ACT SSC Physics · Unit 3
ACT SSC Physics Unit 3: Gravity and Electromagnetism — Flashcards & Quiz
ACT SSC Physics Unit 3 covers gravity and electromagnetism within the BSSS framework. This unit explores gravitational fields, orbital mechanics, Coulomb’s law, electric fields, magnetic fields, electromagnetic induction, motors, generators and transformers. These flashcards and quiz questions help you revise the key concepts tested in ACT assessments.
Key Terms
- Gravitational Field Strength
- The force per unit mass experienced by an object in a gravitational field (g = F/m), measured in N/kg; a key calculation in BSSS Physics Unit 3 assessments on orbital mechanics and satellite motion.
- Coulomb's Law
- The law stating that the electrostatic force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them; central to ACT SSC electrostatics.
- Electric Field
- A region of space around a charged object where another charge would experience a force, with field strength E = F/q measured in N/C; assessed through field line diagrams in BSSS tasks.
- Magnetic Flux
- The total magnetic field passing through a given area, calculated as the product of field strength, area and the cosine of the angle between them; the foundational quantity for electromagnetic induction in ACT SSC Physics.
- Faraday's Law of Induction
- The induced EMF in a circuit equals the negative rate of change of magnetic flux through the circuit; a quantitative relationship tested extensively in BSSS electromagnetism assessments.
- Lenz's Law
- The direction of an induced current always opposes the change in magnetic flux that produced it, ensuring conservation of energy; used alongside Faraday's law in ACT SSC problem-solving.
- Orbital Velocity
- The speed at which a satellite must travel to maintain a stable circular orbit at a given altitude, derived from balancing gravitational and centripetal forces in BSSS gravity problems.
Sample Flashcards
Q1: Define wavelength, frequency, amplitude and period.
Wavelength (λ): distance between two consecutive corresponding points (e.g. crest to crest). Frequency (f): number of complete cycles per second (Hz). Amplitude (A): maximum displacement from equilibrium. Period (T): time for one complete cycle; T = 1/f.
Q2: State the wave equation.
v = fλ, where v is wave speed (m/s), f is frequency (Hz), and λ is wavelength (m). This applies to all types of waves.
Q3: Distinguish between transverse and longitudinal waves.
Transverse: oscillations are perpendicular to the direction of energy transfer (e.g. light, water surface waves). Longitudinal: oscillations are parallel to the direction of energy transfer (e.g. sound, compression waves in a spring).
Q4: State the law of reflection.
The angle of incidence equals the angle of reflection (θ_i = θ_r). Both angles are measured from the normal (perpendicular) to the reflecting surface. The incident ray, reflected ray, and normal all lie in the same plane.
Q5: What is refraction and why does it occur?
Refraction is the bending of a wave as it passes from one medium to another due to a change in speed. When light enters a denser medium it slows down and bends toward the normal; entering a less dense medium it speeds up and bends away.
Q6: State Snell’s law and give the equation.
Snell’s law: n₁ sinθ₁ = n₂ sinθ₂, where n is the refractive index and θ is the angle from the normal. The refractive index n = c/v, where c is the speed of light in vacuum and v is the speed in the medium.
Q7: What is total internal reflection and when does it occur?
Total internal reflection (TIR) occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle (θ_c). All light is reflected back into the denser medium. sinθ_c = n₂/n₁ (where n₁ > n₂).
Q8: What is diffraction?
Diffraction is the spreading of waves as they pass through a gap or around an obstacle. Maximum diffraction occurs when the gap width is approximately equal to the wavelength. All waves can diffract.
Sample Quiz Questions
Q1: Frequency and period are inversely proportional.
Answer: TRUE
T = 1/f, so doubling frequency halves the period. They are inversely related.
Q2: Sound waves are transverse waves.
Answer: FALSE
Sound waves are longitudinal — particles oscillate parallel to the direction of energy transfer (compressions and rarefactions).
Q3: All electromagnetic waves travel at the same speed in a vacuum.
Answer: TRUE
All EM waves travel at c = 3 × 10⁸ m/s in a vacuum, regardless of frequency or wavelength.
Q4: The angle of incidence is measured from the surface, not the normal.
Answer: FALSE
The angle of incidence is always measured from the NORMAL (perpendicular to the surface), not the surface itself.
