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A –Level Physics: Electrical Quantities: Internal Resistance Objectives: 45. Know the definition of electromotive force (e.m.f.) and understand what is meant by internal resistance and know how to distinguish between e.m.f. and terminal potential difference 47. Understand how changes of resistance with temperature may be modelled in terms of lattice vibrations and number of conduction electrons and understand how to apply this model to metallic conductors and negative temperature coefficient thermistors 48. Understand how changes of resistance with illumination may be modelled in terms of the number of conduction electrons and understand how to apply this model to LDRs. Additional skills gained: • Practical Planning • Integrating GCSE content Starter: Riddle (answer revealed next lesson) You have one 16-quart pitcher full of water, one empty nine-quart pitcher, and one empty seven-quart pitcher… 10mins 16 9 7 Using nothing but these three pitchers, can you dividethe water evenly so that the 16-and nine-quart pitchers are each holding exactly eight quarts of water? Internal Resistance Why is it that when a voltmeter is placed around a 1.5V battery, it doesn’t read ‘1.5v’? When a current is flowing through a circuit, the electrons still have to travel through the cell. A small portion of the voltage/energy will be lost here because the electrons have to overcome the internal resistance of the cell! Because the cell has this internal resistance, the convention is to show the cell adjacent to a resistor. This resistor would have a very low value. Power/I.Resistance equations What does V= in Ohm’s law? So, if Power=Current x Voltage, what is the formula relationship between Power and resistance? 1. V= I x R 2. P= I x V (so we can substitute ‘V’ for I x R) 3. P= I x I x R 4. P= I2 x R As electrons are resisted through a cell, they will dissipate power (work done in a set time). This is why each coulomb, will not get the full number of joules (voltage). So, the EMF is actually what is left when we subtract the voltage used in internal resistance. So as V=IR, what is the equation for the EMF of this circuit? Power/I.Resistance equations So the total energy produced by the cell (ε). Is the voltage used against internal resistance added to the voltage supplied to the rest of the circuit. So what’s the equation? ε =IR + Irinternal So, rearranging: • IR = ε - Irinternal • V= ε – Irinternal So this shows that the voltage produced at the terminals of the cell (i.e. outside of the cell) is effectively the EMF – the voltage overcoming internal resistance Typical Exam Q #1 Q1. A battery is connected to a 10ohm resistor as shown. The e.m.f. (electromotive force) of the battery is 12V. (a) (i) Explain what is meant by the e.m.f. of a battery. (ii) When the switch is open the voltmeter reads 12.0V and when it is closed it reads 11.5 V. Explain why the readings are different. (b) Calculate the internal resistance of the battery. Typical Exam Q #2 Q2. A very high resistance voltmeter reads 20V when connected across the terminals of a d.c. power supply. The high resistance meter is disconnected and a second voltmeter of resistance 1.0k is then connected across the supply. The second meter gives a reading of 16V. (i) State the e.m.f. of the power supply. (ii) Calculate the current which flows through the second meter. (iii) Calculate the internal resistance of the power supply. (iv) Show that the current is equal to 0.080A when the supply is short circuited. Typical Exam Q #3 Q3 (a) A steady current of 0.25 A passes through a torch bulb for 6 minutes. Calculate the charge which flows through the bulb in this time. (b) The torch bulb is now connected to a battery of negligible internal resistance. The battery supplies a steady current of 0.25 A for 20 hours. In this time the energy transferred in the bulb is 9.0 × 104 J. Calculate (i) the potential difference across the bulb, (ii) the power of the bulb Typical Exam Q #4 Q4 (a) A student is given three resistors of resistance 3.0 , 4.0 and 6.0 ohms respectively. (i) Draw the arrangement, using all three resistors, which will give the largest resistance. (ii) Calculate the resistance of the arrangement you have drawn. (iii) Draw the arrangement, using all three resistors, which will give the smallest resistance. (iv) Calculate the resistance of the arrangement you have drawn. (b) The three resistors are now connected to a battery of EMF 12 V and negligible internal resistance, as shown in the diagram above (ii) Calculate the voltage across the 6.0 resistor Typical Exam Q #5 Q5. In the circuit shown below, the battery, of emf 6.0V, has negligible internal resistance. (a) Calculate the current through the ammeter when the switch S is (i) open (ii) Closed (b) The switch S is now replaced with a voltmeter of infinite resistance. Determine the reading on the voltmeter. Practice Questions Grab a laptop and go to the following URL: http://www.cyberphysics.co.uk/Q&A/KS5/electricity/circuits/circuits.html For each question: • Answer the question with DETAILED working out (step by step how you’re working it out) not just the numbers • Once completed, then check the solution (bottom right of each question) and mark/correct Extension: http://www.cyberphysics.co.uk/Q&A/KS5/electricit y/resistivity/index_resistivity.html 40m Independent Study