4.5.2 The a.c. generator (3)
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1.
Question 2
A student builds a simple a.c. generator using a coil of wire and permanent magnets. They observe that the current produced is very low. Suggest two improvements the student could make to increase the amount of current produced. Explain the reasoning behind each suggestion, relating it to the principles of electromagnetic induction.
Here are two suggestions to increase the current produced by the a.c. generator, along with the reasoning:
- Increase the number of turns in the coil. The induced EMF in a coil is directly proportional to the number of turns of wire (EMF ∝ N). More turns mean a larger induced voltage, which in turn leads to a higher current if the resistance of the circuit remains constant. This is because a larger voltage drives more current through the circuit (Ohm's Law: I = V/R).
- Increase the speed of rotation of the coil. The frequency of the induced EMF is directly proportional to the speed of rotation (f ∝ ω). A faster rotation means the magnetic flux linking the coil changes more rapidly, resulting in a higher induced voltage and therefore a higher current. This is because a faster change in magnetic flux induces a larger EMF.
2.
Question 2
The e.m.f. produced by an AC generator is plotted with time in the graph below.
a) What is the period of the AC e.m.f.?
b) What is the frequency of the AC e.m.f.?
c) Describe how the position of the generator coil relative to the magnetic field affects the shape of the e.m.f. graph.
a) The period of the AC e.m.f. is 2 seconds. This is the time taken for one complete cycle of the waveform.
b) The frequency of the AC e.m.f. is 1/2 Hz. Frequency is the reciprocal of the period (f = 1/T). Therefore, f = 1/2 s-1.
c) The position of the generator coil relative to the magnetic field directly affects the magnitude of the induced e.m.f. When the coil is aligned with the magnetic field, the e.m.f. is at its maximum. When the coil is perpendicular to the magnetic field, the e.m.f. is zero. As the coil rotates, the e.m.f. varies sinusoidally, reflecting the changing area of the loop exposed to the magnetic field. A larger number of turns in the coil will result in a larger e.m.f. for the same magnetic field and rotation speed.
3.
Question 1
Describe a simple form of an alternating current (a.c.) generator, explaining the function of the rotating coil and the stationary magnets. Explain why slip rings and brushes are necessary in this type of generator.
A simple a.c. generator typically consists of a rotating coil of wire within a stationary magnetic field. The coil is usually a rectangular piece of conducting wire, often wound into coils to increase the voltage output. The magnets are arranged around the coil, creating a magnetic field. These magnets can be permanent magnets or electromagnets.
As the coil rotates, the magnetic flux linking the coil changes continuously. This changing magnetic flux induces an electromotive force (EMF) in the coil, according to Faraday's law of electromagnetic induction. Because the direction of the magnetic flux is constantly changing with the rotation, the induced EMF is also constantly changing in magnitude and direction, resulting in an alternating current (a.c.).
Slip rings and brushes are essential for continuously supplying electricity to the rotating coil. The slip rings are conductive rings attached to the rotating coil. They provide a low-resistance path for electrical contact. Brushes, typically made of carbon, are fixed and make continuous contact with the slip rings. This allows electricity to flow into and out of the rotating coil as it spins. Without slip rings and brushes, the electrical circuit would be broken as the coil rotated, preventing the generation of a continuous a.c. current. The brushes ensure a consistent electrical connection, even as the coil rotates.