(i) Identify the machine. (ii) Name the parts P and Q and R of the machine. (iii) Give the polarities of the magnetic poles. (iv) Write the two ways of increasing the output voltage.
Show Hint
(a) Lenz's law: the induced current opposes the change of flux. (b) The figure is an AC generator: P slip rings, Q brushes, R armature coil. Raise the speed of rotation and the number of turns to get a larger emf.
Step 1: Lenz's law (part a):
Lenz's law gives the direction of an induced emf or current.
It says: the polarity of the induced emf is such that it tries to produce a current which opposes the change in magnetic flux that produced it.
This follows from conservation of energy. If the induced current helped the change, the flux would keep growing by itself and we would get free energy.
The negative sign in \(\varepsilon = -\dfrac{d\Phi_B}{dt}\) shows this opposition.
Step 2: Identify the machine (part b-i):
The figure shows a coil that spins between the poles of a horseshoe magnet. The two ends of the coil are joined to two rings, carbon contacts press on the rings, and a voltmeter is connected across the contacts.
This is an AC generator (alternator). It changes the mechanical energy of rotation into electrical energy by electromagnetic induction.
Step 3: Name the parts P, Q and R (part b-ii):
P is the pair of slip rings. Each ring is joined to one end of the coil and turns with it.
Q is the pair of carbon brushes. They stay fixed, press on the slip rings and carry the current to the outside circuit.
R is the armature coil, the rotating coil of many turns of insulated copper wire wound on a soft iron core. Its rotation in the field makes the flux change.
Step 4: Polarities of the magnetic poles (part b-iii):
The coil sits between the two pole faces of the horseshoe magnet. The field lines run from the north pole to the south pole across the gap.
In the figure the pole on the left of the coil is the north pole (N) and the pole on the right of the coil is the south pole (S).
Step 5: Ways to increase the output voltage (part b-iv):
The emf of the generator is \(\varepsilon = NBA\omega \sin \omega t\), so the peak value is \(\varepsilon_0 = NBA\omega\).
Way 1: rotate the coil faster, that is increase the angular speed \(\omega\).
Way 2: increase the number of turns N of the coil. Using a stronger magnet (larger B) or a coil of larger area A also works. Any two of these give a larger output voltage.
Final Answer:
(a) Induced emf always opposes the change of flux that causes it. (b) (i) AC generator, (ii) P = slip rings, Q = carbon brushes, R = armature coil, (iii) left pole N and right pole S, (iv) increase the speed of rotation and increase the number of turns of the coil.
