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Inquiry Question 2: How is it known that atoms are made up of protons, neutrons and electrons?

Investigate, assess and model the experimental evidence supporting the existence and properties of the electron, including cathode ray tube experiments and Thomson's determination of the charge-to-mass ratio of the electron

Q: 2019 HSC HSC: Outline three observations that established the particle nature of cathode rays, and one observation that ruled against them being electromagnetic waves.

Three observations supporting particle nature: 1. Cathode rays are deflected by electric fields, in a direction consistent with them carrying negative charge. Electromagnetic waves are uncharged and would not deflect. 2. Cathode rays are deflected by magnetic fields. They follow a curved path through the field, as expected for moving charged particles experiencing $qv \times B$. 3. A small object placed in the beam casts a sharp shadow, consistent with particles travelling in straight lines from the cathode. Cathode rays also turn a small paddle wheel placed in their path, transferring momentum as particles do. Observation against electromagnetic waves: the rays are deflected by electric fields. Electromagnetic waves carry no net charge and are not deflected by static electric fields. (Equivalently, the $q/m$ measurement made cathode rays definite particles.) Markers reward three particle-nature observations and one wave-incompatible observation, with clear reasoning.

A focused answer to the HSC Physics Module 8 dot point on the discovery and properties of the electron. Cathode ray tubes and the particle vs wave debate, Thomson's crossed-field experiment to measure the charge-to-mass ratio e/m, and his plum-pudding model of the atom.

Generated by Claude OpusReviewed by Better Tuition Academy8 min answerUpdated 2026-05-18

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What this dot point is asking

NESA wants you to describe the cathode ray tube experiments of the late 19th century, summarise the evidence that cathode rays are negatively charged particles (later called electrons), and describe Thomson's apparatus and reasoning by which he measured the charge-to-mass ratio $e/m$ . You should also state Thomson's plum-pudding model of the atom as the model that the discovery of the electron immediately suggested.

The answer

Cathode ray tubes

A cathode ray tube is a sealed glass tube containing two electrodes and a low-pressure gas. A high voltage applied between the cathode (negative electrode) and anode (positive electrode) produces a stream of "cathode rays" travelling from cathode to anode. The rays make the residual gas glow and produce fluorescence on a screen at the far end of the tube.

By the 1890s the question was: what are cathode rays? Two camps:

the wave camp (mainly German physicists) thought cathode rays were a wave phenomenon in the aether, somewhat like light,
the particle camp (mainly British physicists) thought they were streams of charged particles.

Evidence for particles

Several observations pointed to particles:

A small obstacle placed in the beam casts a sharp shadow, consistent with straight-line travel.
A small paddle wheel placed in the path is set spinning, indicating that the rays carry momentum.
A magnetic field deflects the rays along a curved path, with direction consistent with negatively charged particles.
An electric field (between two parallel plates) deflects them in the direction expected of negative charges.
They are emitted in any direction from the cathode (regardless of where the anode is), suggesting emission from the cathode metal itself.

The deflection by an electric field was particularly damning for the wave model: an electromagnetic wave carries no net charge and is not deflected by static $\vec{E}$ .

Thomson's crossed-field experiment (1897)

J. J. Thomson designed an apparatus to measure properties of the cathode rays themselves. Inside the tube he added two horizontal parallel plates creating a uniform vertical electric field $\vec{E}$ , and a pair of coils producing a horizontal magnetic field $\vec{B}$ perpendicular to both $\vec{E}$ and to the beam direction. The combination is a "velocity selector":

Electric force on an electron moving along the beam: $F_E = qE$ (vertical).
Magnetic force: $F_B = qvB$ (vertical, opposite to $F_E$ when $\vec{E}$ and $\vec{B}$ are arranged correctly).

When the two forces are balanced, the beam passes undeflected. Setting $qE = qvB$ gives:

v = \frac{E}{B}

This is one equation in the wanted ratios, independent of $q$ and $m$ .

Thomson then switched off $\vec{B}$ and measured the vertical deflection $y$ caused by $\vec{E}$ alone over the plate length $L$ . The electron experiences acceleration $a = qE/m$ for a time $t = L/v$ while between the plates. The deflection is:

y = \tfrac{1}{2} a t^2 = \tfrac{1}{2} \frac{q E}{m} \frac{L^2}{v^2}

Solving for the charge-to-mass ratio and substituting $v = E/B$ :

\frac{q}{m} = \frac{2 y v^2}{E L^2} = \frac{2 y E}{B^2 L^2}

His value: $q/m \approx 1.76 \times 10^{11}$ C/kg.

