Radioactivity and Magnetism

You never see it, only what it does

This unit bundles two topics that look unrelated, radioactivity and magnetism. There is one idea that unlocks both, and it is worth having before either. ⭐ You cannot see radiation, and you cannot see a magnetic field. You only ever see what they DO. Radiation is known by its effects: how much it IONISES the air it passes through, and how far it PENETRATES before it is stopped. Those two effects are exactly how alpha, beta and gamma are told apart. A magnetic field is known by its effects too: the force it exerts, turning a compass needle, or pushing on a wire that carries a current. That push has a name, the motor effect. ⭐⭐ So across the whole unit the skill is the same: read an effect back to its cause. What ionised the air? What pushed the wire? The invisible thing is inferred from the visible one. ⚠️ Carry that through both halves: I cannot see it, so what effect tells me it is there, and which kind it is?

Two invisibles, two sets of effects

The two halves of the unit share a shape: an invisible cause, detected only through what it does.

Order the three radiations

Put the three kinds of radiation in order, from the LEAST penetrating (stopped most easily) to the MOST penetrating.

• Alpha, a helium nucleus, stopped by a sheet of paper
• Beta, a fast electron, stopped by a few millimetres of aluminium
• Gamma, a high-energy wave, reduced only by thick lead or concrete

The most damaging up close

A source is swallowed, so the radiation acts inside the body. Which kind is the most dangerous there, and why?

• Alpha, because it is the most ionising, so inside the body, where nothing stops it first, it does the most damage to cells
• Gamma, because it is the most penetrating, so it is always the most dangerous
• Beta, because it travels fastest
• None: radiation inside the body is harmless

Words for the nucleus

Five terms, each defined so you can use it in a sentence rather than only recognise it.

Two half-lives on

A radioactive sample has a mass of 80 grams of undecayed material and a half-life of 6 hours. The undecayed amount halves once every half-life. How many grams of undecayed material remain after 12 hours?

Which nuclear statements hold

Select the TWO statements that are correct.

• Two isotopes of an element have the same number of protons but different numbers of neutrons
• Gamma is the most penetrating radiation and needs thick lead or concrete to reduce it
• Alpha is the most penetrating radiation and passes through lead
• After one half-life, all of the radioactive nuclei have decayed

The other invisible: magnetism

Now the second half of the unit, read with the same idea: a field you cannot see, known by the forces it makes. ⭐ A magnet has a field around it, and so does a wire, but only while a current flows through it. Switch the current off and the field around the wire is gone. ⭐ Wind that wire into a coil and you have an electromagnet, whose field you can strengthen with more current, more turns, or an iron core, and switch on and off at will. ⭐ Put a current-carrying wire into a magnetic field and it feels a force. That force is the motor effect, and it is how an electric motor turns. ⭐ Now run it backwards: MOVE a magnet near a coil, or move a wire through a field, and a voltage appears. This is electromagnetic induction, and it is how a generator makes electricity. ⚠️ The one word that matters for induction is CHANGING. A steady field sitting still induces nothing; the field must be changing, which is why something has to move.

Match each idea to what it is

Find what induction needs

A student writes about a generator. Select the ONE sentence that correctly states what is needed to induce a voltage.

• A generator contains a magnet and a coil of wire.
• A voltage is induced only when the magnetic field through the coil is changing, for example when the magnet moves.
• The stronger the magnet, the shinier the wire.
• Generators are used in power stations.
• I think generators are quite clever machines.

Build the two effects

Assemble a sentence that keeps the motor effect and induction the right way round.

You cannot see the wind

Leave physics aside and stand at a window on a blustery day. ⭐ You never actually see the wind. Air is invisible. And yet you know exactly where it is and how strong it is, because you see what it DOES: the trees bending, the washing flapping, the leaves chased across the grass. ⭐⭐ You can even tell things apart by their effects. A gentle breeze only stirs the leaves; a gale bends whole branches. The size of the effect tells you the strength of the cause. Nobody thinks this is strange. We are completely used to knowing an invisible thing is present, and how strong it is, purely from what it moves. Carry that back to this unit. A ray and a field are exactly like the wind: unseen, but read off confidently from the effects they leave, and told apart by how big those effects are.

Complete the unit facts

An atom with the same number of protons as another but a different number of neutrons is an _____. The time for half the undecayed nuclei in a sample to decay is the _____. Of the three radiations, _____ is the most ionising but the least penetrating. A magnetic field appears around a wire only while a _____ flows through it. A current-carrying wire placed in a magnetic field feels a push called the _____ effect, and moving a magnet near a coil creates a voltage, an effect called electromagnetic _____.

The radioactivity and magnetism run

Five quick decisions across both halves of the unit. Three lives.

Work out the invisible cause

Three situations give you an effect and ask you to name the invisible cause. Work through each.

• A radiation source is fully blocked by a single sheet of paper. Which kind is it, and how do you know?
• A compass needle near a straight wire swings the moment a switch is closed, and returns when it opens. What does the effect tell you?
• A coil produces a voltage while a magnet is pushed into it, but nothing when the magnet is held still inside. Why?

Explain how we know the invisible is there

A friend says radiation and magnetic fields cannot be real science because you cannot see them. Explain how physicists know they are there and can even tell different kinds apart.

• Explain that both are detected by their effects, not by being seen
• Explain how the ionising and penetrating power of radiation tells alpha, beta and gamma apart
• Explain how the force on a compass or a current-carrying wire reveals a magnetic field
• Use the terms half-life and the motor effect correctly
• Finish with the shared idea: an invisible cause can be known, and identified, from what it does