Velocity Selector Calculations

Find the particle speed first, then balance the two forces.

A complete selector calculation

Worked example

Find the required magnetic flux density

Positive ions have kinetic energy 4.1 × 10⁻¹⁷ J and mass 3.2 × 10⁻²⁷ kg. They pass undeviated between plates separated by 3.6 cm with 980 V across them. Find B (9702/42/F/M/25 Q6).

1. Find the speed from kinetic energy.

v=\sqrt{\dfrac{2E_{\mathrm{k}}}{m}}

v=1.6\times10^5\ \mathrm{m\,s^{-1}}

2. Use the selector equation.

B=\dfrac{V}{vd}

B=0.17\ \mathrm{T}

Answer

B=0.17\ \mathrm{T}

Your turnquick check

A selector has electric field strength 2.4 × 10⁴ V m⁻¹ and magnetic flux density 0.15 T. Find the selected speed.

Show worked answer

v=\dfrac{E}{B}

Answer

v=1.6\times10^5\ \mathrm{m\,s^{-1}}

PreviousThe Velocity Selector NextThe Hall Effect

Velocity Selector Calculations

Find the particle speed first, then balance the two forces.

A complete selector calculation

Worked example

Find the required magnetic flux density

Positive ions have kinetic energy 4.1 × 10⁻¹⁷ J and mass 3.2 × 10⁻²⁷ kg. They pass undeviated between plates separated by 3.6 cm with 980 V across them. Find B (9702/42/F/M/25 Q6).

1. Find the speed from kinetic energy.

v=\sqrt{\dfrac{2E_{\mathrm{k}}}{m}}

v=1.6\times10^5\ \mathrm{m\,s^{-1}}

2. Use the selector equation.

B=\dfrac{V}{vd}

B=0.17\ \mathrm{T}

Answer

B=0.17\ \mathrm{T}

Your turnquick check

A selector has electric field strength 2.4 × 10⁴ V m⁻¹ and magnetic flux density 0.15 T. Find the selected speed.

Show worked answer

v=\dfrac{E}{B}

Answer

v=1.6\times10^5\ \mathrm{m\,s^{-1}}

PreviousThe Velocity Selector NextThe Hall Effect