Electromotive force and potential difference
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Electromotive Force (e.m.f.) Electromotive force (e.m.f.) The electrical work done by a source in moving a unit charge around a complete circuit. E = W/Q Where: E = e.m.f. (V), W = work done (J), Q = charge (C) The e.m.f.
is measured in volts (V) . It represents the energy supplied per coulomb of charge by the power source (battery, cell, generator). Potential Difference (p.d.) Potential difference (p.d.) The work done by a unit charge passing through a component.
V = W/Q Where: V = p.d. (V), W = work done (J), Q = charge (C) The Volt The p.d. between two points is 1 volt if 1 joule of energy is transferred when 1 coulomb of charge passes between the points. 1 V = 1 J/C Energy Transfers in Circuits In an electric circuit: The source (battery/cell) supplies electrical energy to the charges Components (lamps, motors, resistors) transfer energy to the surroundings Energy is transferred as heat, light, sound, kinetic energy , etc.
FIG 4.2.18: Energy transfer model in a circuit A simple circuit with battery and lamp. "Mr Coulomb" (representing 1C of charge) shown carrying a bundle of energy. At the battery: charge picks up energy bundle.
At the lamp: charge delivers energy (converted to light and heat). Same charge continues around circuit to collect more energy from battery. Key Point Electrical energy is transferred, not charge. The same amount of charge flows into and out of each component.
Using Voltmeters A voltmeter measures potential difference. It must be connected: In parallel (across) the component + terminal of voltmeter to the side where current enters the component Ammeter Voltmeter Measures current (A) Measures p.d.
(V) Connected in series Connected in parallel Very low resistance Very high resistance Worked Examples Example 1: Calculating p.d. Question: 8 J of work is done when 2 C of charge passes through a lamp.
Calculate the p.d. across the lamp. Answer V = W/Q = 8/2 = 4 V Example 2: Calculating Energy Question: The p.d. across a lamp is 12 V. How much energy is transferred when a current of 2 A flows for 5 s?
Answer Q = I × t = 2 × 5 = 10 C W = Q × V = 10 × 12 = 120 J Or directly: W = I × V × t = 2 × 12 × 5 = 120 J
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