IGCSE Physics: Electricity, Magnetism and Atomic Physics

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IGCSE Physics: Electricity, Magnetism and Atomic Physics

TL;DR

Electric current flows when charge moves through a circuit; voltage drives it and resistance opposes it (Ohm's Law: V = IR). Magnets create fields that exert forces on moving charges and current-carrying wires. Atoms contain nuclei with protons and neutrons orbited by electrons; radioactivity happens when unstable nuclei decay, releasing energy and particles.

1. The Mental Model

You're dealing with three linked ideas: electricity (charges moving and the forces that push them), magnetism (invisible fields around moving charges and magnets), and atoms (the tiny structure underneath it all). Electricity and magnetism are really two sides of one phenomenon—moving charges create magnetic fields, and moving magnets create electric fields. Atomic physics explains why atoms behave the way they do and what happens when they become unstable. Think of electricity as "charge on the move," magnetism as "the push that moving charge creates," and atomic decay as "when nuclei can't hold themselves together."

2. The Core Material

Current, Voltage and Resistance

Electric current is the flow of charge through a circuit. You measure it in amperes (A). The thing that pushes the charge is voltage (measured in volts, V)—think of it as electrical "pressure." Resistance (ohms, Ω) is anything that fights the current.

These three are linked by Ohm's Law:
$$V = I \times R$$

If you double the voltage, current doubles. If you double the resistance, current halves. Real wires have resistance; so do light bulbs and heaters. When current flows through resistance, energy is wasted as heat—that's why a kettle gets hot.

Power (how much energy per second you're using) is:
$$P = V \times I \quad \text{or} \quad P = I^2 R$$

A 60 W bulb uses 60 joules of energy every second.

Circuits: Series and Parallel

In a series circuit, everything is in one loop. If one bulb burns out, the whole thing dies. Current is the same everywhere. Voltages add up: if you have two 3V batteries in series, you get 6V total. Resistances add: $R_{\text{total}} = R_1 + R_2 + R_3$.

In a parallel circuit, components sit on separate branches. If one bulb fails, the others still work. Voltage is the same across each branch (it's what the battery "sees"). Current splits between branches. Resistances behave oddly:
$$\frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}$$

Your home uses parallel wiring—every outlet gets full voltage, and adding more devices doesn't dim the others.

Magnetism and Magnetic Fields

A magnet creates an invisible magnetic field (measured in tesla, T). Field lines come out the north pole and go in the south pole. If you move a wire through a magnetic field, or move a magnet past a wire, you induce a voltage—this is electromagnetic induction, the basis of generators and transformers.

When current flows through a wire in a magnetic field, the wire experiences a force (the motor effect):
$$F = B \times I \times L$$

where B is field strength, I is current, and L is the length of wire in the field. Direction? Use the left-hand rule: thumb = current, fingers = field, palm = force.

A solenoid (coil of wire) creates a magnetic field inside itself when current flows through it. More coils or more current = stronger field. This is how electromagnets work.

Atomic Structure

An atom has a tiny nucleus (containing protons and neutrons) surrounded by electrons orbiting at different energy levels. Protons have positive charge (+), electrons negative (−), neutrons neutral. Most of an atom is empty space.

• Proton number = number of protons = atomic number (defines the element).
• Mass number = protons + neutrons.
• Atoms are normally neutral because the number of electrons equals the number of protons.

If an atom gains or loses electrons, it becomes an ion (charged). If it gains an electron, it's negatively charged; if it loses one, positively charged.

Radioactivity and Decay

Some nuclei are unstable—they have too many neutrons or protons. To reach stability, they emit radiation:

• Alpha decay: nucleus ejects a helium nucleus (2 protons + 2 neutrons). Mass number drops by 4, proton number by 2.
• Beta decay: a neutron converts to a proton, ejecting an electron (and an antineutrino). Mass number stays the same, proton number increases by 1.
• Gamma decay: nucleus emits high-energy electromagnetic radiation. No change in proton or mass number.

