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Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits Click on a question number 3, 4, 5, 6, 7, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 3 An electron volt is a unit of energy. It represents the change in potential energy of an electron which moves through a potential difference of 1 volt. If the charge on an electron is 1.6 x 10 -19 C, what is the equivalent energy in joules? E QV E 1.6 10 19 1 E 1.6 10 19 J Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 4 Mass of an electron = 9.1 x 10 -31 kg. Charge on an electron = 1.6 x 10 -19 C. The electron shown below is accelerated across a p.d. of 500 V. (a) How much electrical work is done? W QV W 1.6 1019 500 W 8 1017 J (b) How much kinetic energy has it gained? Work done Energy gained EK 8 1017 J 1 17 (c) What is its final speed? 8 10 9.110 31 v 2 2 1 2 17 EK mv 2 8 10 14 2 v2 1 . 76 10 9.110 31 v 1.33 107 m s -1 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 5 Electrons are ‘fired’ from an electron gun at a screen. The p.d. across the gun is 2000 V. After leaving the positive plate the electrons travel at a constant speed to the screen. Assuming the apparatus is in a vacuum, at what speed will the electrons hit the screen? W QV W 1.6 10 19 2000 W 3.2 10 16 J 1 2 EK mv 2 3.2 10 16 1 9.110 31 v 2 2 16 2 3 . 2 10 14 v2 7 10 9.110 31 v 2.65 107 m s -1 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 6 What would be the increase in speed of an electron accelerated from rest by a p.d. of 400 V? W QV W 1.6 10 19 400 W 6.4 10 17 J 1 EK mv 2 2 1 6.4 10 17 9.11031 v 2 2 17 2 6 . 4 10 14 v2 1 . 41 10 9.110 31 v 1.2 107 m s -1 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 7 An X-ray tube is operated at 25 kV and draws a current of 3 mA. (a) Calculate (i) the kinetic energy of each electron as it hits the target EK QV EK 1.6 1019 25 103 EK 4 1015 J (ii) the velocity of impact of the electron as it hits the target 1 EK mv 2 2 4 10 15 1 9.110 31 v 2 2 2 4 10 15 14 v 88 10 9.110 31 v 9.4 107 m s -1 2 (iii) the number of electrons hitting the target each 3 second. Q It Q 3 10 1 3 103 16 No. of electrons 1 . 875 10 1.6 1019 (b) What happens to the kinetic energy of the Click the mouse to continue electrons? It is transferred to the Ek of the X ray photon Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 11 In the circuit opposite: (a) what is the total resistance of the RP R1 R2 RP 12 4 12 4 circuit P R 1 1 1 1 1 1 4 12 (b) what is the resistance between X 3 and Y (c) find the readings on the ammeters I V R I 12 6 I 2 A (A1 ) 3 RS 3 3 RS R1 R2 RS 6 The current splits in the ratio of 12 : 4 or 3 : 1 The current through the 4 resistor is 3/4 of 2 A or 1.5 A (d) calculate the p.d. between X and Y (e) what power is supplied by the battery? P 12 2 24 W P VI V IR V 23 6 V Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 12 The circuit opposite uses the 230 V alternating mains supply. Find the current flowing in each resistor when: (a) switch S is open (b) switch RP R1 R2 RP 8 24 24 S is closed. S S 1 2 R R R R 20 1 1 1 1 1 1 4 I V R I I 11.5 A 4 R 6 24 RS R1 R2 RS 6 12 18 230 20 P V 230 I 12.8 A 12 18 R Voltage across 12 IR 12.8 12 153.3 V I Voltage across parallel branch 230 153.3 76.7 V V 76.7 V 76.7 (8 ) I 9.6 A (24 ) I 3.2 A R 8 R 24 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 13 An electric cooker has two settings, high and low. It takes 1 A at the low setting and 3 A at the high setting. (a) Find the resistance of R1 and R2 At the low setting, the switch is open R1 I V 1 230 230 RP R1 R2 76.7 230 R2 At the high setting, the switch is closed I 3 1 1 1 1 1 1 RP 76.7 2 R2 76.7 230 R2 230 230 V 230 2 R 115 230 1 1 1 1 3 1 2 (b) What is the power consumption at each setting? Open P VI P 230 1 230 W Closed P VI P 230 3 690 W Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 14 (a) Find the value of the series resistor which would allow the bulb to operate at its normal rating. Finding the current through the bulb P VI I P V I 36 3A 12 The current through the bulb is the same as the current through the resistor R V I R 12 4 3 (b) Calculate the power dissipated in the resistor P VI P 12 3 36 W Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 15 In the circuit below, r represents