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4.3.2Electricity and Magnetism

Current in Circuits — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 37 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.3.2 Current in Circuits — IGCSE PHYSICS notes: State and use the rules for current in series and parallel circuits, including that current is conserved at a junction.Single loop — the components are connected one after another in one complete loop.Current — it is the same at ev…

  • Voltage of the supply
  • Resistance of the components
  • More lamps in series
  • Itotal = I1 + I2 + I3
  • Resistance and current
  • Extra branch

Section 1 of 10Series circuits

Single loop
  • Single loop the components are connected one after another in one complete loop.
Current
  • Current it is the same at every point: I = I1 = I2 = I3.
  • The number of electrons per second passing one point equals that passing any other point.
Break
  • Break if one component breaks, the circuit is broken and every component stops.

Section 2 of 10Factors affecting current in a series circuit

Factors affecting current in a series circuit
Voltage of the supply
  • Voltage of the supply increasing it drives more current and decreasing it reduces the current.
Resistance of the components
  • Resistance of the components adding components increases the total resistance, so less current flows.
More lamps in series
  • More lamps in series each lamp is dimmer because the current falls.

Section 3 of 10Parallel circuits

Branches
  • Branches the components are on separate loops between the same two points.
Splitting
  • Splitting the current from the supply splits at a junction, flows through each branch and recombines.
Itotal
I1 + I2 + I3
  • the total current is the sum of the branch currents (Extended).
Resistance and current
  • Resistance and current the branch with the smaller resistance carries the larger current.
  • Branches have identical currents only if their resistances are identical.
Extra branch
  • Extra branch it raises the current from the supply, but every branch keeps the same p.d.

Section 4 of 10Lighting circuits

Construction
  • Construction lighting circuits are wired in parallel.
Same p.d.
  • Same p.d. all bulbs have the same p.d. and so the same brightness.
Failure
  • Failure if one bulb breaks, the rest continue because the current passes along each branch independently.
Switching
  • Switching lamps can be switched on and off individually.

Section 5 of 10Conservation of charge at a junction (Extended)

Rule
  • Rule the current into a junction equals the current out of it.
Reason
  • Reason electrons cannot be created or destroyed, so the total current must stay the same.
Direction
  • Direction current flows from the positive to the negative terminal of the cell.
Example
  • Example if 5.0 A flows into a junction and one branch carries 3.0 A, the other carries 2.0 A.

Section 6 of 10Measuring currents

Measuring currents
Place one ammeter in the main supply wire.
Place one ammeter in each branch.
Compare the readings; the branch readings should add up to the main reading.
  • Series circuit
    • ammeters in different positions read the same, which shows current is not used up.

Section 7 of 10Short circuits

Cause
  • Cause a wire of almost no resistance connected across a component gives the current an easy path.
Effect
  • Effect nearly all the current goes through the wire, the component goes out, and the current from the cell rises sharply.
Danger
  • Danger cells or wires can overheat, so a fuse is placed in the circuit.

Section 8 of 10More from the specification

1
Kirchhoff's first law validation
  • Kirchhoff's first law validation summing junction currents to zero provides a direct empirical means to verify conservation of electric charge.
2
Electric Current Distribution
  • Current represents the rate of charge flowing from a power source;
    • in a series circuit current is identical everywhere,
  • whereas in parallel circuits it divides among branches.
3
Fuse and Electrical Safety
  • A safety fuse incorporates a thin metallic wire calibrated to melt and isolate the power source whenever excessive current threatens to overheat appliance wiring.
4
Earth Wire and Protective Insulation
  • Metal-cased appliances use an earth wire to divert fault current harmlessly into the ground to blow the fuse,
  • while double insulation prevents external chassis contact with live parts.
5
Appliance Power Ratings
  • The electrical power rating indicates the rate of energy consumption from the mains supply (P = VI), dictating the appropriate rating for protective fuses.
    What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
6
Source
  • Reservoir at TH providing heat QH
7
Insulation
  • Material that reduces thermal energy transfer.
8
Potential difference
  • Work done by a unit charge passing through a component.
9
Potential divider
  • Series resistors that share the supply p.d.
10
Mains supply
  • Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
11
Circuit diagram
  • Drawing of a circuit using standard symbols.
12
Circuit symbol
  • Standard picture for a component.

Section 9 of 10Key terms

TermMeaning
Series circuitCircuit with a single loop where the current is the same everywhere.
Parallel circuitCircuit with several branches between the same two points.
JunctionPoint where wires meet and current splits or joins.
Conservation of chargeCharge is neither created nor destroyed, so current in equals current out.
Short circuitLow-resistance path across a component.

Section 10 of 10Exam technique

1
Saying the current is smaller after a lamp in series — it is the same all the way round.
2
Thinking the current splits equally at a junction — it divides according to the resistance of each branch.
3
Adding the currents of series components — they are equal, not added.
4
Believing a break in one parallel branch stops the whole circuit.

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