Diagram Decoding
Head-On Collision Mystery
P and Q are identical positive charges moving left at equal speed.
Your task: deduce the unseen magnetic field from their curved tracks.
Key Points:
- Lorentz force: \( \mathbf{F}=q\mathbf{v}\times\mathbf{B} \) ⟂ velocity, so paths turn circularly.
- Use Fleming’s left-hand rule to decide if \( \mathbf{B} \) is into or out of the page.
- Field magnitude formula: \( B = \frac{m v}{q r} \).
Section C Sketch
Grazing Ray in Right-Angle Prism
Goal: translate prism geometry into its refractive index.
A horizontal ray meets the 45° face and refracts to travel along the base.
Key Points:
- Snell’s law at play: \(1\sin 45^{\circ}=n\sin 90^{\circ}\Rightarrow n=\sqrt{2}\).
- Critical angle check: grazing ray implies \(r=90^{\circ}\) so \(C=45^{\circ}\).
- Identical reasoning earns 3 marks in CBSE Section C.
Case Study Tactics
Electron path in uniform magnetic field
Electron’s Motion in Magnetic Field
Decide if motion is circular or helical. Then predict how speed alters radius, pitch, but never period.
Key Points:
- \(v \perp B\) only ⇒ circle; \(r=\frac{mv}{qB}\).
- Add \(v_{\parallel}\) ⇒ helix; pitch \(p=v_{\parallel}T\).
- Doubling \(v\) doubles \(r\) & \(p\); \(T=\frac{2\pi m}{qB}\) stays same.
- Spot these patterns to ace motion-in-field questions.
Graph Questions
Reading the Binding-Energy Peak
Binding energy per nucleon rises and peaks near iron, then slowly falls.
Reactions that move nuclei uphill on this curve release the energy gap.
Key Points:
- Heavy nuclei (A > 200) split—fission drives fragments toward the peak, releasing ≈ 200 MeV.
- Light nuclei (A < 20) fuse—fusion climbs the slope and powers stars.
- Iron–nickel peak is a stability plateau; further change yields almost no energy.