Short on time? This page is the recap. For explanations, Figures 1 and 2 and the worked examples, use the full study guide.
Recap
- Current is the rate at which charge passes through a cross-section of a conductor. It is measured at a place, not stored in a component.
- Charge moves around a circuit because a source keeps a potential difference across it. The potential difference a source provides is its emf (ℰ), measured in volts. emf is energy per unit charge, not a force.
- The carriers are already in the wire. The source supplies energy, not charge.
- Free electrons move randomly at about 10⁶ m/s. With no current their net motion is zero. A potential difference adds a slow drift in one direction, and only the drift carries net charge.
- Current is a scalar with a direction along the wire. It does not change when a wire bends.
- Conventional current = the direction positive charge would move: from + to − outside the source. In metals, electrons carry the current and drift the opposite way.
- Circuit diagrams in this course use conventional current unless told otherwise.
Key relationships
| Relationship | Symbols and units | Use it to |
|---|---|---|
| I = ΔQ / Δt | I in A, ΔQ in C, Δt in s | find current, charge or time for a steady current |
| N = ΔQ / e | e = 1.60 × 10⁻¹⁹ C | count the electrons (or singly charged ions) that pass |
| ΔQ = area under I–t graph | A × s = C | find charge when the current changes |
| I_average = total ΔQ / total Δt | A | summarise a changing current |
Assumptions behind the numbers
- Time is in seconds.
- Each electron or singly charged ion carries charge of size e.
- The current is the net flow: carriers of opposite sign moving in opposite directions both add to the current in the same direction.
Mistakes to avoid
- Minutes not converted. 2 minutes is 120 s.
- Electron direction given as the current direction. Reverse it for conventional current.
- Subtracting ion currents when positive and negative ions move in opposite directions. They add.
- “Current is used up.” The same current enters and leaves a component.
- Peak × time for a changing current. Use the area under the I–t graph.
- “emf is a force.” It is a potential difference, in volts.
Quick self-check
- A current of 2.0 A flows for 30 s. How much charge passes? (60 C)
- Electrons in a wire drift to the left. Which way is the conventional current? (To the right)
- About how many electrons carry 1.0 C of charge? (6.25 × 10¹⁸)
Next: practice questions.