What Thomson learned

The measured $q/m$ for cathode rays is about 1800 times larger than for the lightest known ions (hydrogen). Two interpretations were possible: cathode-ray particles either have much larger charge or much smaller mass than hydrogen ions. The same ratio was obtained from any cathode metal (aluminium, platinum, iron), so the particles were a universal constituent. Charge measurements (later refined by Millikan) confirmed the small-mass interpretation.

Thomson concluded:

Cathode rays are streams of negatively charged particles.
These particles (electrons) are much lighter than any atom.
They are present in every kind of matter.

The electron was the first known sub-atomic particle, and the result implied that atoms have internal structure.

Thomson's plum-pudding model

If atoms are electrically neutral and contain negatively charged electrons, they must also contain positive charge. With no clearer picture available, Thomson proposed that atoms consist of a diffuse positively charged sphere with electrons embedded in it like plums in a pudding (or raisins in a bun). The model:

explained neutrality (total charge cancels),
accommodated the small mass of the electron compared to the atom,
predicted that atoms should respond to applied fields in simple ways.

The plum-pudding model survived only until 1909, when Geiger and Marsden's gold foil experiment (under Rutherford) revealed that the positive charge and almost all the mass of the atom are concentrated in a tiny central nucleus.

Worked example: deflection in a CRT

A beam of electrons enters a 4.0 cm region between parallel plates that produce a uniform field of $1.0 \times 10^4$ V/m. The electrons enter with speed $1.0 \times 10^7$ m/s. Find the vertical deflection while between the plates. ( $e/m = 1.76 \times 10^{11}$ C/kg.)

Acceleration: $a = (e/m) E = 1.76 \times 10^{11} \times 1.0 \times 10^4 = 1.76 \times 10^{15}$ m/s $^2$ .

Time inside plates: $t = L/v = 0.040 / 1.0 \times 10^7 = 4.0 \times 10^{-9}$ s.

Deflection: $y = \tfrac{1}{2} a t^2 = 0.5 \times 1.76 \times 10^{15} \times (4.0 \times 10^{-9})^2 = 1.4 \times 10^{-2}$ m = 1.4 cm.

Common traps

Confusing $e/m$ with $e$ . Thomson measured the ratio. The charge $e$ on its own required Millikan's oil-drop experiment.

Calling cathode rays X-rays. X-rays are high-frequency electromagnetic waves, produced when cathode rays strike the anode. Cathode rays themselves are electrons.

Saying the plum-pudding model is wrong because the nucleus exists. The plum-pudding model is wrong, but it was a reasonable model in 1897 given the data, and it correctly predicted neutrality with electrons inside the atom. It was overturned by direct experimental evidence (Geiger-Marsden), not by armchair reasoning.

Mixing up the directions of $\vec{E}$ and $\vec{B}$ in the velocity selector. They must be perpendicular to each other and to the beam, and arranged so that their forces are opposite. Picture the right-hand rule for the magnetic case and choose $\vec{E}$ to oppose it.

In one sentence

J. J. Thomson showed cathode rays are negatively charged particles with the same charge-to-mass ratio regardless of the cathode material, measured $e/m \approx 1.76 \times 10^{11}$ C/kg using crossed electric and magnetic fields, and proposed the plum-pudding model of the atom (negative electrons embedded in a positive sphere) as the immediate consequence.

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

2022 HSC5 marksDescribe how Thomson used crossed electric and magnetic fields to measure the charge-to-mass ratio of the electron. Include the key equations and the role of each field.

Show worked answer →

Thomson accelerated cathode rays through a potential difference and passed them between two parallel plates in a region with both an electric field $E$ (between the plates) and a magnetic field $B$ (perpendicular to $E$ and to the beam direction).

Step 1: balance the two forces so that the beam goes through undeflected. The electric force on an electron is $qE$ ; the magnetic force is $qvB$ . Setting them equal gives the beam velocity:

$v = E/B$ .

This velocity selector lets him measure $v$ from the field strengths alone.

Step 2: switch off the magnetic field. The beam is now deflected by $E$ only. The vertical deflection $y$ over a horizontal length $L$ inside the field, with no force after, gives:

$y = \frac{1}{2} a t^2 = \frac{1}{2} \frac{qE}{m} \left( \frac{L}{v} \right)^2$ .

Solving for the charge-to-mass ratio:

$\frac{q}{m} = \frac{2 y v^2}{E L^2} = \frac{2 y E}{B^2 L^2}$ .

Thomson obtained $q/m \approx 1.76 \times 10^{11}$ C/kg, far larger than the corresponding value for hydrogen ions. He concluded cathode rays are charged particles much lighter than the lightest atom.

Markers reward both fields and their roles, the velocity selector, the deflection equation, and the conclusion (small mass relative to atoms).

2019 HSC3 marksOutline three observations that established the particle nature of cathode rays, and one observation that ruled against them being electromagnetic waves.