Physics: Electromotive Force and Potential Difference PDF

Physics Ms.Kaneeshka 4.2.3 Electromotive force and potential difference Electromotive force Electromotive force (e.m.f.), symbol E, is defined as the electrical work, w, done by a source in moving a unit charge aro...

Physics Ms.Kaneeshka 4.2.3 Electromotive force and potential difference Electromotive force Electromotive force (e.m.f.), symbol E, is defined as the electrical work, w, done by a source in moving a unit charge around a complete circuit: Electromotive force (e.m.f.) is defined as, The electrical work done by a source in moving a unit charge around a complete circuit. e.m.f. = work done (by a source) Charge E = W/ Q The electromotive Force (e.m.f.) is the name given to the potential difference of the power source in a circuit Electromotive force (e.m.f.) is measured in volts (V) The e.m.f. of a car battery is 12 V and the domestic mains supply in many countries is 240 V. Potential Difference or voltage As charge flows around a circuit energy is transferred from the power source to the charge carriers, and then to the components This is what makes components such as bulbs light up The potential difference between two points in a circuit is related to the amount of energy transferred between those points in the circuit Potential difference is defined as The work done by a unit charge passing through a component p.d. = work done (on a component) /charge V=W/Q Potential difference is measure in volts (V) 1 Physics Ms.Kaneeshka The p.d. between two points in a circuit is 1 volt if 1 joule of energy is transferred when 1 coulomb passes from one point to the other. That is, 1 volt = 1 joule per coulomb (1V = 1J/C). If 2J is transferred by each coulomb, the p.d. is 2V. V = W/Q W=Q×V W=I×t×V Voltmeters A voltmeter is used to measure potential differences. It should always be placed in parallel with the component across which the p.d. is to be measured. The potential difference is the difference in electrical potential between two points, therefore the voltmeter has to be connected to two points in the circuit Voltmeter can be either,  Digital (with an electronic read out)  Analogue (with a needle and scale) 2 Physics Ms.Kaneeshka Analogue voltmeter Analogue voltmeters are subject to parallax error Always read the meter from a position directly perpendicular to the scale Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary They should be checked for zero errors before using Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements Digital voltmeters Digital voltmeters can measure very small potential differences, in mV or µV Digital displays show the measured values as digits and are more accurate than analogue displays They’re easy to use because they give a specific value and are capable of displaying more precise values However digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down Digital voltmeters should be checked for zero error Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results Analogue and Digital Voltmeters When making a measurement on either an analogue or digital voltmeter a suitable range must first be chosen. If the reading is off-scale, the sensitivity should be reduced by changing to the higher range. 3 Physics Ms.Kaneeshka Calculating Total EMF In series When several cells are connected together in series, their combined EMF is equal to the sum of their individual EMFs Total Electromotive Force In parallel When cells with the same EMF are connected in parallel, the resultant EMF will be equal to the EMF of any one of the cells. CHECKPOINT 4.2.3 Electromotive force and potential difference 1. Define e.m.f. (electromotive force) as the electrical work done by a source in moving a unit charge around a complete circuit; recall and use the equation e.m.f. = work done (by a source) / charge E=W/Q 2 Define p.d. (potential difference) as the work done by a unit charge passing through a component; recall and use the equation p.d. = work done (on a component) / charge V=W/Q 3 Know that e.m.f. and p.d. are measured in volts and that the volt is given by joule per coulomb (J/C) 4 Describe the use of voltmeters (analogue and digital) with different ranges 5 Calculate the total e.m.f. where several sources are arranged in series 6 State that the e.m.f of identical sources connected in parallel is equal to the e.m.f. of one of the sources 4