Science Forces Outcomes Assessed AT3 Yr7 2025 MHS

Atom structure

Atoms consist of a nucleus (protons +, neutrons 0) and electrons (−) in shells/orbits around the nucleus.

Electron

A negatively charged subatomic particle that can be transferred between atoms to create static charge.

Static electricity

Net electric charge that builds on an object when electrons are added or removed; the charge is not flowing.

Electrostatic force

Non-contact force between charged objects: like charges repel; opposite charges attract.

Charging by friction

Transfer of electrons by rubbing two neutral materials together (e.g., ebonite + wool → ebonite negative).

Ebonite rod

Becomes negatively charged when rubbed with wool (electrons transferred onto rod).

Perspex rod

Becomes positively charged when rubbed with silk/cloth (electrons transferred off rod).

Charging by contact

When a charged object touches a neutral object and transfers charge, leaving the second object with net charge.

Charging by induction

A charged object brought near a neutral object causes charge separation without touching, producing attraction.

Conductor

Material with free charge carriers (usually electrons) that allow charge to move through it (e.g., metals).

Insulator

Material with no free charge carriers (e.g., plastic, glass); any charge placed remains at that spot.

Grounding (earthing)

Connecting a charged object to Earth so electrons flow to/from the ground and the object becomes neutral.

Van de Graaff generator

Device that builds a large static charge on a metal dome using an insulating belt and combs; causes hair to stand and sparks.

How Van de Graaff works

A charged comb removes or deposits electrons on a moving belt; the charge transfers to the dome where it accumulates (high voltage, low current).

Electroscope

Instrument with a plate and metal leaves/needle that shows presence of charge: leaves diverge when net charge is present.

Electroscope behaviours (induction/contact/grounding)

Induction near plate causes temporary leaf divergence; contact (touch) transfers charge and leaves stay diverged; grounding discharges leaves to collapse.

Balloon–wall (induced adhesion) example

Rubbing a balloon adds electrons; when near a neutral wall it induces opposite surface charge and the balloon sticks by electrostatic attraction.

Lightning

Catastrophic electrostatic discharge caused by charge separation in storm clouds; can be cloud-to-cloud or cloud-to-ground.

Cloud charge separation in thunderstorms

Collisions of hydrometeors separate charge: positive at top, negative at base; ground becomes induced positive beneath the cloud.

Role of ground in lightning

Ground supplies or accepts charge and provides a return path for cloud-to-ground discharges; tall objects concentrate fields.

Electrostatic safety tips

During storms: go indoors or into a car, avoid tall isolated objects, stay away from water and exposed metal structures.

Gravitational force

Non-contact attractive force between masses; magnitude depends on mass and separation distance.

Mass vs weight

Mass = amount of matter (kg). Weight = gravitational force on that mass (N); weight changes with local gravity.

Weight formula (F = m×g)

The weight (force) on an object is F = m × g where g is local gravitational acceleration (units: N = kg × m/s²).

g values

Earth: ≈ 9.8 m/s². Moon: ≈ 1.63 m/s². Use given g value for calculations as required.

Worked example F = m×g

Show working: e.g., W (Earth) = 100 kg × 9.8 m/s² = 980 N; W (Moon) = 100 kg × 1.63 m/s² = 163 N.

Universal law of gravitation

Newton: F = G·(m1·m2) / d² where G ≈ 6.67×10⁻¹¹ N·m²/kg²; force ∝ (m1×m2) and ∝ 1/d².

Effect of mass on gravity

Increase one or both masses → gravitational force increases (directly proportional).

Effect of distance on gravity

Increase distance between centres → gravitational force decreases by the square of the distance (inverse-square law).

Gravitational field

Region around a mass where another mass experiences gravitational force; field strength at a point = g there.

Orbit

An object’s curved path around a larger body caused by the object’s forward motion and gravity providing centripetal force.

Why satellites stay in orbit

Forward velocity makes the satellite ‘fall around’ Earth; gravity provides the centripetal force so it remains in orbit.

Centripetal force

Force directed toward the centre of circular motion that keeps an object moving in that curve; gravity supplies this for satellites.

Inertia (orbit context)

The tendency of an object to keep moving in a straight line; combined with gravity, leads to orbital motion.

Orbit sketching

Label arrows: gravity (toward centre) and velocity (tangent) to explain continuous free-fall/orbit.

Electrostatic vs gravitational forces

Compare: both non-contact; electrostatic can attract/repel depending on charge, gravity only attracts and depends on mass.

Magnetism

A non-contact force from magnetic poles; opposite poles attract, like poles repel.

Magnetic poles

Regions where a magnet’s effect is strongest; typically labelled north (N) and south (S).

Magnetic field

The region around a magnet where magnetic forces act; field lines show direction and strength (outside: N→S).

Mapping magnetic fields

Use iron filings or a small compass on a grid to trace field direction; denser lines indicate stronger field.

Effect of distance on magnetism

Magnetic force and field strength decrease rapidly with increasing distance from the magnet.

Electromagnet

A magnet formed by electric current through a coil; strength amplified by a ferromagnetic core (iron).

Factors increasing electromagnet strength

More coil turns, higher current, and an iron core all increase electromagnet strength.

Constructing an electromagnet (practical)

Wind insulated wire tightly around iron core, connect to power supply, vary one factor (coils or current), measure strength (e.g., clips) and repeat.

Investigating magnet strength vs distance (method)

Measure magnetic effect (e.g., number of lifted paper clips or compass deflection) at set distances; keep other variables constant; repeat and average.

Practical: mapping magnetic fields method

Place magnet under paper, use compass at grid points to record direction, draw arrows and smooth field lines; denser arrows = stronger field.

Interpreting graphs (working scientifically)

Read axes, identify variables, describe trends (increase, decrease, plateau), estimate values and calculate gradients if needed.

Variables in investigations

Independent variable: changed by you; dependent: measured; control variables: kept constant to ensure a fair test.

Experimental reliability

Improve by repeating trials, calculating averages, controlling variables, and recording anomalies.

Exam checklist (forces only)

Be able to: explain static charging methods (friction/contact/induction), describe electroscope/VdG behaviour, describe lightning, do F = m×g calculations, explain orbits, map magnetic fields and design a simple electromagnet practical.