Reliable circuit work begins by identifying what must be measured or calculated. Write the quantity, symbol and unit before choosing a meter or equation.
Quantity and unit check
Current I → ampere (A)
Potential difference V and emf ε → volt (V)
Resistance R and internal resistance r → ohm (Ω)
Power P → watt (W)
Energy W → joule (J) or kilowatt-hour (kWh)
Meter connection check
Ammeter: connect in series so the circuit current passes through it.
Voltmeter: connect in parallel across the two points being compared.
For direct current, observe terminal polarity.
Begin with a safe range and never connect an ammeter directly across a source.
Reading and recording check
Find the value of one smallest scale division.
Read an analogue pointer at eye level to reduce parallax error.
Record sensible significant figures and attach the unit.
Repeat readings where appropriate and check for zero error.
Equation check
Key equation
ε=V+Ir
Key equation
P=VI=I2R=V2R
Key equation
W=Pt=VIt
Final physical check
Confirm that all values use compatible SI units.
Check the sign and assumed current direction.
Test whether the size of the answer is reasonable.
For a loaded discharging cell, terminal voltage should be less than emf.
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Exam preparation
Exam Focus
Important points
Meter connections
Range and parallax
Circuit units
Source, power and energy equations
Common mistakes
Connecting an ammeter across a component.
Connecting a voltmeter in series.
Selecting an equation before identifying the quantity.
Mixing minutes with seconds or watts with kilowatts.
Recording more digits than the scale supports.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.