Half-life is the time it takes for half the nuclei in a sample to decay. After one half-life, you have half left; after two, one quarter; after three, one eighth. This is exponential decay.

graph TD
    A["Unstable nucleus"] -->|Alpha decay<br/>loses 2p, 2n| B["New nucleus<br/>mass−4, proton−2"]
    A -->|Beta decay<br/>neutron→proton + e| C["New nucleus<br/>mass unchanged<br/>proton+1"]
    A -->|Gamma decay<br/>emits high-E photon| D["Same nucleus<br/>lower energy"]

3. Worked Example

Problem: A 12 V battery powers a circuit with three resistors: 2 Ω, 3 Ω, and 6 Ω. The 2 Ω and 3 Ω are in series with each other, and that pair is in parallel with the 6 Ω resistor. Find the total current from the battery and the power dissipated.

Step 1: Combine the series pair.
$$R_{\text{series}} = 2 + 3 = 5 \text{ Ω}$$

Step 2: Combine the 5 Ω with the 6 Ω parallel resistor.
$$\frac{1}{R_{\text{total}}} = \frac{1}{5} + \frac{1}{6} = \frac{6+5}{30} = \frac{11}{30}$$
$$R_{\text{total}} = \frac{30}{11} \approx 2.73 \text{ Ω}$$

Step 3: Use Ohm's Law to find total current.
$$I = \frac{V}{R} = \frac{12}{2.73} \approx 4.4 \text{ A}$$

Step 4: Calculate power.
$$P = V \times I = 12 \times 4.4 = 52.8 \text{ W}$$

The battery supplies about 4.4 amperes and dissipates roughly 53 watts as heat across the resistors.

Radioactivity example: A sample contains 800 nuclei of an isotope with a half-life of 10 years. How many nuclei remain after 30 years?

• After 10 years (1 half-life): $800 ÷ 2 = 400$
• After 20 years (2 half-lives): $400 ÷ 2 = 200$
• After 30 years (3 half-lives): $200 ÷ 2 = 100$

So 100 nuclei remain; 700 have decayed.

4. Examiner's Breakdown

4.1 What Examiners Actually Reward

1. State clearly whether components are in series or parallel before calculating. Examiners mark this first; if you mix them up, everything after is wrong.
2. Always rearrange Ohm's Law correctly. Write "$I = V ÷ R$" or "$V = IR$" explicitly in working—don't jump straight to a number.
3. For combined resistors, show the reciprocal formula step-by-step (especially for parallel). "$1/R_t = 1/R_1 + 1/R_2$" then "$R_t = ...$" gets full marks; skipping steps loses marks.
4. In radioactivity, write the decay equation with all particles. Don't just say "alpha decays." Write: "$^{210}_{84}\text{Po} \to ^{206}_{82}\text{Pb} + ^{4}_{2}\text{He}$" to show you conserved mass and proton numbers.
5. Define half-life as "the time for half the nuclei to decay" in your own words, not a textbook copy—examiners can tell.
6. When describing the left-hand rule, name all three directions: "Thumb points in the direction of current, fingers point in the direction of the magnetic field, and the palm pushes in the direction of the force."

4.2 Trapdoor Mistakes

4.3 Score-Boosting Comparisons

5. Now Try It

Exam-style question (25 mins):

A student sets up a circuit with a 6 V battery and three resistors: 10 Ω, 15 Ω, and 30 Ω. The 10 Ω and 15 Ω resistors are connected in series, and this combination is connected in parallel with the 30 Ω resistor.

(a) Calculate the total resistance of the circuit.
(b) Calculate the total current supplied by the battery.
(c) Calculate the power dissipated in the 30 Ω resistor.
(d) A radioactive source has a half-life of 50 days. Starting with 960 nuclei, how many remain after 150 days?

Model answer must include:
- Step 1: Series combination ($R = 10 + 15 = 25$ Ω).
- Step 2: Parallel formula ($1/R_t = 1/25 + 1/30$), then $R_t ≈ 13.6$ Ω.
- Step 3: Total current ($I = 6 ÷ 13.6 ≈ 0.44$ A).
- Step 4: Voltage across the 30 Ω branch (same as battery, 6 V).
- Step 5: Power in 30 Ω resistor ($P = V^2 ÷ R = 36 ÷ 30 = 1.2$ W).
- Part (d): Three half-lives (150 ÷ 50 = 3), so $960 ÷ 2 ÷ 2 ÷ 2 = 120$ nuclei remain.

Success looks like: All five calculation steps shown with correct rearrangement of formulas, series and parallel clearly identified, and radioactivity calculation split into individual half-lives.

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