Physics · Electromagnetic Induction · NEET
No. This is the single most tested trap. A sine curve is the shape a simple AC generator produces because flux = NBA cos(wt), so EMF = NBAw sin(wt). But the definition of AC is only that the EMF is periodic and reverses its sign each cycle. A triangular wave, a sawtooth, or a square wave all count as AC because they too go positive then negative repeatedly. So do not reject a non-sine graph as 'not AC'.
Check one thing: does the curve cross the time axis and go both above (positive) and below (negative) it, over and over? If yes, it is AC. If the whole curve stays on one side (for example always positive, like a bumpy line that touches zero but never goes negative), it is DC (pulsating DC). Rule: AC = Axis-Crossing (goes + and -). DC = stays one side.
The coil rotates at steady angular speed w, so the angle is theta = wt. Flux through the coil is Phi = NBA cos(wt). Faraday's law gives EMF = -dPhi/dt = NBAw sin(wt). The sin(wt) is what makes it a smooth sine curve. Peak EMF is e0 = NBAw. But a generator with a different design could give a different periodic shape and still be AC.
No. Pulsating DC (what you get after passing AC through a diode/rectifier, or from a DC generator with a split-ring commutator) rises and falls but never goes negative. It stays above the time axis. Because it does not reverse sign, it is DC, not AC. Only when the graph goes below the axis (reverses polarity) is it AC.
The output ring. An AC generator uses two slip rings, so the coil output keeps its natural sign changes and the graph crosses the axis (AC). A DC generator uses a split-ring commutator that flips the connection every half turn, folding the negative halves up to positive, so the graph stays on one side (pulsating DC). Same rotating coil, different graph because of how the current is collected.
The variation of EMF with time for four types of generators are shown in the figures. Which amongst them can be called AC?
An emf is generated by an ac generator having 100 turn coil, of loop area 1 m^2. The coil rotates at one revolution per second in a uniform magnetic field of 0.05 T perpendicular to the axis of rotation. The maximum value of emf is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
e = e0 sin(wt), where e0 = NBAw is the peak EMF, w = 2*pi*f is the angular frequency, and t is time. It is a sine curve that repeats every period T = 1/f.
An AC graph crosses the time axis and goes both above (positive) and below (negative) it, repeating each cycle. A DC graph stays on one side of the axis (does not reverse sign); steady DC is a flat horizontal line and pulsating DC is bumps that never go negative.
Yes. A square wave that jumps between +V and -V is periodic and reverses sign, so it is AC even though it is not a sine curve. NEET tests exactly this idea.
For e = e0 sin(wt): EMF is zero when the coil's plane is parallel to B (flux maximum, changing slowest at that instant gives EMF=0 at wt=0, pi) and EMF is maximum when the coil's plane is along the field so flux changes fastest (at wt = pi/2, 3pi/2). Note EMF is greatest when flux is momentarily zero and changing fastest.
Flux is Phi = NBA cos(wt) (a cosine), and EMF = -dPhi/dt = NBAw sin(wt) (a sine). The derivative of cosine is sine, which is shifted by 90 degrees, so the EMF peaks exactly where the flux is zero and vice versa.
