CIE iGCSE Co-ordinated Sciences-P4.5.2 The a.c. generator- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-P4.5.2 The a.c. generator – Study Notes

CIE iGCSE Co-ordinated Sciences-P4.5.2 The a.c. generator – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

Supplement

Describe a simple form of a.c. generator (rotating coil) and the use of slip rings and brushes where needed
Sketch and interpret graphs of e.m.f. against time for simple a.c. generators

Simple a.c. Generator (Rotating Coil)

An a.c. (alternating current) generator is a device that converts mechanical energy into electrical energy by electromagnetic induction.

Basic Construction:

A rectangular coil of wire placed between the poles of a strong magnet.
The coil is rotated mechanically (by a turbine, hand crank, engine, etc.).
Two slip rings are connected to the ends of the coil. They rotate with the coil.
Carbon brushes press against the slip rings to maintain electrical contact, transferring the induced current to the external circuit.

Working Principle:

As the coil rotates, it cuts the magnetic field lines.
An e.m.f. is induced in the coil according to Faraday’s Law of electromagnetic induction.
The direction of current is given by Fleming’s Right-Hand Rule.
Because the coil continuously changes orientation, the induced current reverses direction every half-turn → producing an a.c. (alternating current).

Role of Slip Rings and Brushes:

Slip Rings: Keep the connections from the rotating coil to the external circuit continuous, but allow the coil to rotate freely without twisting the wires.
Carbon Brushes: Press against the slip rings to provide a low-friction electrical contact, carrying current out to the external circuit.

Output:

The output is an a.c. waveform (sine wave), with the voltage and current reversing direction every half turn of the coil.

Applications of a.c. Generators:

Example :

Why does an a.c. generator produce alternating current instead of direct current?

▶️ Answer/Explanation

Step 1: As the coil rotates, the sides of the coil cut the magnetic field lines in opposite directions.

Step 2: This causes the induced current to flow in opposite directions on each half-turn.

Step 3: Therefore, the current reverses direction every half rotation of the coil.

Final Answer: The generator produces an alternating current because the induced e.m.f. changes direction as the coil rotates.

Graph of e.m.f. vs Time for a Simple a.c. Generator

Key Idea: As the coil in an a.c. generator rotates in a magnetic field, the induced e.m.f. varies with time in a sine wave pattern.

Shape of Graph:

The e.m.f. is zero when the coil is vertical (parallel to field lines, no flux cut).
The e.m.f. is at a maximum positive value when the coil is horizontal and cutting field lines most rapidly.
As the coil continues to rotate, the e.m.f. decreases back to zero, then reaches a maximum negative value (reversing direction).
This cycle repeats every full rotation, producing a sine wave.

Interpretation:

One complete cycle of the sine wave corresponds to one full revolution of the coil.
The frequency of the a.c. depends on the speed of rotation of the coil.
The peak (amplitude) of the sine wave depends on the strength of the magnetic field, the number of coil turns, and the speed of rotation.

Graph Features:

y-axis: Induced e.m.f. (positive = one direction of current, negative = opposite direction).
x-axis: Time.
The graph is a smooth sine wave alternating between positive and negative values.

Example :

How does doubling the rotation speed of the generator affect the e.m.f.–time graph?

▶️ Answer/Explanation

Step 1: Faster rotation → more flux is cut per second → larger induced e.m.f.

Step 2: Frequency of the sine wave doubles because the coil completes more rotations per second.

Final Answer: The graph has larger amplitude and shorter wavelength (higher frequency) when rotation speed is doubled.