Topic 11.110 min read
Electric Current
Electric current is the rate of flow of charge through a cross-section.
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Electric current tells us how quickly electric charge passes through a cross-section of a conductor. A larger amount of charge passing each second means a larger current.
- I
- electric current (A)
- \Delta Q
- charge passing through a cross-section (C)
- \Delta t
- time interval (s)
One ampere means that one coulomb of charge passes a cross-section each second.
In a metal, electrons actually drift from the negative terminal toward the positive terminal. Conventional current is defined in the opposite direction, from higher potential toward lower potential in the external circuit.
Recommended for Class 11
Electric current is the rate at which charge crosses an imaginary surface drawn through a conductor. It describes charge transfer through that surface and is represented by I.
Average electric current
If a net charge ΔQ passes through a cross-section during a time interval Δt, the average current is:
- I_{\mathrm{avg}}
- average electric current (A)
- \Delta Q
- net charge transferred (C)
- \Delta t
- time interval (s)
For a steady current, equal amounts of charge pass every cross-section in equal time intervals, so I=Qt can be used directly.
Instantaneous current
When the rate of charge flow changes with time, the current at one instant is the limiting rate of change of charge.
When to use: Use when charge Q is given as a function of time.
SI unit: ampere
The SI unit of electric current is the ampere, symbol A. A current of one ampere transfers one coulomb of charge per second.
Electric current is one of the seven SI base quantities. Charge can therefore also be expressed as current multiplied by time.
When to use: Use for constant current over time t.
Charge carriers in different materials
| Material | Main moving charge carriers |
|---|---|
| Metal | Free electrons |
| Electrolyte | Positive and negative ions |
| Ionised gas | Electrons and ions |
| Semiconductor | Electrons and holes |
A material conducts when it contains charge carriers that can move through it. Metals contain many free electrons, whereas the charges in an insulator are strongly bound and cannot move easily through the material.
Conventional-current direction
By convention, current points in the direction in which positive charge would move. In the external circuit connected to a cell, this direction is from the positive terminal toward the negative terminal.
The mobile carriers in a metal are negatively charged electrons. Their drift direction is from the negative terminal toward the positive terminal, opposite to conventional current.
| Description | Direction in a metallic external circuit |
|---|---|
| Conventional current | Positive terminal → negative terminal |
| Electron drift | Negative terminal → positive terminal |
Scalar quantity with an assigned direction
Current is treated as a scalar quantity. At a junction, currents are added algebraically according to their assigned directions; current does not follow vector addition.
Measuring current
An ammeter measures current. It is connected in series so that the same charge flow passes through the ammeter and the component being measured.
An ideal ammeter has zero resistance. A practical ammeter has very low resistance so that inserting it causes only a very small change in circuit current.
Worked example
A charging current of 10.0 A flows for 2.0 h. Find the charge transferred.
Connection to electron count
Charge is quantised. If N electrons pass a cross-section, the magnitude of transferred charge is Q=Ne, where e=1.60×10⁻¹⁹ C.
- N
- number of electrons (none)
- e
- elementary-charge magnitude (C)
Exam-ready method
- Write I=ΔQΔt and identify all SI units.
- For constant current, rearrange to Q=It.
- Convert minutes or hours into seconds.
- State conventional current and electron drift in opposite directions for metals.
- Name the correct charge carrier for the material.
- Connect an ammeter in series and explain its very low resistance.
Exam Focus
Exam Focus
Important points
- I=ΔQΔt
- I=dQ/dt
- Q=It
- 1 A=1 C·s⁻¹
- Conventional current versus electron drift
- Ammeter connection
Important formulas
- I=Δ QΔ t
- I=dQdt
- Q=It
- N=Ite
Common exam traps
- Convert time to seconds.
- Do not use electron-flow direction as the conventional-current arrow.
Long question structure
Definition → average and instantaneous current → ampere → charge carriers → directions → measurement → example
Common Mistakes
Common Mistakes
- Mistake
Writing current as charge instead of charge per unit time.
- Mistake
Using hours or minutes without converting them to seconds.
- Mistake
Treating electron drift as the conventional-current direction.
- Mistake
Connecting an ammeter in parallel.
- Mistake
Writing 1 A=1 C·s instead of 1 A=1 C·s⁻¹.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
Average current is Iavg=ΔQΔt; instantaneous current is I=dQ/dt.
For constant current, transferred charge is Q=It.
The SI unit is the ampere: 1 A=1 C·s⁻¹.
Conventional current follows the direction of positive-charge flow.
Electric current is the rate of flow of charge through a cross-section.
I=Δ QΔ t, Q=It, 1 A=1C\,s^{-1}
- Definition
- Electric current is the rate of flow of charge through a cross-section.
- Formula
- I=Δ QΔ t, Q=It, 1 A=1C\,s^{-1}
- Example
- At 10.0 A for 2.0 h, Q=7.20×10⁴ C.
- Critical warning
- Conventional current is opposite to electron drift in a metal.
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Exam Practice
Definition
Q1. Define electric current and state its SI unit.
Concept
Q2. Differentiate between conventional current and electron flow in a metal.
Numerical
Q3. A current of 10.0 A charges a battery for 2.0 h. Calculate the charge transferred.
Measurement
Q4. How is an ammeter connected and why must its resistance be low?
Reasoning
Q5. Why can metals conduct electricity while insulators generally cannot?