Two horizontal copper rails are 6.0 cm apart. A light copper rod of mass 12 g rests across the rails, at right angles to them.
The rails and the rod are in a vertical magnetic field of flux density 0.15 T0.15\ \text{T}0.15 T. The rails are connected to a power supply, so that there is a current of 4.0 A in the rod.
The rod starts to roll along the rails.
Use the equation: force=magnetic flux density×current×length\text{force} = \text{magnetic flux density} \times \text{current} \times \text{length}force=magnetic flux density×current×length
Calculate the force on the rod.
Calculate the initial acceleration of the rod.
Ignore friction.
The student reverses the direction of the current and also turns the magnet upside down so that the magnetic field is reversed.
State the direction in which the rod now rolls. Explain your answer.
48 exam-style questions on OCR GCSE Physics 4.2 Uses of magnetism, covering 4.2.1 The motor effect, 4.2.2 Fleming's left-hand rule, 4.2.3 The force on a current-carrying conductor, 4.2.4 The electric motor, 4.2.5 Electromagnetic induction, 4.2.6 Alternators and dynamos, 4.2.7 How transformers work, 4.2.8 Transformer turns ratio, 4.2.9 Potential differences and turns in transformers, and 4.2.10 Microphones, loudspeakers and headphones. Each one has a worked solution and a mark scheme showing where the marks go.