9702 · 18.4
Electric field of a point charge flashcards
Revision flashcards for Cambridge 9702 Electric field of a point charge (syllabus 18.4). Flip, recall, then mark a real past-paper question.
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What defines an electric field?
A region surrounding a charged object where another charge would experience an electrostatic (non-contact) force.
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How do electric field lines show the direction of force for a positive test charge?
The field lines point in the exact direction a positive test charge would be pushed or pulled.
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What does the density of electric field lines visually represent?
The strength of the electric field; denser (closer) lines indicate a stronger field.
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State the formula for Coulomb's Law.
$F = \frac{1}{4\pi\epsilon_0} \frac{Q_1Q_2}{r^2}$, where $F$ is force, $Q_1, Q_2$ are charges, $r$ is separation, and $\epsilon_0$ is permittivity of free space.
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Define electric field strength, E, for a point charge.
The force experienced per unit positive test charge ($E = F/q$). For a point charge $Q$, it's $E = \frac{1}{4\pi\epsilon_0} \frac{Q}{r^2}$.
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How does electric field strength change if the distance from a point charge is tripled?
It becomes one-ninth (1/9) of its original value, due to the inverse square relationship ($E \propto 1/r^2$).
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What is electric potential (V) at a point due to a point charge?
The work done per unit positive test charge to bring it from infinity to that point within the field.
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Write the formula for electric potential due to a point charge Q at distance r.
$V = \frac{1}{4\pi\epsilon_0} \frac{Q}{r}$.
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What is an equipotential line?
A line (or surface) connecting all points in an electric field that have the same electric potential.
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What is the key geometric relationship between electric field lines and equipotential lines?
They are always perpendicular to each other.
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How can electric field strength be found from a potential-distance graph?
It is the magnitude of the negative gradient of the potential-distance graph: $E = |\frac{\Delta V}{\Delta r}|$.
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What is the Principle of Superposition for electric fields?
The total electric field at any point due to a group of charges is the vector sum of the electric fields produced by each charge individually.
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How is the total electric potential at a point due to multiple charges calculated?
It is the algebraic sum (scalar sum) of the potentials due to each individual charge. Signs (+ or -) are crucial.
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Why is calculating total potential often simpler than calculating total field strength?
Potential is a scalar quantity, so you just add the numbers (with their signs). Field strength is a vector, requiring vector addition (e.g., using components or considering directions), which is more complex.
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What is the work done, W, in moving a charge q between two points with a potential difference of ΔV?
The work done is given by the formula W = qΔV, where ΔV is the change in electric potential (V_final - V_initial).