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Class 9–10 · Physics

Modern physics and electronics

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Modern physics and electronics

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.
Lessons in this chapter
  1. Nuclei and isotopes
  2. Photons and photoelectric effect
  3. Radioactivity
  4. Half-life
  5. Fission, fusion and energy
  6. Semiconductors and doping
  7. Diodes and rectification
  8. Transistors
  9. Digital signals and gates
  10. Electronics in communication

Lessons

1

Nuclei and isotopes

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

Atomic number Z counts protons; mass number A counts protons plus neutrons. Isotopes share Z but differ in neutron count. Electron-orbit sketches are simplified models of quantum states, not literal tracks.

Key relationship

A = Z+N

Worked example

Carbon-14: Z = 6, A = 14 ⇒ 8 neutrons; neutral atom has 6 electrons

2

Photons and photoelectric effect

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

A photon carries energy proportional to frequency. Photoelectric emission requires enough energy to overcome a material's work function. Below threshold, raising intensity does not produce emission in the ordinary single-photon process.

Key relationship

E = hf; Kmax = hf−φ

Worked example

Double frequency ⇒ double photon energy; higher intensity at same frequency mainly adds photons

3

Radioactivity

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

Unstable nuclei decay spontaneously. Alpha particles are helium nuclei, beta-minus particles are electrons created in decay, and gamma rays are electromagnetic photons. Penetration and ionisation differ; gamma is uncharged and is not deflected by an electric field.

Key relationship

α: A→A−4, Z→Z−2; β⁻: A unchanged, Z→Z+1

Worked example

Uranium-238 (Z = 92) emits α ⇒ A = 234, Z = 90

4

Half-life

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

Half-life is the time for half a large population of unstable nuclei to decay. Individual events are unpredictable. Equal intervals remove the same fraction, not the same number. Activity is decays per second.

Key relationship

N = N₀(½)^(t/T½); 1 Bq = 1 decay/s

Worked example

Half-life 5 days: 80 units after 15 days ⇒ 80/2³ = 10 units

5

Fission, fusion and energy

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

Fission splits heavy nuclei; fusion combines light ones. In energy-releasing reactions, a decrease in total rest mass corresponds to released energy. Fission neutrons may sustain a chain reaction. Fusion powers the Sun. Reactors require control and cooling.

Key relationship

E = Δmc²

Worked example

Δm = 10⁻⁶ kg ⇒ E = 9×10¹⁰ J

6

Semiconductors and doping

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

Semiconductor conductivity can be controlled. Pentavalent doping produces electron-majority n-type material; trivalent doping produces hole-majority p-type. The bulk remains electrically neutral despite the carrier labels.

Key relationship

n-type: electrons; p-type: holes, majority carriers

Worked example

Silicon + phosphorus → n-type; silicon + boron → p-type

7

Diodes and rectification

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

A p–n junction develops a depletion region and barrier. Forward bias lowers the barrier; reverse bias normally permits little current until limits are exceeded. Rectifiers produce unidirectional pulsating output; filters smooth it.

Key relationship

forward bias: p positive, n negative

Worked example

Single-diode half-wave rectifier mainly passes one AC half-cycle.

8

Transistors

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

A bipolar transistor has emitter, base and collector. A small base signal controls collector current. Amplification draws energy from a supply. Switching uses near-off and saturated states, where a simple active-region gain formula may not apply.

Key relationship

Ie = Ib+Ic; active region β ≈ Ic/Ib

Worked example

β = 100, Ib = 20 μA ⇒ Ic ≈ 2 mA in active region

9

Digital signals and gates

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

Digital circuits assign voltage ranges to 0 and 1. AND requires both high; OR at least one; NOT inverts. NAND and NOR invert AND and OR. XOR is high when its two inputs differ. Check every input combination in a truth table.

Key relationship

AND 00→0,01→0,10→0,11→1; OR 00→0,01→1,10→1,11→1

Worked example

A = 1, B = 0 ⇒ AND 0, OR 1, NAND 1, NOR 0, XOR 1

10

Electronics in communication

NtimeN₀N₀/2N₀/4
Each half-life halves the expected remaining population.

A microphone converts sound into an electrical signal. A transmitter encodes information onto a carrier and a receiver recovers it. Analogue signals vary continuously; digital systems use defined levels. LEDs and solar cells convert energy in opposite directions.

Key relationship

information → signal → transmission → reception → recovered information

Worked example

Phone speech can cross space as encoded electromagnetic signals, not as unaided sound in vacuum.

Check your understanding

Try answering first, then open the question to compare your reasoning.

1. A = 23, Z = 11: neutron number?

N = 23−11 = 12

2. Fraction left after three half-lives?

(½)³ = 1/8

3. Which is undeflected electrically: alpha, beta or gamma?

Gamma: uncharged electromagnetic radiation

4. Majority carriers in n-type material?

Electrons; material remains approximately neutral

5. Inputs 1 and 0: AND and OR?

AND = 0; OR = 1

Sources and scope

Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.