Physics · Current Electricity · NEET
Both are correct because V = IR, so I²Rt = (V²/R²)Rt = V²t/R = VIt. They are the same law written three ways. The trick is to pick the form where the quantity that stays FIXED is on top. If the SAME CURRENT flows (series circuit), use H = I²Rt, so more R means more heat. If the SAME VOLTAGE is applied (parallel across a supply), use H = V²t/R, so more R means LESS heat. Choosing the right form for the situation is what NEET tests.
When voltage V is the same across both (they are in parallel), use H = V²t/R. Heat is inversely proportional to R, so the SMALLER resistance gets hotter. Example (NEET 2022): 100 Ω and 200 Ω in parallel — heat ratio H₁/H₂ = R₂/R₁ = 200/100 = 2:1, so the 100 Ω resistor produces twice the heat.
When the SAME current I flows (resistors in series), use H = I²Rt. Heat is directly proportional to R, so the LARGER resistance gets hotter. This is the opposite of the same-voltage case. Series → bigger R hotter; Parallel → smaller R hotter. Mixing these two up is the single most common mistake in this topic.
Power dissipated is P = VI, and by Ohm's law V = IR, so P = (IR)·I = I²R. The I comes twice — once as the charge flow rate and once as the voltage drop it causes — so it appears squared. Because I² is always positive, heating happens whether the current is positive or negative (this is why AC still heats, and why the ampere is defined by heating effect).
Heat needs current, not charge. First get i = dQ/dt. Then heat over the time the current flows is H = ∫i²R dt. You cannot use H = I²Rt directly because the current is not constant — you must integrate. This exact style appeared in NEET 2016.
The charge through a resistance R varies with time as Q = at − bt² (a, b are positive constants). The total heat produced in R is:
Two resistors 100 Ω and 200 Ω are connected in parallel. The ratio of thermal energy developed in 100 Ω to that in 200 Ω in a given time is:
A room heater is rated 400 W, 220 V. If the supply voltage drops to 200 V, the power consumed (approximately) is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It states that the heat produced in a resistor equals H = I²Rt, where I is the current, R the resistance, and t the time. Equivalently H = VIt = V²t/R. Heat is directly proportional to the square of current, to resistance, and to time.
H = I²Rt (use when current is fixed, i.e. series), H = V²t/R (use when voltage is fixed, i.e. parallel across a supply), and H = VIt (general). All three are equal because V = IR.
The joule (J), since H is energy. Power of heating P = H/t is in watts (W). One kilowatt-hour (unit of electricity) = 3.6 × 10⁶ J.
A fuse is a thin wire with high resistivity and a low melting point. When current becomes too large, the I²Rt heating melts the fuse and breaks the circuit, protecting appliances. This was directly asked in NEET 2019.
Yes. Since heat depends on I² which is always positive, current in either direction produces heat. This is why AC bulbs and heaters work, and why the rms value of AC current is defined through its heating effect.
