The shape of the lesson
By the end of the lesson, learners can
- Inside the wire: Watch the charge carriers drift along the conductor.
- Two ways to count the charge: One relation for the circuit, one for the microscopic picture.
- Drift speed is slow: The I = Anvq equation hides a surprise about how fast electrons actually move.
- Current: I = Q/t.
- Carriers: I = Anvq.
- Quantum: e = 1.6 × 10⁻¹⁹ C.
One picture every idea returns to
Electric current is the rate of flow of charge, I = Q / t, so the charge passing in a time t is Q = It. Charge is quantised in units of e = 1.6 × 10⁻¹⁹ C. Inside a conductor the current is carried by moving charge carriers, and I = Anvq, where A is the cross-sectional area, n the number of carriers per unit volume, v their mean drift speed and q the charge on each.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Charge on the move.. An electric current is nothing more mysterious than charge flowing past a point. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Inside the wire | Watch the charge carriers drift along the conductor. Project tab 1 of the lab (Charge carriers, I = nAvq) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Two ways to count the charge | One relation for the circuit, one for the microscopic picture. | Pairs |
| 21 to 30 min | Teach and check: Drift speed is slow | The I = Anvq equation hides a surprise about how fast electrons actually move. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Drift speed is slow, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Keep the symbols in I = Anvq straight: n is the number of carriers per unit volume, not the total number in the wire. | The drift speed is genuinely slow; the current appears instantly because the field is established almost at the speed of light, not because the electrons race along. And remember to convert time to seconds in Q = It. |
Electric current: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Define electric current and state its SI unit.
A steady current of 0.50 A flows for 2.0 minutes. Find the charge transferred.
A wire of cross-sectional area 1.0 × 10⁻⁶ m² carries charge carriers of density 1.0 × 10²⁹ m⁻³ drifting at 1.0 × 10⁻⁴ m s⁻¹ (q = 1.6 × 10⁻¹⁹ C). Find the current.
How many electrons make up a charge of 4.0 C? (e = 1.6 × 10⁻¹⁹ C)
A current of 3.2 A flows in a wire of area 2.0 × 10⁻⁶ m² with carrier density 1.0 × 10²⁹ m⁻³ (q = 1.6 × 10⁻¹⁹ C). Find the drift speed.
The drift speed of electrons in a wire is less than a millimetre per second. Explain why a lamp lights almost instantly when the switch is closed.
Answer key and marking notes
- Current is the rate of flow of charge, I = Q/t ✓
- The SI unit is the ampere (A) ✓
- Q = It = 0.50 × (2.0 × 60) ✓
- Q = 0.50 × 120 = 60 C ✓
- I = Anvq = (1.0 × 10⁻⁶)(1.0 × 10²⁹)(1.0 × 10⁻⁴)(1.6 × 10⁻¹⁹) ✓
- I = 1.6 A ✓
- N = Q/e = 4.0 / (1.6 × 10⁻¹⁹) ✓
- N = 2.5 × 10¹⁹ electrons ✓
- v = I/(Anq) = 3.2 / ((2.0 × 10⁻⁶)(1.0 × 10²⁹)(1.6 × 10⁻¹⁹)) ✓
- v = 3.2 / (3.2 × 10⁴) = 1.0 × 10⁻⁴ m s⁻¹ ✓
- The electric field is set up along the wire at nearly the speed of light ✓
- so charge carriers everywhere begin to drift almost at once, even though each moves slowly ✓
14 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Electric current is the:
- Arate of flow of charge
- Btotal charge in a wire
- Cenergy per unit charge
- Dcharge times time
A steady current of 4.0 A flows for 10 s. The charge transferred is:
- A2.5 C
- B0.40 C
- C40 C
- D14 C
The SI unit of electric charge is the:
- Aampere
- Bcoulomb
- Cvolt
- Djoule
In I = Anvq, the symbol n is the:
- Atotal number of carriers
- Bdrift speed
- Ccharge per carrier
- Dnumber of charge carriers per unit volume
The number of electrons in 1.0 C of charge is (e = 1.6 × 10⁻¹⁹ C):
- A6.25 × 10¹⁸
- B1.6 × 10⁻¹⁹
- C1.0
- D6.25 × 10⁻¹⁸
A current of 0.25 A flows for 4.0 minutes. The charge is:
- A1.0 C
- B960 C
- C60 C
- D16 C
The drift speed of electrons in a metal is very small because the:
- Acharge on each electron is large
- Bnumber density of carriers is very large
- Cwire is very long
- Dcurrent is small
For a fixed current, halving the cross-sectional area of a wire makes the drift speed:
- Ahalf as large
- Bunchanged
- Cfour times as large
- Dtwice as large
A current of 2.0 A means that the charge passing each second is:
- A2.0 C
- B0.50 C
- C2.0 A
- D1.0 C
To measure the current in a component, an ammeter is connected:
- Ain parallel with it
- Bacross the supply
- Cin series with it
- Dto earth
The magnitude of the charge on an electron is:
- A1.6 × 10¹⁹ C
- B1.6 × 10⁻¹⁹ C
- C9.1 × 10⁻³¹ C
- D6.0 × 10²³ C
Conventional current is defined as flowing:
- Ain the same direction as electron flow
- Bfrom low to high potential
- Conly in a vacuum
- Din the opposite direction to electron flow
A charge of 90 C passes a point in 30 s. The current is:
- A3.0 A
- B2700 A
- C0.33 A
- D60 A
In a metal conductor, the charge carriers are:
- Aprotons
- Bpositive ions
- Cfree electrons
- Dneutrons
Quiz key
I = Q/t.
Q = It = 4.0 × 10 = 40 C.
Charge is measured in coulombs.
n is the carrier number density.
N = Q/e = 1.0/(1.6 × 10⁻¹⁹) = 6.25 × 10¹⁸.
Q = It = 0.25 × 240 = 60 C.
With huge n, a small v gives the observed current in I = Anvq.
I = Anvq is constant, so halving A doubles v.
I = Q/t, so 2.0 A is 2.0 C per second.
An ammeter goes in series so the same current flows through it.
e = 1.6 × 10⁻¹⁹ C.
Conventional current is the direction positive charge would flow, opposite to the electrons.
I = Q/t = 90/30 = 3.0 A.
Current in a metal is carried by free (delocalised) electrons.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.