Current
High-Yield Summary
- Conventional current is defined as positive-charge flow, even though electrons (negative) are the actual movers in most circuits.
- Current: I = Q/Δt, measured in amperes (A) = 1 C/s.
- DC (constant direction) is what the MCAT tests; AC (periodically reversing direction) is not tested.
- Voltage drives current; emf is the energy per unit charge a source supplies, present even with no current flowing (open circuit).
- Kirchhoff's Junction Rule (Iin = Iout, conservation of charge) and Loop Rule (Vsource = Vdrop, conservation of energy) let you analyze any circuit.
Current
I = Q/Δt
- I = Current, in amperes (A = C/s)
- Q = Charge that flows past a point
- Δt = Time interval
Kirchhoff's Two Laws
| Rule | Statement |
|---|---|
| Junction Rule | Iin = Iout at any junction (conservation of charge) |
| Loop Rule | Vsource = Vdrop around any closed loop (conservation of energy) |
DC vs. AC
| Direct Current (DC) | Alternating Current (AC) |
|---|---|
| Constant direction | Direction reverses periodically |
| Constant voltage over time | Sinusoidal voltage over time |
| Batteries, flashlights, phones | Generators, power grids |
| MCAT-tested | Not MCAT-tested |
Common MCAT Trap
- Conventional current direction is opposite to actual electron flow — don't assume electrons move 'with' the current arrow.
- emf is not actually a force despite the name — it's energy per unit charge (volts), and it exists even in an open circuit with zero current.
Quick Recall
3 A enters a junction and splits into two branches; one carries 1 A. What does the other carry?
Why does emf exist even when no current is flowing?