All chapters / 11
Class 9–10 · Physics
Current electricity
Change it and see
Current electricity
Lessons in this chapter
Lessons
1Current and direction
Current is charge crossing a section per unit time. Conventional current follows positive-charge motion; metal electrons drift oppositely. A source maintains the driving potential difference in a closed circuit. Devices transform energy rather than consuming charge.
Key relationship
I = Q/t; 1 A = 1 C/s
Worked example
12 C in 4 s ⇒ 3 A
Next lesson →2emf and terminal voltage
emf is energy supplied per charge, measured in volts rather than newtons. A discharging cell's internal resistance causes an internal voltage drop, so terminal voltage is below emf.
Key relationship
ε = energy supplied/Q; discharging: V = ε−Ir
Worked example
ε = 1.5 V, r = 0.2 Ω, I = 0.5 A ⇒ V = 1.4 V
Next lesson →3Ohm's law
An ohmic conductor has proportional voltage and current under constant physical conditions, especially temperature. Its V–I slope, with V vertical, is resistance. A heating filament need not remain ohmic over its full operating range.
Key relationship
V = IR; R = V/I; unit Ω
Worked example
V = 6 V, I = 0.3 A ⇒ R = 20 Ω
Next lesson →4Resistivity
Wire resistance increases with length and decreases with cross-sectional area. Resistivity depends on material and temperature. Metals usually gain resistance when heated. Diameter changes affect area quadratically.
Key relationship
R = ρL/A; resistivity unit Ω m
Worked example
Double length and halve area ⇒ resistance becomes 4R
Next lesson →5Series resistance
Series components share one current path. Voltage drops add to the supplied voltage and resistances add. Opening any component breaks that path.
Key relationship
Rs = R₁+R₂+…
Worked example
2 Ω + 4 Ω at 12 V ⇒ I = 12/6 = 2 A; drops 4 V and 8 V
Next lesson →6Parallel resistance
Parallel branches connect across the same two nodes and share voltage. Branch currents add; lower resistance draws more current. The equivalent resistance is below the smallest branch resistance.
Key relationship
1/Rp = 1/R₁+1/R₂+…; I = I₁+I₂
Worked example
6 Ω and 3 Ω across 6 V ⇒ currents 1 A, 2 A; equivalent 2 Ω
Next lesson →7Circuits and meters
Standard symbols clarify circuit structure. Connect an ammeter in series and a voltmeter across the measured component. An ideal ammeter has negligible resistance; an ideal voltmeter has extremely high resistance.
Key relationship
ammeter: series; voltmeter: parallel
Worked example
Measure V and I across/through one resistor ⇒ R = V/I
Next lesson →8Joule heating and power
Electrical power is VI. For an ohmic resistor this becomes I²R or V²/R. Whether greater resistance means greater heating depends on whether current or voltage is held fixed.
Key relationship
H = I²Rt; P = VI = I²R = V²/R
Worked example
I = 2 A, R = 5 Ω, t = 10 s ⇒ H = 200 J, P = 20 W
Next lesson →9Electrical energy use
One billing unit is 1 kWh of energy. Multiply kW by hours. A rating describes operation at specified voltage. This calculation gives energy, not a complete bill with tariffs or fixed charges.
Key relationship
E(kWh) = P(kW)t(h); 1 kWh = 3.6 MJ
Worked example
4 × 60 W × 5 h/day × 30 days = 36 kWh
Next lesson →10Fuses, earthing and safety
A short circuit can draw excessive current and heat wiring. Fuses and breakers interrupt unsafe currents. Protective earthing provides a fault path for exposed conductive parts. Use only the online demonstration; mains work belongs to trained people.
Key relationship
heating ∝ I²R
Worked example
Adding several high-power devices to one outlet raises total current.
Next lesson →11Cells and junction rules
Series cells add emf and internal resistance. Identical parallel cells retain emf and reduce equivalent internal resistance. Junction current balance expresses charge conservation; closed-loop voltage balance expresses energy conservation.
Key relationship
series εtotal = nε; identical parallel rtotal = r/n; ΣIin = ΣIout
Worked example
3 A enters; one outgoing branch has 1 A ⇒ other outgoing branch 2 A
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. 12 V across 4 Ω: current?
I = 12/4 = 3 A
2. Two 6 Ω resistors in parallel?
1/R = 1/6+1/6 ⇒ R = 3 Ω
3. How is a voltmeter connected?
In parallel across the measured component
4. 2 kW device for 3 h: energy?
E = 2 × 3 = 6 kWh
5. Double current at fixed R: heating-power factor?
P = I²R ⇒ 4
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.