Two magnets are held on a track inside a sealed box, with like poles facing each other. The magnets, track, air and box form a closed system. One magnet is released and moves along the track. At a later instant, the magnetic energy store of the magnets is 6.0 J. Since the magnet was released, the kinetic energy store of the moving magnet has increased by 12 J and the thermal energy stores of the track and air have increased by 4.0 J. No other energy stores have changed.
What was the magnetic energy store of the magnets when the magnet was released?
10 J10\ \mathrm{J}10 J
16 J16\ \mathrm{J}16 J
22 J22\ \mathrm{J}22 J
18 J18\ \mathrm{J}18 J
115 exam-style questions on Edexcel GCSE Physics 8.1 Energy transfers and work, covering 8.1.1 Energy stores and transfer diagrams, 8.1.2 Ways of changing a system's energy, and 8.1.3 Work done by forces. Each one has a worked solution and a mark scheme showing where the marks go.