Lesson 7 of 12 · 7 min
Electrical energy and power
NCERT § "Electrical Energy, Power"
Riya's mother joins the 60 W and 100 W bulbs one after the other in a single line to save a socket. One of them glows brighter, and it is the one you would not guess.
The lesson in notes
In short
When charge moves through a potential difference V, the energy transferred per unit time is P = VI.
For a resistor, the power dissipated as heat is P = I²R = V²/R (Joule heating).
Two appliances rated for the same voltage: the one with the higher power rating has the lower resistance, since R = V²/P.
In series, the same current flows, so the higher-resistance device dissipates more power; a lower-rated bulb glows brighter when connected in series with a higher-rated one.
In parallel, the same voltage is applied, so the lower-resistance device dissipates more power.
Power is sent over long distances at high voltage: for a fixed power P delivered through cables of resistance R_c, the loss P²R_c/V² falls sharply as V is raised.
The kilowatt hour is the commercial unit of electrical energy, 1 kWh = 3.6 × 10⁶ J.