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is electrical circuits on 32 Questions 1 What is the internal resistance of a GV battery If it supplies 58V to a lamp when the current flowing is 0.40 A? 2 What is the internal resistance of a 1.5V dry cell, that delivers 13% to a lamp which has a resistance of 5.0417 The starter motor for a car engine needs a very high current, for example 100 A. resistance. (a) Explain why a car battery should have a very low internal (b) Explain why the car's headlights are observed to dim if they are on when the car's starter motor is used. 4 A student investigating the emf and internal resistance of a dry cell used the circuit in fig B, with the cell in the place marked by the dashed lines. She collected the results in table A Plot a graph to find both the internal resistance and the emf of the cell that the student tested. Ammeter reading, I/A Voltmeter reading, V/V 0.10 1.45 0.20 1.42 fig B Circuit for demonstrating the effect of internal resistance 0.30 1.39 0.40 1.37 0.80 1.18 1.10 7.09 table A Current and voltage readings from a student experiment to internal resistance of a dry cell. Questions 1 How much work as done by the reshtor in the cincut in fig [ if the crcut in sanched on for 11 secondal power supply Penister volumeter nig E A resistor circus 2 what is the power dissipated by the reviitor in fig E? 3 what power is dissipated by a 470 fl realtor where (al current of 1 76 A passes through it? my it is measured with a pd of 90VT 4 (1) What is the efficiency of the electric heater in fig Fi (b) Why would your answer to part (a) be the same if you consider the heater switched on for one hour, `and if you consider it switched on for two hours? 100 J of 1 J of light 4 J of sound energy electrical energy energy useful energy output - energy input 95J of heat energy fig F Energy flows through an electric heater. 3 Compact fluorescent lamps have a tube containing a gas, which glows when electricity passes through it. These are much more efficient than filament, lamps in which a metal wire glows when it is red hot due to the ohmic heating current flow through it. Explain why compact fluorescent lamps are much more efficient than filament lamps.Emf and interna Complete electrical circuits on, you should be able to.. by internal resistance By considering energy conservation, we can draw up an equation for the circuit that incorporates the internal resistance. on to determine an emf's internal resistance Questions Vload + Vinternal ternal resistance * = Vload + Ir = IR + Ir What is the internal resistance of a 6 V battery if it supplies 5.8 V to a the proponents have come meistance The best ammeters hit is not actually zero, Southat effect on their current measurements can be ignored, but it is not actually zero, Soy that The circuit shown in fig B demonstrates what the mathematics lamp when the current flowing is 0.40 A? means practically. The voltmeter will be measuring the voltage 2 What is the internal resistance of a 1.5 V dry cell, that delivers 1.3 V tomotive force (em. such as batteries, will also have a small, but non-zero, resistances of actually supplied by the cell, so that will be its pure emf minus the pd across the internal resistance, or lost volts. However, as the to a lamp which has a resistance of 5.0!? resistance of an emf source is called its internal resistance. In most situations, the ance. Howstance of an emf source will not affect the performance of a circuit. It may not eveninternet ammeter should have zero pd, the voltmeter will also be reading 3 The starter motor for a car engine needs a very high current. for as the power lost - energy wasted as heat - through a component increases with now E - V, = VR the value of the potential difference across the variable resistor, R example 100A. ough it (see Section 3.2.5). there can be a significant power loss when large currents in the (a) Explain why a car battery should have a very low internal net effect of this energy usage by the emf source itself will be for it to supply a smaller vote us Or, in terms of the ammeter reading. /, and the voltmeter reading. resistance. of the circuit, Which could hamper the circuit's performance. A large current through gole to. e - Ir = V b) Explain why the car's headlights are observed to dim if they are resistance could also damage the emf source as a result of ohmic (resistance) heating E = V + Ir on when the car's starter motor is used. which is, again, a statement of energy conservation in this circuit. A student investigating the emf and internal resistance of a dry cell used the circuit in fig B, with the cell in the place marked by the dashed lines. She collected the results in table A. internal Investigation resistance, r h to find both the internal resistance and the emf of the Investigating internal resistance cell that the student tested. ED emf. E We can find the internal resistance for a source of emf such as a cell Ammeter reading, I/A experimentally. Using a circuit like that shown in fig B, we adjust the Voltmeter reading, V/V 8 A Any emf source could be considered to be a pure emf with a small resistive potential difference acting variable resistor to change the current through the circuit, and at 0.10 each current value we record the voltmeter reading. 0.20 145 142 with the emf. The equation we have previously seen for the quantities in this 0.30 1.39 cllect of having an internal resistance is that an emf will never be able to fully supply in circuit is: 0.40 B - Ir = V 0.80 1.31 wuinum voltage. There will always be a small drop in voltage over the internal resistance not Will be bigger with a higher current. The pd over the internal resistance, r, can be four ney or V = -rl+e 1.10 1.18 1.09 Comparing this with the equation for a straight-line graph y = mx + C table A Current and voltage readings from a student experiment to m's law: shows us that a graph plotted with V on the y-axis and / on the x-axis internal resistance of a dry cell Vinternal = Ir pu Is sometimes referred to as 'lost volts', as the circuit appears to have fewer volts in you, will have a gradient equal in magnitude to the internal resistance, r. The y-intercept, C. here represents the voltmeter reading when no Key definition odd than the emf should be supplying. A measurement of voltage across the terminals e by current flows. If there is no current in the circuit, the internal resistance can take no energy from the circuit, so there will be zero The resistance of an emf source is called its internal ring a circuit would not measure the emf, it would measure the emf minus the lost vol to resistance. ost volts and the voltmeter will read the full value of the emf. So, the inal pd' would then be: y-intercept is the value of the emf, &. terminal = 8 - It Parallel branches have the same voltage across them, the terminal pd would also equally he load resistance. erminal = Vload = 8 - Ir gradient = -r HO fig C Graph of internal resistance investigation results. demonstrating the effect of internal resistance