Current as Charge Flow
Conventional current is positive-charge flow. For a specified orientation, instantaneous current is dQ/dt and transferred charge is the time integral of current.
Why this shows up in the exam
Time-varying current waveforms · Charge delivered by a battery · Reading current from a Q-versus-time graph
Learn the idea
Current is the signed rate at which charge crosses a chosen surface. Imagine counting charge packets crossing a gate. A changing flow rate must be integrated over time; multiplying by time works only when the current is constant.
🧠 Memory hook: Differentiate charge for current; integrate current for charge.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- I = dQ/dt — instantaneous current for differentiable Q(t)
- Delta Q = integral(t1 to t2) I(t) dt — charge crossing the section; constant I is not assumed
How to approach it
- 1Identify whether Q(t) or I(t) is given
- 2Differentiate or integrate with limits
- 3Check that ampere-second gives coulomb
Common slip-ups that cost marks
- •Using Q = It for a nonconstant current
- •Ignoring the sign set by the chosen surface
- •Reading graph height instead of slope for current
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A cell of emf 6 V and internal resistance 1 ohm is connected to a 2 ohm resistor. Find the circuit current.
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