Lesson 2 of 4
Current, charge and electron flow
I can define current as charge flow per second, measured in amperes.Syllabus
Syllabus K223 / K224, 10(c). Define current as charge flow per second, measured in amperes.
I can distinguish conventional current from electron flow.Syllabus
Syllabus K223 / K224, 10(d). Distinguish conventional current from electron flow.
I can apply charge = current x time to new situations and problems.Syllabus
Syllabus K223 / K224, 10(e). Apply charge = current x time to new situations and problems.
In short
- Current is the rate of flow of charge: I = Q / t, in A.
- Measure current with an ammeter connected in series.
- Charge is measured in coulombs, C.
Current tells you how quickly charge passes a point in a circuit. A larger current means more charge passes each second.
Charge is measured in coulombs, C. Electrons carry negative charge. In a metal wire, mobile electrons can drift through the material while the positive ions remain in its structure.
Electric current is the rate of flow of charge. It is measured in amperes, A. One ampere means one coulomb passes a cross-section each second: 1 A = 1 C/s. Equivalently, 1 C = 1 A s: a coulomb is an ampere-second. Current is represented by I; charge by Q or q.
Imagine marking one cross-section of a wire and counting the total charge that passes it during an interval. A steady 0.30 A means 0.30 C passes each second. It does not mean the whole wire contains only 0.30 C of charge.
Follow charge through the external metal circuit
The cell is discharging into the resistor. Green arrows show conventional current; blue arrows show electron drift. Arrows give direction, not speed.
Steady current: 0.30 A. Time: 2.0 min = 120 s.
Charge passing X: Q = It = 0.30 × 120 = 36 C.
The resistor transfers energy. It does not use up the charge passing through it.
Conventional current and electron drift
Conventional current is defined in the direction in which positive charge would move. In the external circuit of a discharging cell, it goes from the positive terminal, through the components, towards the negative terminal.
Electrons are negative, so their drift in the metal wire is in the opposite direction: from the negative terminal towards the positive terminal through the external circuit. The cell symbol's long line marks its positive terminal and its short line the negative terminal.
A lamp transfers energy as charge passes through it; it does not use up that charge. Electrons are already present throughout the metal circuit. Do not picture the lamp waiting for one electron to leave the cell and complete a whole journey before it responds.
Charge passing a point
A current flows for two minutes
A steady current of 0.30 A flows for 2.0 min. First convert the time: 2.0 x 60 = 120 s.
Q = It = 0.30 x 120 = 36 C.
In a separate example, 18 C passes in 45 s. Its average current is I = Q/t = 18/45 = 0.40 A.
If the current is given as 300 mA, use 300/1000 = 0.300 A. The prefix milli means one thousandth.
Try it step by step
Work it out in steps: Time in s, then Charge Q = It in C.
Show the answer
Problem 1.
Time in s: t = 3.0 x 60 = 180 s
Charge Q = It in C: Q = It = 0.50 x 180 = 90 C (2 sf)
Problem 2.
Time in s: t = 1.5 x 60 = 90 s
Charge Q = It in C: Q = 2.0 x 90 = 180 C (2 sf)
Problem 3.
Time in s: t = 4.0 x 60 = 240 s
Charge Q = It in C: I = 250 mA = 0.25 A, so Q = 0.25 x 240 = 60 C (2 sf)
More detail (optional)
Q = It applies directly when I is constant over the interval. For a changing current, use the average current for that interval, or find the charge in each stated constant-current part and add the amounts. An instantaneous reading does not establish the current throughout a long interval.
Measure current through a component
An ammeter, shown as a circle containing A, connects in series with the branch being measured. Open the circuit at the chosen position and include the meter in the path, so the branch's charge passes through it.
For a d.c. measurement, connect its positive terminal on the side from which conventional current enters. Choose a suitable range; if the expected value is uncertain, start with a sufficiently high range and reduce it when appropriate for a clearer reading. Keep the reading within the selected range.
An ammeter has very low resistance. Connecting it directly across a cell would provide a low-resistance path, rather than measure the current through the intended component. The resistance arrangement shows the correct current and voltage connections together.
An ammeter reads amperes, not coulombs. To determine charge for a steady current, also measure the time interval, for example with a stopwatch, then use Q = It.
Watch out for this
A student says: "Current is used up as it flows through a lamp, so less current returns to the cell." What is wrong with this?
Show the answer
Charge is not used up. In a series circuit the current is the same at every point. What the lamp takes is energy, which the charge carries from the cell.
Check your understanding
A steady current of 0.30 A flows through a wire for 2.0 min. How much charge passes a marked cross-section?
Show the answer
36 C
The current carries 0.30 C past the cross-section each second. In 120 s, Q = 0.30 x 120 = 36 C.
Practise an exam question
Write your answer first, then tick the mark points it makes.
Exam-style question
Define electric current. A current of 0.40 A flows through a lamp for 5.0 minutes. Calculate the charge that passes through the lamp. [3]
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