IB Physics HL D 4 3 Hl Motional Emf Questions

Apply motional emf to moving rods, rails, cables and coils, relating magnetic force, charge separation, induced voltage, current and terminal speed.

Exam points

derive and calculate motional emf using V equals vBL for a conductor moving through a magnetic field with the relevant orientation
explain charge separation and force on conduction electrons from velocity across a magnetic field, and interpret emf graphs from motion or displacement data
analyse induced current, magnetic drag, terminal speed and energy transfer in moving rods, rails, cables, coils and resistive circuits

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Study D.4.3 (HL)—Motional emf conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

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A conducting rod of length L is moved with speed v at right angles to a uniform magnetic field of flux density B. The field is directed into the plane of the page.

Figure for Question IB Physics HL D 4 3 Hl Motional Emf Questions question 1 — IB Physics HL

Show, using Faraday's law or otherwise, that the potential difference, V, established between the ends of the rod is V=v B L.

ALTERNATE 1

In time $\Delta t$ , rod moves a distance $v \Delta t$

Flux increases by $B(L v \Delta t)^{}$

«By Faraday» induced emf is the rate of change of flux: $\frac{B L v \Delta t}{\Delta t}$

«=BLv»

ALTERNATE 2

<<From $\varepsilon=-\frac{N \Delta \Phi}{\Delta t} \gg$

Recognition that $\Phi=B A$

Recognition that N = 1

Recognition that $\frac{\Delta A}{\Delta t}=L v \vee$

<<Leading to V=vBL OR $\varepsilon=\mathrm{BLv}>$

ALTERNATE 3

«In steady state» electrons stop drifting and so net force is zero q v B=q E

Substitution of $E=\frac{V}{L}$ to get result

ALT 1: Correct MP2 scores MP1

ALT2:

V and $\varepsilon$ are interchangeable. Ignore negative sign.

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IB Physics HL D 4 3 Hl Motional Emf Questions question 1