Science Understanding: Energy, Momentum and Fields
What you must understand
Science Understanding covers the big ideas unifying the syllabus: conservation laws (energy, momentum, charge), fields (gravitational, electric, magnetic) and models (particles, waves, rays). Know that energy transfers between stores (kinetic, gravitational potential, elastic, thermal, chemical, nuclear) and is conserved overall, while energy becomes more dispersed. Understand momentum as a vector conserved in isolated systems, and impulse as change in momentum.
For fields, distinguish field strength from potential, and know that field lines show direction of force on a positive test charge or north pole, with density indicating strength. Uniform fields have parallel, equally spaced lines. Recognise that work done moving a charge or mass depends on potential difference, not path, in a conservative field.
Traps
- Saying energy is "used up" rather than transferred or dissipated.
- Ignoring direction in momentum problems; total momentum is a vector sum.
- Confusing field strength (force per unit mass/charge) with potential (energy per unit mass/charge).
- Assuming a satellite in circular orbit has constant velocity; speed is constant but velocity changes.
Worked example
Two trolleys collide and stick: 2.0 kg at 3.0 m/s hits a stationary 1.0 kg trolley. Total momentum before = 6.0 kg m/s. After: (2.0 + 1.0)v = 6.0, so v = 2.0 m/s. Kinetic energy before = 9.0 J; after = 6.0 J. Momentum is conserved but 3.0 J is dissipated as heat and sound: an inelastic collision.