For an electric bulb operated from a given supply, the brightness of the bulb effectively depends on which of the following electrical quantities?

Introduction / Context:
The brightness of an electric bulb is related to the electrical power it consumes. Power, in turn, depends on both the voltage applied across the bulb and the current flowing through it. This question checks whether you can connect brightness with these basic electrical quantities rather than thinking of voltage and current in isolation.

Given Data / Assumptions:

• We consider an ordinary filament or incandescent type bulb or any resistive lamp.
• The bulb is connected to an electrical supply with some voltage V and current I.
• Brightness is assumed to be proportional to the electrical power converted into light and heat.
• We must choose whether brightness depends on current, voltage, both or none.

Concept / Approach:
The electrical power P consumed by a device is given by formulas such as P = V * I, P = I^2 * R or P = V^2 / R, where V is voltage, I is current and R is resistance. Brightness of a bulb increases with the power dissipated in its filament. Because power can be expressed using both voltage and current together, it is clear that brightness depends on both quantities rather than on only one of them in a simple way. In practice, if voltage increases across a fixed resistance, current also increases and thus power goes up, making the bulb brighter. The key idea is that voltage and current are linked and together determine power and brightness.

Step-by-Step Solution:
Step 1: Write the basic relation for electrical power: P = V * I. Step 2: Observe that this formula directly involves both voltage V and current I, and power is what determines brightness. Step 3: If we hold the resistance constant, increasing voltage increases current, and both changes together cause an increase in power. Step 4: Similarly, if for some reason current is increased at the same voltage, the power and brightness increase. Step 5: Therefore, brightness cannot be described as depending solely on current or solely on voltage; it depends on both voltage and current.

Verification / Alternative check:
Consider a bulb rated at 60 W, 230 V. Using P = V * I, the rated current is I = P / V. If the applied voltage is reduced significantly, the current will fall and the bulb will glow dimly because power is lower. If voltage is increased beyond the rating, current will rise and the bulb glows brighter until it risks burning out. These examples show that both voltage and current together set the power level. Therefore, treating brightness as a function of only one of these quantities is incomplete for general questions like this.

Why Other Options Are Wrong:
Current: Focusing only on current ignores the fact that current itself depends on voltage and resistance, and power is the product V * I.

Voltage: Voltage alone does not determine brightness because different bulbs with different resistances can draw different currents at the same voltage.
None: Brightness clearly depends on electrical quantities, so this is not correct.

Common Pitfalls:
A frequent mistake is to memorize that “brightness depends on current” without considering the role of voltage. Another is to think that standard mains voltage is fixed, so only current matters. In truth, the correct physical relationship for power includes both V and I, and exam questions at this level expect the answer that reflects the complete dependency. Keeping the formula P = V * I in mind provides a quick and reliable way to answer similar questions.

Final Answer:
The brightness of a bulb effectively depends on Both A & B, that is, on both current and voltage.