9702 · 10.2
Kirchhoff's laws flashcards
Revision flashcards for Cambridge 9702 Kirchhoff's laws (syllabus 10.2). Flip, recall, then mark a real past-paper question.
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What fundamental principle underpins Kirchhoff's First Law?
Conservation of electric charge.
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What does Kirchhoff's Second Law state regarding the potential differences in a closed loop?
The algebraic sum of all potential differences (voltages) around any closed loop must be zero.
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Write the formula relating e.m.f., terminal potential difference, and lost volts.
\\epsilon = V + v
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If 2A enters a junction and 0.8A leaves through one path, what is the total current leaving through all other paths?
1.2 A (since 2 A - 0.8 A = 1.2 A)
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Which of Kirchhoff's laws is a direct consequence of the conservation of energy?
Kirchhoff's Second Law.
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What is a 'junction' in the context of Kirchhoff's First Law?
A point in an electrical circuit where three or more conductors meet, allowing current to split or combine.
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How are sources of e.m.f. treated when applying Kirchhoff's Second Law?
As potential rises (positive voltage) when traversing from the negative to positive terminal.
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What causes 'lost volts' within a power source?
The potential drop across the internal resistance of the power source when current flows through it.
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What is the mathematical representation of Kirchhoff's First Law?
\\Sigma I_{in} = \\Sigma I_{out}
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In a series circuit, what can be said about the current through each component?
The current is the same through every component.
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State the formula for terminal potential difference (V) in terms of e.m.f. (ε), current (I), and internal resistance (r) for a discharging cell.
V = ε - Ir
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In a circuit loop, if you traverse a resistor in the opposite direction to the current flow, is the potential difference (IR) considered positive or negative in your KVL equation?
Positive. Moving against the current is like walking 'uphill' in potential, so it's a potential rise.
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A cell has an e.m.f. of 1.5 V and internal resistance of 0.2 Ω. If it supplies a current of 0.5 A, what are its 'lost volts'?
Lost volts v = Ir = 0.5 A * 0.2 Ω = 0.1 V.
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What happens to the terminal potential difference of a discharging battery as the current it supplies increases?
It decreases, because the 'lost volts' (v = Ir) increase with current, and V = ε - Ir.
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Can the terminal potential difference of a battery ever be greater than its e.m.f.?
Yes, if the battery is being charged. In this case, current flows into the positive terminal, and the equation becomes V = ε + Ir.