Common misconceptions › Class 12 › Physics › Alternating Current Class 12 Physics · Chapter 7 · NCERT (leph107)
Alternating Current: common misconceptions
See the tempting mistake students make in Alternating Current, why it feels right at first glance, and a worked visual example that shows the right idea.
Worked visual “If your child says…” (8) Teach & practice The tempting mistake & worked visual Can AC with zero average current still heat a resistor? Wrong
A resistor on AC cannot heat because the current averages to zero.
Why it feels right. I assume positive and negative half cycles cancel their heating.
Right
Resistive heating depends on I²R, which is positive for nonzero current in either direction.
Worked example
Heating ∝ I² Current I The dashed current changes sign. The solid heating curve stays at or above zero in both half cycles. Through 2 Ω, the instantaneous currents +3 A and −3 A each produce 18 W. A sinusoid with peak current 3 A has root-mean-square current 3/√2 A and average power 9 W. If your child says… (8 concept checks in this chapter)
As you work through Alternating Current , listen for these everyday misunderstandings and use the prompt alongside each one.
If your child says…
“The average current is zero, so there is no heating.”
Ask together Does the power curve fall below zero when current reverses? What’s true Heating depends on the average squared current, which stays positive for a nonzero sine wave. If your child says…
“A higher frequency makes more current flow through the inductor.”
Ask together If opposition doubles while voltage stays unchanged, what happens to current? What’s true Higher frequency increases inductive reactance when inductance stays fixed. If your child says…
“A higher frequency makes less current flow through the capacitor.”
Ask together Does changing the plate charge faster require more current or less? What’s true Higher frequency means faster voltage changes and more current at unchanged rms voltage and capacitance. If your child says…
“I can add all three voltage readings to get the source voltage.”
Ask together If equal coil and capacitor arrows cancel, which arrow remains? What’s true Coil and capacitor voltages oppose each other, while resistor voltage differs by a quarter-cycle. If your child says…
“At resonance, nothing limits the current.”
Ask together Which component still turns electrical energy into heat when the two voltage arrows cancel? What’s true Only the coil and capacitor effects cancel, leaving the resistor to limit current. If your child says…
“Voltage times current always gives average power.”
Ask together For an ideal coil, can rms voltage times rms current equal its zero average power? What’s true Multiplying rms voltage by rms current gives average power only when power factor is one. If your child says…
“A transformer doubles voltage and current together.”
Ask together Would doubling both voltage and current leave their product unchanged? What’s true For unchanged power and a resistive load, doubling voltage halves current. If your child says…
“Halving the current only halves the heating.”
Ask together What happens when both factors in current times current are halved? What’s true At fixed resistance, halving current quarters heating because current is squared. Use this chapter three ways Previous chapter Electromagnetic Induction Next chapter Electromagnetic Waves
In the app, your child answers a short check-in with confidence taps, and you see which of these misunderstandings showed up—and which ones are gone.
Start free 
