All questions of Electricity for Class 10 Exam

25 A

To calculate the electrical energy consumed by an electric bulb, we need to use the formula:

Energy (in kilowatt-hours) = Power (in kilowatts) × Time (in hours)

Given:
Power of the electric bulb = 200 watts
Time = 15 hours

Using the formula, we can calculate the energy consumed as follows:

Energy = (Power × Time)/1000

Substituting the given values:

Energy = (200 × 15)/1000
= 3000/1000
= 3 kilowatt-hours (kWh)

Therefore, the electrical energy consumed by the electric bulb in 15 hours is 3 kWh.

Explanation:
- The formula for calculating electrical energy is Energy = Power × Time.
- In this case, the power of the electric bulb is given as 200 watts and the time is given as 15 hours.
- To use the formula, we need to convert the power from watts to kilowatts by dividing it by 1000.
- After substituting the values into the formula, we calculate the energy consumed as 3 kilowatt-hours (kWh).
- This means that the electric bulb consumes 3 kilowatt-hours of electrical energy over a period of 15 hours.

To find the equivalent resistance of a combination of resistors, we need to consider the rules for resistors connected in series and parallel.

Resistors in Series:
When resistors are connected in series, their resistances add up. The equivalent resistance (Rₑ) of resistors in series can be calculated using the formula:

Rₑ = R₁ + R₂ + R₃ + ...

Resistors in Parallel:
When resistors are connected in parallel, their equivalent resistance is given by the formula:

1/Rₑ = 1/R₁ + 1/R₂ + 1/R₃ + ...

Combination:
To achieve an equivalent resistance of 2 ohms using three 3-ohm resistors, we need to connect them in a combination of series and parallel.

Connecting the resistors:
By connecting two resistors in series, we can achieve an equivalent resistance of 6 ohms (3 + 3 = 6). Let's call this combination A.

By connecting the third resistor in parallel with combination A, we can calculate the equivalent resistance.

1/Rₑ = 1/6 + 1/3

Simplifying the equation:

1/Rₑ = 1/6 + 2/6

1/Rₑ = 3/6

1/Rₑ = 1/2

Rₑ = 2 ohms

Therefore, connecting two resistors in series and the third resistor in parallel with the combination will give us an equivalent resistance of 2 ohms. The correct answer is option 'D'.

Note: It is important to understand the concepts of series and parallel connections of resistors to solve such problems.

Understanding Potential Difference and Current
When the potential difference (voltage) across the ends of a conductor is altered, it directly influences the current flowing through it. This relationship is described by Ohm's Law.
Ohm's Law Explained
Ohm's Law states that:
- V = I × R
Where:
- V = Potential difference (voltage)
- I = Current
- R = Resistance
From this equation, we can derive that:
- I = V / R
This implies that current is directly proportional to the voltage applied across the conductor, given that the resistance remains constant.
Effect of Doubling Voltage
When the potential difference is doubled:
- New Voltage (V') = 2V
Substituting into Ohm's Law:
- I' = V' / R
- I' = (2V) / R
- I' = 2(V / R)
- I' = 2I
This shows that the current also doubles when the voltage is doubled, assuming that resistance remains unchanged.
Conclusion
In summary, if the potential difference across a conductor is doubled, the current flowing through it will also double. Thus, the correct answer is:
- a) It gets doubled.
Understanding this fundamental relationship is crucial for analyzing electrical circuits and their behavior.

To determine the amount of work done in moving a charge from one point to another in an electric circuit, we can use the formula:

Work = Charge × Voltage

Here, the charge is given as 5 C (coulombs), and the voltage difference between the two points is given as 240 volts - 220 volts = 20 volts.

Hence, the work done can be calculated as:

Work = 5 C × 20 V = 100 Joules

Therefore, the correct answer is option B, 100 Joules.

Explanation:
To understand why the work done is 100 Joules, let's break down the concept behind it.

The work done in an electric circuit is given by the product of the charge and the voltage. When a charge moves from a point with lower potential (220 volts) to a point with higher potential (240 volts), work is done on the charge by an external agent.

The work done on a charge can be thought of as the energy transferred to the charge. In this case, as the charge moves against the electric field, energy is required to overcome the potential difference.

The unit of work is Joules, which represents the amount of energy transferred. In this scenario, the charge of 5 C is moved across a potential difference of 20 volts. Multiplying these values together gives us the amount of work done, which is 100 Joules.

Therefore, the correct answer is option B, 100 Joules.

Understanding Charge and Electrons
The question revolves around the relationship between electric charge and the number of electrons. To find the number of electrons in one coulomb of charge, we need to know the charge of a single electron.
Charge of an Electron
- Each electron carries a fundamental charge of approximately -1.6 × 10^-19 coulombs.
Calculating the Number of Electrons
To find the total number of electrons in one coulomb, we divide the total charge (1 coulomb) by the charge of one electron.
- Formula:
Number of electrons = Total charge / Charge of one electron
- Using the values:
Number of electrons = 1 C / (1.6 × 10^-19 C/electron)
- Performing the calculation:
Number of electrons = 1 / (1.6 × 10^-19)
≈ 6.25 × 10^18 electrons
Conclusion
Thus, one coulomb of charge is constituted by approximately 6.25 × 10^18 electrons, confirming that the correct answer is option 'D'.
This relationship underscores the concept of charge at the atomic level and illustrates how a seemingly large unit of charge like a coulomb is composed of an exceedingly vast number of elementary particles like electrons. Understanding this helps in grasping the fundamentals of electric charge and its quantization.