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QLDPhysicsQuick questions
Unit 3: Gravity and electromagnetism
Quick questions on Electric fields, point charges and parallel plates (QCE Physics Unit 3)
12short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is coulomb's law?Show answer
The force between two point charges $q_1$ and $q_2$ separated by distance $r$:
What is electric field of a point charge?Show answer
The electric field $\vec{E}$ at a point is the force per unit positive test charge placed there:
What is uniform field between parallel plates?Show answer
Two parallel conducting plates separated by distance $d$ with a potential difference $V$ between them set up a uniform field in the region between the plates (away from the edges):
What is charged particle deflected sideways?Show answer
A charged particle entering a parallel-plate region with a horizontal velocity $v_0$ perpendicular to the field experiences a force only in the field direction. The motion is exactly analogous to a horizontally launched projectile in gravity:
What is field-line diagrams?Show answer
QCAA frequently asks for field-line sketches. Conventions:
What is iA1 data test?Show answer
Expect a parallel-plate problem with a charged particle accelerated or deflected, asked for the final speed or deflection. Alternatively, a two-charge geometry with a question on the net field at a point. Markers focus on candidates who forget the direction of $\vec{E}$, treat the field of a negative charge as positive, or confuse the field direction with the force direction on a negative test charge.
What is iA2 student experiment?Show answer
A field-only IA2 is rare in QCE Physics (typically Topic 2 IA2s use induction or transformers), but design discussions sometimes reference a Millikan-style charged-droplet apparatus or a parallel-plate beam-deflection demo. The theory section then uses $E = V/d$ and $qV = \tfrac{1}{2} m v^2$ to predict speeds and deflections.
What is confusing $\vec{E}$ direction with force direction on a negative charge?Show answer
$\vec{E}$ points from positive to negative plate. A negative charge accelerates against $\vec{E}$, that is, from the negative plate toward the positive plate.
What is inverse vs inverse-square for parallel plates?Show answer
Between parallel plates $E = V/d$ (uniform; inversely proportional to plate separation). For a point charge $E$ falls off as $1/r^2$. Do not mix the two.
What is substituting microcoulombs instead of coulombs?Show answer
Always convert charges to SI coulombs (and distances to metres) before substituting into Coulomb's law.
What is forgetting that field is a vector?Show answer
Two point charges produce fields that add vectorially. Use components when the directions are not collinear.
What is treating $E$ as if it were a potential?Show answer
$E$ is field strength (N/C). $V$ is potential difference (V or J/C). They differ by a factor of distance: $V = E d$ for a uniform field.