Question 1
Write true or false for each statement:
(a) The image formed by a plane mirror is real.
(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the angle of reflection.
(c) The image of the right hand in a plane mirror looks like that of a left hand.
(d) The image formed by a plane mirror is upright.
(e) The image formed by a plane mirror can be obtained on a screen.
(f) The objects around us are seen due to irregular reflection of light.
(g) The speed of light in vacuum is 3 x 108 m s-1.
(h) A rose appears red in light of all the colours.
(i) A black paper absorbs light of all the colours and reflects none.
(j) The primary colours are red, blue and green.
(a) False
Correct Statement — The image formed by a plane mirror is virtual.
(b) False
Correct Statement — When a light ray is reflected from a wall, the angle of incidence is equal to the angle of reflection.
(c) True
(d) True
(e) False
Correct Statement — The image formed by a plane mirror cannot be obtained on a screen.
(f) True
(g) True
(h) False A rose appears red in white light. Correct Statement —
(i) True
(j) True
ICSE Class 7 Physics — Energy: Complete Study Guide
Energy is a central concept in ICSE Class 7 Physics, carrying approximately 12 marks. This chapter introduces students to the universal principle that energy drives all processes in nature and technology. From the food we eat to the devices we use, every activity involves energy transformation.
Students learn nine forms of energy: kinetic, potential, heat, light, sound, chemical, electrical, nuclear, and mechanical. The distinction between kinetic energy (energy of motion) and potential energy (stored energy due to position or shape) is fundamental. Understanding energy transformation chains — how a car converts chemical energy in fuel to heat to mechanical motion — helps students see physics at work in everyday life. The law of conservation of energy, which states that energy can neither be created nor destroyed but only transformed, is one of the most important principles in all of science.
The chapter also addresses the crucial real-world topic of energy sources — renewable (solar, wind, hydro) versus non-renewable (coal, oil, gas) — and the importance of energy conservation in the face of growing global demand and environmental concerns.
| Concept | Key Points |
|---|---|
| Kinetic Energy | Energy of motion; depends on mass and speed |
| Potential Energy | Stored energy; gravitational (height) or elastic (stretched/compressed) |
| Conservation of Energy | Total energy remains constant; only changes form |
| SI Unit | Joule (J) |
Must-Know Concepts
- Energy = ability to do work. SI unit: Joule (J)
- KE depends on mass and speed; PE depends on height/mass or stretch/compression
- Energy transformation happens in every device: electrical → light (bulb), chemical → heat (burning)
- In a pendulum: PE is max at extremes, KE is max at centre; total is constant
- Renewable sources can be replenished; non-renewable took millions of years to form
Common Mistakes
- Saying energy is "used up" or "destroyed" — it only changes form
- Forgetting elastic PE — it is not only about height; stretched objects also have PE
- Incomplete transformation chains — always include heat and sound as by-products
- Confusing energy sources (solar, coal) with energy forms (light, chemical)
Exam Tips
- Draw energy transformation chains using arrows for full marks
- Use the pendulum or falling ball as your go-to example for conservation of energy
- For renewable vs non-renewable, give at least 3 examples of each in a table
- When discussing "energy crisis," clarify that it means shortage of useful forms, not destruction of energy
What is the law of conservation of energy?
Energy can neither be created nor destroyed. It can only be transformed from one form to another. The total energy of an isolated system remains constant. For example, in a swinging pendulum, potential energy converts to kinetic energy and back, but the total remains the same.
Why does a bouncing ball not return to its original height?
When a ball bounces, some of its kinetic energy is converted to heat (from deformation) and sound (the bouncing noise). Less energy is available to convert back to potential energy, so the ball reaches a lesser height. Total energy is still conserved — it has just changed to less useful forms.
What is the difference between renewable and non-renewable energy sources?
Renewable sources (solar, wind, hydro, biomass) can be replenished naturally in a short time and will not run out. Non-renewable sources (coal, petroleum, natural gas, nuclear fuel) took millions of years to form, exist in limited quantities, and will eventually be exhausted.