Unit 18: Electromagnetic Induction & EM Waves

Lenz's Law: The direction of induced current is always such that it opposes the cause which produces it.

Relation with the Law of Conservation of Energy: This law is a direct consequence of the conservation of energy. If the induced current were to reinforce the change instead of opposing it, it would lead to a perpetual increase in energy, which is impossible.

When a magnet is pushed toward a coil, the coil's induced current will produce a magnetic field that repels the magnet, resisting its motion.

Conversely, if the magnet is pulled away, the induced current will create a field that attracts the magnet, again opposing the movement.

Real-Life Example: In a bicycle dynamo, the magnet rotates near a coil, generating electricity. The induced current creates a magnetic field that opposes the motion of the magnet. This opposition means the rider has to work harder to keep pedaling, which is energy being converted into electrical energy. This demonstrates the law of conservation of energy—the mechanical energy from pedaling is converted into electrical energy, not created from nothing.

Transformers are used to either increase or decrease voltage in A.C. circuits.

Feature	Step-up Transformer	Step-down Transformer
Definition	A step-up transformer is a transformer in which the secondary coil has more turns than the primary coil, so it increases the output voltage.	When the secondary coil has fewer turns than the primary coil, the transformer reduces the output voltage. It is called a step-down transformer.
Number of Turns	Secondary coil has more turns than the primary coil. \(N_s > N_p\)	Secondary coil has fewer turns than the primary coil. \(N_s < N_p\)
Voltage Output	Output voltage is greater than the input voltage. \(V_s > V_p\)	Output voltage is less than the input voltage. \(V_s < V_p\)
Use	Used to increase voltage for long-distance transmission.	Used to reduce voltage to safe levels in homes.

Turns-Ratio Equation:

This equation shows that the voltage ratio is directly proportional to the number of turns in the transformer coils.

Sunlight is made up of all the colours of the visible spectrum; red, orange, yellow, green, blue, indigo, and violet. As this light passes through the atmosphere, it interacts with tiny gas molecules. Blue light, having a shorter wavelength, scatters more easily than red or green light.

Blue Sky During the Day: During daytime, the scattered blue light reaches us from all directions, making the sky appear blue.

Red Sky at Sunset: During sunrise or sunset, the sunlight has to pass through a longer path in the atmosphere. By the time the light reaches us, much of the blue light has already been scattered out of the path. What remains are the longer wavelengths like red and orange, which scatter less.

These colours dominate the sky near the horizon, giving it the warm, reddish hues we often see in the evening.

CRO: A Cathode Ray Oscilloscope (CRO) is a widely used electronic instrument designed to display, observe, and measure how electrical signals change with time.

It is used to display the waveform of a signal and analyze its characteristics.

Main Components

Electron Gun: The electron gun generates a focused beam of electrons. It contains a heated filament \(F\) that heats the cathode \(C\), causing it to emit electrons. These electrons are accelerated and focused by anodes, while a control grid \(G\) controls how many electrons reach the screen and thus adjusts the brightness of the spot formed.

Deflecting System: The deflecting system consists of two pairs of plates: the \(Y\)-plates, which control the vertical movement of the electron beam according to the input signal, and the \(X\)-plates, which control its horizontal movement.

Fluorescent Screen: The fluorescent screen is coated with a material that glows when the electron beam strikes it, leaving a visible trace that allows the waveform of the signal to be seen.

How it Helps in Analyzing Electrical Signals: CROs are extremely useful for measuring and analyzing electrical signals. They can:

• Show the waveform of a signal.
• Measure its amplitude.
• Determine its time period and frequency.
• Allow comparison of two signals to measure their phase difference.

A magnetic field exerts force on electrically charged particles that are in motion. Cathode rays generated from an electron gun are a stream of negatively charged electrons. Electrons, while passing through the magnetic field, are deflected as shown in Fig. 18.10. To determine the direction of force and thus the deflection of the electron beam, Fleming's left-hand rule can be used.

Note: The provided chapter does not mention the shape of the path followed by an electron moving at right angles to a uniform magnetic field.