Paper 3 Magnetic Force Answers

Q1 Circular Motion of Ions

1. $B=mv/(qr)=3.3\times 10^{-26}(2.5\times 10^5)/(1.60\times 10^{-19}\times 0.18)=0.286\mathrm{T}$. [3]
2. It is always perpendicular to velocity and directed toward the centre of the circular path. [2]
3. Since $r=mv/(Bq)$, doubling $v$ doubles $r$ if $B$ and $q$ are unchanged. [2]

Q2 Parallel Currents

1. Attractive. [1]
2. $F/L=(4\pi \times 10^{-7})(8.0)(5.0)/(2\pi \times 0.040)=2.0\times 10^{-4}\mathrm{N}{\mathrm{m}}^{-1}$. [3]
3. Each wire produces a magnetic field at the other wire; the current in that field experiences a motor-effect force. For same-direction currents the forces are toward each other. [3]