Electric Potential & Potential Difference — Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain the concept of electric potential with the help of an example. How does it relate to work and charge?
Answer:
Electric potential at a point is defined as the work done in bringing a unit positive charge from infinity to that point, without acceleration. For instance, imagine moving a 1 coulomb charge toward an isolated positive sphere. If you do 5 joules of work to bring it close, the electric potential at that point is . Electric potential shows energy per unit charge at a point. The formula tells us how much work (W) is needed to move a specific charge (q) to that location. This concept is similar to raising an object to a certain height where it gains gravitational potential energy.
Q2. What is potential difference? Why is it more important practically than the concept of absolute electric potential?
Answer:
Potential difference is the work done in moving a unit charge from one point to another in an electric field. It is given by . This difference is more important in real life because charges only flow when there’s a difference in potential—just like water flows only when there is a difference in height. While absolute electric potential at a point is hard to measure or use, potential difference between two points can be easily measured using a voltmeter. Current flows in wires and devices because of this potential difference, making it crucial in circuits, batteries, and electrical appliances.
Q3. Describe how a voltmeter is used to measure potential difference in a circuit. Why should it be connected in parallel?
Answer:
A voltmeter measures the potential difference between two points in a circuit. To do this, you connect the voltmeter across (in parallel to) the component (like a bulb or resistor) you wish to measure. This is because parallel connection ensures both ends of the voltmeter are at the same potential as the component’s ends. If connected in series, the voltmeter would severely limit current flow due to its high resistance, disrupting the circuit. Thus, parallel placement ensures accurate measurement of the energy lost by each charge passing through that component.
Q4. Relate potential difference to the energy conversion that happens in everyday electrical appliances. Give at least two examples.
Answer:
Potential difference provides the “push” that moves charges through devices, making them convert electrical energy into other forms. For example:
- In a bulb, a 6 V potential difference means every coulomb of charge loses 6 J of energy, which is converted into light and heat.
- In an electric kettle powered by a 220 V household supply, every coulomb loses 220 J, heating the water.
The greater the potential difference, the more energy each charge can provide for useful work inside the device.
Q5. Using water tank analogy, explain why potential difference is necessary for current to flow.
Answer:
Imagine a water tank at height. Water flows down from the tank due to the difference in its levels (height)—this difference causes pressure. Similarly, electric potential difference between two points in a wire is like a height difference for charges. If both ends are at the same potential, no charge will “flow”, just as water wouldn’t flow between two tanks at the same height. Only a difference in these levels (i.e., potential) creates the "push" (voltage) that moves current through the circuit.
High Complexity (Analysis & Scenario-Based)
Q6. A student is using a 12 V battery to light up two bulbs in parallel. Explain what potential difference each bulb experiences, and why they glow with equal brightness even if the battery is far away.
Answer:
When bulbs are connected in parallel to a 12 V battery, both ends of each bulb are directly connected to the battery’s terminals. This means each bulb gets the full 12 V potential difference, regardless of their position in the circuit. Even if the battery is physically far, the wires ensure the voltage at each end is the same as the battery terminals. Thus, energy per charge provided to each bulb is equal, so both receive the same amount, leading to equal brightness (assuming identical bulbs). The path length does not affect the potential difference each gets.
Q7. Suppose you have a 1.5 V cell. Describe what happens at the microscopic (charge movement) and macroscopic (circuit behavior) level when you connect it across a resistor.
Answer:
At the microscopic level, the 1.5 V cell creates a potential difference across the resistor, causing free electrons in the wire to drift from the negative terminal to the positive (conventional current flows from positive to negative). Each coulomb of charge receives 1.5 joules of energy from the cell, moves through the resistor, and delivers that energy—converted to heat.
At the macroscopic level, the circuit is completed, current starts to flow, and the resistor gets warm. The flow continues as long as the cell can maintain the 1.5 V difference, demonstrating direct relationship between potential difference and current.
Q8. Two different appliances, a 60 W bulb and a 100 W bulb, are connected in parallel to a 220 V supply. Both bulbs have the same potential difference across them. Explain why their brightness is different and what role potential difference plays here.
Answer:
Both bulbs experience the same potential difference (220 V) because they are connected in parallel. However, their power ratings (60 W and 100 W) mean the 100 W bulb allows more current to flow (it has less resistance). The potential difference provides the same energy per coulomb to both, but the 100 W bulb lets more charges pass every second, consuming more total energy per second. Hence, it glows brighter. The difference in current, not voltage, causes one bulb to convert more energy (thus, be brighter) than the other.
Q9. A boy says, “If there is a potential difference of 0 V, there can still be energy supplied to a circuit.” Analyze and correct his statement.
Answer:
The boy is incorrect. If the potential difference is 0 V, then no work is done in moving charges between those points (). This means no energy is transferred to the circuit, thus, no current will flow and no appliance will work. For electrical energy to be supplied, a non-zero potential difference is necessary. Only then do charges flow, transporting energy to do useful work (like lighting a bulb or running a fan).
Q10. Imagine an electric fence with a potential difference of 10,000 V between the fence and the ground. Explain in detail what happens if a person accidentally touches the fence while standing on the ground, with respect to potential difference, charge flow, and energy transfer.
Answer:
If a person stands on the ground (0 V) and touches a fence at 10,000 V, there is a huge potential difference (10,000 V) across the parts of the body touching both. This difference causes current to flow through the person's body, because charges move rapidly from high potential (fence) to low (ground). Every coulomb transfers 10,000 joules of energy through the body, which is felt as a strong electric shock. This large energy per charge can disrupt bodily functions, leading to injury or worse. Thus, potential difference directly determines the severity of the energy transfer in such accidents.