressure (voltage) pushes more water (current) through the same narrow pipe (resistance).

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4. V-I Graph and Resistance

• For ohmic conductors, plotting potential difference ($V$) on the y-axis and current ($I$) on the x-axis produces a straight line through the origin.
• The slope (V/I) of the line gives the resistance of the conductor.
• Steepness:
  • A flatter line means higher resistance (needs more voltage to get same current).
  • A steep line means lower resistance (gets more current for same voltage).

Examples:

• A copper wire and a nichrome wire are tested. The copper wire's graph is steeper than nichrome because copper has less resistance.
• For a device like a bulb, the graph is not a straight line. As bulb heats up, resistance changes, so it is non-ohmic.
• A diode’s graph allows current only after a certain minimum voltage, not a straight line.

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5. Practical Applications of Ohm’s Law

• Designing Circuits:
  Engineers use Ohm’s law to decide what size resistor to use with LEDs. Too much current can burn out components.
• Home Electric Wiring:
  Wires are chosen based on the expected current and Ohm’s law to avoid overheating and fire.
• Measuring Resistance:
  Simple multimeters use Ohm’s law to calculate the resistance of any object.

Examples:

• You want to reduce current in a heater for safety, so you use a resistor based on Ohm’s law.
• Making a phone charger, manufacturers calculate resistance needed for safe charging.
• When replacing a fuse in your home, you choose its rating based on the normal current and Ohm's law.

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Scenario Based Questions

1. Scenario: You increase the voltage across a metallic wire from 3 V to 6 V and notice the current doubles.

   • Question: Is the wire obeying Ohm’s law? Explain.
   • Answer: Yes, because the current increases in direct proportion to voltage, showing constant resistance.

2. Scenario: You replace a resistor with a glowing bulb in a circuit and notice the V-I graph is curved.

   • Question: What could be the reason for this?
   • Answer: The bulb is heating up; its resistance changes with temperature, so it does not strictly follow Ohm’s law.

3. Scenario: In a lab, you plot current (I) on the x-axis and voltage (V) on the y-axis for an unknown wire and get a straight line through the origin.

   • Question: What does the slope of this line represent?
   • Answer: The slope ($V/I$) represents the resistance of the wire.

4. Scenario: A student adds a resistor in series with a lamp, and the lamp becomes dimmer.

   • Question: Why does this happen?
   • Answer: Adding resistance reduces the current according to Ohm’s law, so less current flows through the lamp, making it dimmer.

5. Scenario: You are told a component does not obey Ohm’s law.

   • Question: What kind of graph would you expect for its V-I characteristics?
   • Answer: You would see a curve or nonlinear relationship, not a straight line.

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Remember, Ohm’s law is simple and magical! It’s like a relationship between pressure (voltage), flow (current), and roadblock (resistance) in an electric circuit. Play with safe circuits and see it yourself!

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