Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.2.2Electricity and Magnetism

Electric Current — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 14 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

4.2.2 Electric Current — IGCSE PHYSICS notes: Define electric current, use I = Q/t, measure current with an ammeter and distinguish conventional current from electron flow and d.c.

  • Definition
  • Charge carriers
  • Electron flow (Extended)
  • Analogue ammeter
  • Digital ammeter
  • Zero error

Part 1 of 7Electric current

Definition
  • Definition electric current is the rate of flow of electric charge.
Unit
  • Unit current is measured in amperes (A), where 1 A = 1 coulomb per second.
Charge carriers
  • Charge carriers in metals the carriers are free electrons.

Part 2 of 7Current equation (Extended)

  • I = Q / t
    What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
    • current (A) equals charge (C) divided by time (s).
    • Rearranged, Q = I t and t = Q / I.
    • Convert mA to A (1 mA = 10−3 A) and minutes to seconds before calculating.

Part 3 of 7Direction of current

1Conventional current
  • Conventional current flows from the positive terminal to the negative terminal of the supply.
2Electron flow (Extended)
  • Electron flow (Extended) electrons are negative, so they actually move from the negative terminal to the positive terminal.
  • This is opposite to the direction of conventional current.

Part 4 of 7Measuring current

Ammeter
  • Ammeter measures current and is connected in series with the component.
Analogue ammeter
  • Analogue ammeter has a needle and scale and must be read from directly in front to avoid parallax error.
Digital ammeter
  • Digital ammeter gives a direct numerical reading and can measure very small currents.
Zero error
  • Zero error check the meter reads zero before use, or subtract the zero value from every reading.
1
Switch off the circuit and break it at the point where the current is to be measured.
2
Connect the ammeter in series with its + terminal towards the + side of the supply.
3
Choose a suitable range, starting with the highest.
4
Switch on and read the value in amperes.

Part 5 of 7Current in series and parallel circuits

Series
  • Series the current is the same at every point in the loop, because charge is not used up.
Parallel
  • Parallel the currents in the branches add up to the current in the main line.
Adding bulbs in series
  • Adding bulbs in series more resistance, so the current decreases and the bulbs are dimmer.
Adding bulbs in parallel
  • Adding bulbs in parallel each branch has its own path, so each bulb stays bright and the main current increases.

Part 6 of 7Direct and alternating current

1Direct current (d.c.)
  • Direct current (d.c.) flows in one direction only, as from a cell or battery.
2Alternating current (a.c.)
  • Alternating current (a.c.) repeatedly changes direction, as in the mains supply.
    • (50 Hz in many countries)

Part 7 of 7More from the specification

1Electric current
  • Electric current the rate of flow of charge, I = Q/t, measured in amperes
2Potential difference
  • Potential difference the energy transferred per unit charge between two points, measured in volts
3Charging by friction
  • Charging by friction rubbing a polythene rod transfers electrons to it, leaving it negatively charged and the cloth positive
4Electroscope
  • Electroscope a gold-leaf electroscope detects charge because the plate and leaf gain the same charge and repel
5Conductors and insulators
  • Conductors and insulators metals have free electrons that carry charge, while insulators hold their electrons so charge cannot flow
6Electric field
  • Electric field the region around a charge where another charge experiences a force; power = current x voltage
7Electric current
  • Electric current the rate of flow of charge, I = Q/t, measured in amperes
8Potential difference
  • Potential difference the energy transferred per unit charge between two points, measured in volts
9Charging by friction
  • Charging by friction rubbing a polythene rod transfers electrons to it, leaving it negatively charged and the cloth positive
10Electroscope
  • Electroscope a gold-leaf electroscope detects charge because the plate and leaf gain the same charge and repel
11Conductors and insulators
  • Conductors and insulators metals have free electrons that carry charge, while insulators hold their electrons so charge cannot flow
12Electric field
  • Electric field the region around a charge where another charge experiences a force; power = current x voltage
13Insulator
  • Material that does not allow current to flow.
14Potential difference
  • Work done by a unit charge passing through a component.
15Energy transferred
  • W = V Q, in joules.
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
16Electric field
  • Region of space in which a charge experiences a force.
17Field lines
  • Lines showing the direction and strength of a magnetic field.
18Electromotive force
  • Energy transferred from other forms to electrical energy per unit charge.
19Electrical energy
  • Energy carried by electrons in a circuit.
20Gold-leaf electroscope
  • A device in which a charged gold leaf falls when photoelectrons discharge the plate.
21Electroscope
  • Instrument with a metal cap, stem and gold leaf used to detect charge.
22Conductor
  • Material that allows current to flow.

Key terms

TermMeaning
Electric currentRate of flow of electric charge, I = Q / t.
Ampere (A)Unit of current; 1 A = 1 C per second.
Conventional currentCurrent drawn from the positive to the negative terminal.
AmmeterInstrument that measures current, connected in series.
Direct current (d.c.)Current in one direction only.
Alternating current (a.c.)Current that repeatedly reverses direction.

Avoid these mistakes

1
Saying current is used up by a lamp — the same current leaves as enters; energy is transferred, not charge.
2
Saying electrons flow from positive to negative — conventional current does, electrons go from negative to positive.
3
Connecting an ammeter in parallel — it must be in series.
4
Forgetting to convert mA to A before using I = Q / t.

Last updated:

\n