A Hall flow probe on an artery applies a 0.100 T field. The vessel is 4.00 mm across and blood moves at 20.0 cm/s. What Hall emf appears?

1. B, v and l are mutually perpendicular here, so use emf = Blv. 2. Convert the width: 4.00 mm = 4.00 x 10^-3 m. 3. Convert the speed: 20.0 cm/s = 0.200 m/s. 4. Substitute: emf = (0.100 T)(4.00 x 10^-3 m)(0.200 m/s). 5. Multiply: (0.100)(4.00 x 10^-3) = 4.00 x 10^-4, and (4.00 x 10^-4)(0.200) = 8.00 x 10^-5 V. 6. So the Hall emf is 80.0 microvolts. 7. An answer in millivolts comes from leaving the width in millimetres instead of metres. 8. Voltages from heart action are themselves of the order of millivolts, which is why this small signal is hard to pick out. _Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.6 The Hall Effect_