Question

A torch bulb is rated $5V$ and $500mA$. Calculate:
A) Its power
B) Its resistance and
C) The energy consumed if the bulb is lighted for $4hours$.

Answer 1

Recall that the power is defined as the amount of energy that is transferred or converted in a time of one second. It is also known as activity. It is the rate of doing work. It is written as the product of voltage and current. It is measured in Watts.

Complete step by step solution:
The formula for power can be written as
$Power = Voltage \times Current$
The given voltage is $V = 5V$
And the value of current is $I = 500mA = \dfrac{{500}}{{1000}}A = 0.5A$
$\therefore P = 5 \times 0.5 = 2.5W$
$\Rightarrow$The power of torch bulb is$= 2.5W$
Resistance is defined as the blockage offered to the flow of current in an electric circuit. It is the property that is possessed by the electrical components. It is measured in ohms.
The formula of resistance is written as
$R = \dfrac{{Voltage}}{{Current}}$
$R = \dfrac{5}{{0.5}}$
The resistance of the bulb is$R = 10\Omega$
Time taken is $t = 4hours$
Converting hours into seconds, the value of time becomes
$\Rightarrow t = 4 \times 60 \times 60\sec$
$\Rightarrow t = 14400\sec$
The amount of charge that is moved by applying a voltage in a time interval is called the energy. It is measured in Joules.
The formula for energy consumed is given by
$E = Power \times Time$
$\Rightarrow E = 2.5 \times 14400$
The energy of the bulb is
$\Rightarrow E = 36000Joules$

Note: It is important to remember that the energy of an isolated system is always constant. This means that the energy of a system can never be created nor be destroyed. But it can be transferred from one form to another form. There are different forms of energy. Also it is different from power. It is often confused with power. Power is the rate at which energy is transferred but energy is the ability to make a change.