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Electric and Magnetic Fields · Paper 2 — Free Physics HL Practice Questions

What this topic covers

Fields is where gravity, electricity and magnetism finally get shown to be the same trick played three times: a source (mass, charge, or moving charge), a field that falls off as $1/ r^{2}$ or sits uniform between plates, and a potential that tells you the energy cost of moving through it. HL bolts on two whole subtopics that SL never sees — motion in combined electric and magnetic fields, and electromagnetic induction — and a huge fraction of the marks in this theme come from just two calculation types: transformer turns ratios and induced emf from $ΔΦ/Δ t$ . Get those automatic, then spend the rest of your revision on the sign conventions ( $V_{g}$ is always negative), the vector-vs-scalar traps ( $g$ vs $V_{g}$ , $E$ vs $V$ ), and the direction rules (Lenz's law, $q v \times B$ ) that separate a 6 from a 7.

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1MasterySAQ-SElectric fields and potentials7 marksPaper 2~11 min

A point charge of

+4.0 \times 10^{-9}\,\text{C}

is placed at the origin of a coordinate system. A second point charge of

-2.0 \times 10^{-9}\,\text{C}

is placed on the x-axis at

x = 0.30\,\text{m}

. Point P is on the x-axis at

x = 0.10\,\text{m}

. Coulomb constant:

k = 8.99 \times 10^{9}\,\text{N\,m}^{2}\,\text{C}^{-2}

(a)

Explain why the electric potential at point P is positive. [2 marks]

(b)

Calculate the magnitude and direction of the net electric field at point P. [3 marks]

(c)

A third charge of

-1.5 \times 10^{-9}\,\text{C}

is placed at point P. Deduce whether the net electrostatic force on this third charge acts in the positive or negative x-direction, and justify your answer without further calculation. [2 marks]

Solutions

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