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
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
Part 1 of 8Series 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.
Part 2 of 8Factors 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.
Part 3 of 8Parallel 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.
Part 4 of 8Lighting 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.
Part 5 of 8Conservation 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.
Part 6 of 8Measuring 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.
Part 7 of 8Short 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.
Part 8 of 8More 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.
Key terms
| Term | Meaning |
|---|---|
| Series circuit | Circuit with a single loop where the current is the same everywhere. |
| Parallel circuit | Circuit with several branches between the same two points. |
| Junction | Point where wires meet and current splits or joins. |
| Conservation of charge | Charge is neither created nor destroyed, so current in equals current out. |
| Short circuit | Low-resistance path across a component. |
Avoid these mistakes
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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