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What is the Internal Resistance of a Cell?
Grade Level:
Class 10
AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine
Definition
What is it?
The internal resistance of a cell is the opposition offered by the materials inside the cell (like its chemicals and electrodes) to the flow of electric current. It's like a tiny 'speed breaker' within the battery itself that reduces the actual voltage available at its terminals.
Simple Example
Quick Example
Imagine you have a new cricket bat. When you hit a ball, some energy is lost as vibrations in the bat itself, not all of it goes to the ball. Similarly, a cell's internal resistance 'absorbs' some of the energy it produces, so the voltage you measure outside the cell is slightly less than what it actually generates.
Worked Example
Step-by-Step
Let's say a cell has an electromotive force (EMF) of 1.5V. When it supplies a current of 0.5A to an external circuit, the voltage measured across its terminals (terminal voltage) drops to 1.3V. We want to find the internal resistance (r) of the cell.
Step 1: Identify the given values.
EMF (E) = 1.5V
Terminal Voltage (V) = 1.3V
Current (I) = 0.5A
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Step 2: Recall the formula relating EMF, terminal voltage, current, and internal resistance.
E = V + Ir
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Step 3: Rearrange the formula to solve for internal resistance (r).
Ir = E - V
r = (E - V) / I
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Step 4: Substitute the values into the formula.
r = (1.5V - 1.3V) / 0.5A
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Step 5: Calculate the difference in voltage.
r = 0.2V / 0.5A
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Step 6: Perform the division to find the internal resistance.
r = 0.4 Ohm
Answer: The internal resistance of the cell is 0.4 Ohm.
Why It Matters
Understanding internal resistance is crucial for designing efficient batteries for electric vehicles and mobile phones, ensuring they deliver consistent power. Engineers and scientists in fields like battery technology, electronics, and even space technology use this concept to optimize power sources for rockets and satellites.
Common Mistakes
MISTAKE: Confusing EMF with terminal voltage, thinking they are always the same. | CORRECTION: EMF is the total voltage generated by the cell, while terminal voltage is the actual voltage available outside the cell, which is always less than or equal to EMF due to internal resistance.
MISTAKE: Forgetting to include the internal resistance in circuit calculations when current is flowing. | CORRECTION: Always consider internal resistance as a small resistor connected in series with the ideal cell when current is drawn from it.
MISTAKE: Believing that internal resistance is zero for all practical cells. | CORRECTION: All real-world cells have some internal resistance, though it can be very small for new, high-quality cells.
Practice Questions
Try It Yourself
QUESTION: A cell has an EMF of 2V and an internal resistance of 0.1 Ohm. If it supplies a current of 1A, what is its terminal voltage? | ANSWER: 1.9V
QUESTION: A battery with an EMF of 12V delivers 2A of current to an external circuit. If the terminal voltage measured is 11.5V, calculate the internal resistance of the battery. | ANSWER: 0.25 Ohm
QUESTION: A cell with an EMF of 6V and internal resistance 'r' supplies 0.5A current when connected to a 10 Ohm resistor. What is the value of 'r'? (Hint: First find the terminal voltage across the 10 Ohm resistor). | ANSWER: 2 Ohm
MCQ
Quick Quiz
What happens to the terminal voltage of a cell when it delivers current, compared to its EMF?
It increases
It remains the same
It decreases
It becomes zero
The Correct Answer Is:
C
When a cell delivers current, some voltage is lost across its internal resistance, causing the terminal voltage to be less than the EMF. Options A, B, and D are incorrect because voltage is always 'dropped' internally.
Real World Connection
In the Real World
When your mobile phone battery gets old, it charges slower and drains faster. This is often because its internal resistance has increased. This higher internal resistance reduces the actual power delivered to your phone, making apps run slower or even causing the phone to shut down unexpectedly, especially during heavy usage like gaming or watching videos.
Key Vocabulary
Key Terms
EMF: The maximum potential difference a cell can provide when no current is drawn from it. | Terminal Voltage: The actual voltage measured across the terminals of a cell when current is flowing through the external circuit. | Ohm: The unit of electrical resistance. | Current: The flow of electric charge per unit time.
What's Next
What to Learn Next
Now that you understand internal resistance, you're ready to explore 'Ohm's Law' in more detail and learn about 'Series and Parallel Combinations of Resistors'. These concepts will help you analyze more complex circuits, just like engineers design real electronic gadgets!
