SET 05

🎯 Objectives: TOWARD NECTA Physics - Set 5 Questions and Answers

SET 5: NECTA Form Four Conceptual Physics Questions and Answers

Explain the term thermionic emission and give its application.
Answer: Thermionic emission is the release of electrons from the surface of a heated metal. When metals are heated, the thermal energy enables electrons to overcome the attractive forces holding them in the atom.

Example: Used in cathode ray tubes (CRTs) and vacuum tubes.
What is meant by electric field?
Answer: An electric field is a region in which an electric charge experiences a force. It is represented by electric field lines showing the direction of the force on a positive test charge.
Explain how a transformer works.
Answer: A transformer operates on the principle of electromagnetic induction. When alternating current flows in the primary coil, it creates a changing magnetic field, inducing a voltage in the secondary coil.

Example: Used in power distribution to step up or step down voltage.
State two differences between concave and convex lenses.
Answer:
- Concave lenses diverge light rays; convex lenses converge light rays.
- Concave lenses are used to correct short-sightedness; convex lenses are used to correct long-sightedness.
What is Brownian motion and what does it prove?
Answer: Brownian motion is the random movement of particles suspended in a fluid, resulting from collisions with the fast-moving molecules of the fluid.

Proof: It proves the kinetic theory of matter — that molecules are in constant motion.
Describe how a simple microphone works.
Answer: Sound waves cause a diaphragm in the microphone to vibrate, which in turn moves a coil or varies a capacitor. These variations are converted into an electrical signal representing the sound.
What is the function of a fuse in an electrical circuit?
Answer: A fuse protects electrical appliances by breaking the circuit if the current exceeds a safe level, preventing damage or fire.
Why is the sky blue?
Answer: Due to Rayleigh scattering — shorter blue wavelengths scatter more than longer red wavelengths in the Earth's atmosphere, making the sky appear blue.
What is the function of a vacuum in a thermos flask?
Answer: The vacuum prevents heat transfer by conduction and convection, keeping the contents hot or cold for longer.
Explain why metals are good conductors of electricity.
Answer: Metals contain free electrons that move easily throughout the metal, allowing electric current to flow efficiently.
State the principle of moments.
Answer: For a body in equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments about the same point.
Why are X-rays dangerous?
Answer: X-rays are a form of ionizing radiation that can damage or kill living cells, potentially causing cancer or mutations.
Give two uses of ultrasonic waves.
Answer: Ultrasonic waves are used in medical imaging (ultrasound scanning) and in cleaning delicate objects like jewelry or electronic components.
State the law of conservation of energy.
Answer: Energy cannot be created or destroyed, only transformed from one form to another, but the total energy remains constant.
What is the role of a capacitor in an electronic circuit?
Answer: A capacitor stores electrical energy and can be used to smooth voltage fluctuations or filter signals in circuits.
Describe how lightning occurs.
Answer: Lightning occurs due to the discharge of static electricity built up between clouds and the ground or between clouds, creating a sudden spark and thunder.
Why does a current-carrying conductor experience a force in a magnetic field?
Answer: Due to the interaction between the magnetic field and the electric current, a force (Lorentz force) is exerted on the conductor — basis of electric motors.
What causes echoes and how can they be reduced?
Answer: Echoes are caused by sound waves reflecting off hard surfaces. They can be reduced by using sound-absorbing materials like curtains or foam panels.
Differentiate between series and parallel circuits.
Answer: In a series circuit, components share the same current; in a parallel circuit, each component has its own branch and voltage.
Why does a balloon inflate when heated?
Answer: Heating air inside the balloon increases the kinetic energy of molecules, causing expansion and inflation.
Explain why pressure increases with depth in a liquid.
Answer: As depth increases, the weight of the overlying liquid column increases, leading to greater pressure.
Why is mercury used in barometers?
Answer: Mercury has a high density, so it can measure atmospheric pressure with a short column, and it does not evaporate easily at room temperature.
State Newton’s third law of motion.
Answer: For every action, there is an equal and opposite reaction.

Example: When a gun is fired, the bullet moves forward, and the gun recoils backward.
What is latent heat?
Answer: Latent heat is the energy required to change the state of a substance without changing its temperature.
Explain how a periscope works.
Answer: A periscope uses mirrors placed at 45° angles to reflect light from an object into the viewer’s eye, allowing vision over obstacles.
What is the difference between real and virtual images?
Answer: Real images can be projected on a screen and are inverted; virtual images cannot be projected and are upright.
State two uses of a Geiger-Müller tube.
Answer:
- Detecting radiation from radioactive sources.
- Measuring background radiation levels.
Define refraction of light.
Answer: Refraction is the bending of light as it passes from one medium to another due to a change in speed.

Example: A straw appears bent when placed in water.
Why is tungsten used as a filament in bulbs?
Answer: Tungsten has a very high melting point and resists oxidation, making it ideal for producing light when heated.
Describe how a seismograph detects earthquakes.
Answer: A seismograph has a suspended mass that remains stationary as the ground shakes. The relative motion is recorded on paper, showing earthquake intensity and duration.

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