Q5: Light bends toward the normal when it enters a denser medium.
Answer: TRUE
When entering a denser medium (higher n), light slows down and bends toward the normal.
Why It Matters
Gravity and electromagnetism in ACT SSC Physics Unit 3 explores the fundamental forces that govern motion on cosmic and everyday scales. BSSS assessments test your ability to apply Newton’s law of universal gravitation, analyse orbital mechanics, calculate electric and magnetic field strengths, and explain electromagnetic induction. This unit connects gravitational theory to satellite motion and planetary orbits, while linking electric and magnetic phenomena to the operation of motors, generators and transformers. Students who develop strong quantitative skills in field calculations and can explain how these forces shape technology and the universe consistently achieve the highest results. Gravitational and electromagnetic field concepts bridge to the modern physics unit, where photon energy and de Broglie wavelength calculations depend on understanding field interactions at the quantum scale. BSSS exam questions on gravity and electromagnetism commonly require you to compare gravitational and electric field equations, so practise identifying the structural parallels between Newton's law of gravitation and Coulomb's law.
Key Concepts
Gravitational Fields
Newton’s law of universal gravitation describes the attractive force between masses. Understanding gravitational field strength, calculating orbital velocities and periods, and analysing satellite motion and planetary orbits are fundamental skills assessed throughout this unit.
Electric Fields
Coulomb’s law quantifies the force between charged objects. Understanding electric field strength, potential difference, and the behaviour of charges in uniform and radial fields allows you to solve quantitative problems and explain electrostatic phenomena tested in BSSS assessments.
Magnetic Fields
Moving charges and current-carrying conductors create and experience magnetic fields. Understanding the motor effect, the force on a current in a magnetic field, and the operation of DC motors connects electromagnetic theory to practical applications.
Electromagnetic Induction
Faraday’s law and Lenz’s law explain how changing magnetic flux induces an EMF. Understanding how generators, transformers and alternators work, and performing calculations involving flux, EMF and turns ratios, is essential for BSSS assessments.
Common Mistakes to Avoid
- Confusing gravitational field strength (g) with the gravitational constant (G) in ACT SSC calculations — g varies with location while G is a universal constant; always check which value a question requires.
- Forgetting the negative sign in Faraday's law when determining the direction of induced EMF — BSSS examiners expect application of Lenz's law to establish that the induced current opposes the flux change.
- Drawing electric field lines that cross each other — field lines never intersect; this error immediately signals a misunderstanding in ACT Board of Senior Secondary Studies assessments.
- Using the wrong formula for force in a magnetic field — BSSS assessments distinguish between F = qvB for a moving charge and F = BIL for a current-carrying conductor; identify the scenario before selecting the equation.
Study Tips
- Practise gravitational and electric field calculations in varied contexts, paying careful attention to units and the direction of forces and fields.
- Create flashcards for key equations including Newton’s law of gravitation, Coulomb’s law, and Faraday’s law, reviewing with spaced repetition.
- Draw labelled field diagrams for gravitational, electric and magnetic fields, clearly showing field lines, direction and relative strength.
- Work through electromagnetic induction problems by always identifying the changing flux first, then applying Faraday’s and Lenz’s laws systematically.
- Connect theoretical concepts to real technologies such as satellites, electric motors, generators and transformers to build applied understanding.
- Before your exam, work through the practice questions in this set at least twice using spaced repetition. Testing yourself repeatedly is the most effective revision strategy for long-term retention.
Related Topics
Frequently Asked Questions
What does ACT SSC Physics Unit 3 cover?
Unit 3 covers gravitational fields, Newton’s law of universal gravitation, orbital mechanics, electric fields, Coulomb’s law, magnetic fields, electromagnetic induction, motors, generators and transformers.
How many flashcards are in this set?
This free set contains 20 flashcards and 20 true/false quiz questions covering all key gravity and electromagnetism concepts, aligned to the BSSS Physics framework.
Are these flashcards aligned to the ACT curriculum?
Yes — every flashcard and quiz question is mapped to the BSSS Science Framework for ACT SSC Physics Unit 3.
Last updated: March 2026 · 20 flashcards · 20 quiz questions · Content aligned to the BSSS Framework