Calculate energy per charge and identify correct meter and protection functions.
Method and worked examples
Potential difference is energy transferred per unit charge: V=QΔE. One volt means one joule per coulomb. A source supplies energy to charges; components transfer that energy to other forms. Voltage is measured between two points, whereas current is measured through a component. Do not confuse the coulomb of charge with the joule of energy.
An ideal ammeter has negligible resistance and connects in series. An ideal voltmeter has very high resistance and connects in parallel across the component. A fuse or circuit breaker interrupts excessive current. A residual current device detects an imbalance between outgoing and returning current; it serves a different protection function and does not make contact with electricity safe.
Example 1
A component transfers 48J when 4.0C passes. Find potential difference.
V=QΔE=4.048=12V.
Example 2
A 9.0V source moves 20C. Find supplied electrical energy.
ΔE=VQ=9.0×20=180J. Voltage is joules per coulomb, not coulombs per second.
Watch out:Voltage is a difference between two points. A voltmeter connects across them; current flows through a branch.
7. Potential difference and circuit meters | Year 11 Physics | Ascent Arcade