the internal resistance of the cell and R represents the external resistance of the circuit. When S is open, the voltmeter reads 2.0 V. When S is closed, it reads 1.6 V and the ammeter reads 0.8 A. 2.0 V (a) What is the e.m.f. of the cell? (b) What is the terminal potential difference when S is 1.6 V closed? (c) Calculate the values of r and V 1.6 R. R 2 R I E IR Ir 0.8 2 1.6 0.8 r r 2 1.6 0.5 0.8 (d) If R was halved in value, calculate the new readings on the ammeter and voltmeter. E I (R r) 2 I (1 0.5) 2 I 1.3 A 1.5 V IR V 1.3 1 1.3 V Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 16 The cell in the diagram has an e.m.f. of 5 V. The current through the lamp is 0.2 A and the voltmeter reads 3 V. Calculate the internal resistance of the cell. E IR Ir IR t. p.d . 3 V 5 3 0.2 r r 53 10 0.2 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 17 A cell of e.m.f. 4 V is connected to a load resistor of 15 . If 0.2 A flows round the circuit, what must be the internal resistance of the circuit? V IR V 0.2 15 3 V IR t. p.d . 3 V Lost volts 4 V 3 V 1 V Lost volts Ir 1 0.2 r r 1 5 0.2 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 18 A signal generator has an e.m.f. of 8 V and internal resistance of 4 . A load resistor is connected to its terminals and draws a current of 0.5 A. Calculate the load resistance. E IR Ir 8 0.5 R 0.5 4 R 82 12 0.5 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 19 (a) What will be the terminal p.d. across the cell in the circuit below. t.p.d. 1.5 - 0.2 1.3 V (b) Will the current increase or decrease as R is increased? As R increases, the current decreases (c) Will the terminal p.d. then increase or decrease? Explain your answer. The t.p.d. increases because the lost volts will be smaller. Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 20 A cell with e.m.f. 1.5 V and internal resistance 2 is connected to a 3 resistor. What is the current? E I (R r) 1.5 I (3 2) I 1.5 0.3 A 5 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 21 A pupil is given a voltmeter and a torch battery. When he connects the voltmeter across the terminals of the battery it registers 4.5 V, but when he connects the battery across a 6 resistor, the voltmeter reading decreases to 3.0 V. (a) Calculate the internal resistance of the battery. E 4.5 V t. p.d . IR 3 V I 6 3 I 0.5 A E IR Ir 4.5 3 0.5 r r 4.5 3 3 0.5 (b) What value of resistor would have to be connected across the battery to reduce the voltage reading to 2.5 V ? Lost volts Ir 2 I 3 I 0.67 A E IR Ir 4.5 0.67 R 2 R 4.5 2 3.75 0.67 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 22 In the circuit shown, the cell has an e.m.f. of 6.0 V and internal resistance of 1 . When the switch is closed, the reading on the ammeter is 2 A. What is the corresponding reading on the voltmeter ? Lost volts Ir Lost volts 2 1 2 V E t. p.d . Lost volts t. p.d . 6 V - 2 V 4 V Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 23 In order to find the internal resistance of a cell, the following sets of results were taken. (a) Draw the circuit diagram used. Graph of Voltage against Current 1.2 (b) Plot a graph of these results and from it determine (i) the e.m.f. (ii) the internal resistance of the cell. 1 Voltage (V) 0.8 0.6 0.4 0.2 E IR Ir E Voltage (V ) Ir V E Ir V rI E E 1.1 V 0 0 0.02 0.04 y = -4.1714x + 1.1053 0.06 0.08 0.1 0.12 0.14 Current (A) r 4.2 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 23(cont) (c) Use the e.m.f. from part (b) to calculate the lost volts for each set of readings and hence calculate 6 values for the internal resistance. E t. p.d . Lost volts Lost volts E t. p.d . Lost volts Lost volts Ir r Lost volts I r () 0.08 0.16 0.25 0.32 0.41 0.50 4.0 4.0 4.17 4.0 4.10 4.17 (d) Calculate the mean value of internal resistance and the r approximate randomMean uncertainty. value 4.07 n Approximat e random uncertaint y Maximum value - minimum value number of measuremen ts Approximat e random uncertaint y 4.17 - 4.0 0.03 6 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 24 The voltage across a cell is varied and the corresponding current noted. The results are shown in the table below. Graph of voltage against current Plot a graph of V against I. 6.2 Voltage (V) 6.1 6 5.9 5.8 5.7 5.6 (a) What is the open circuit p.d? Open circuit p.d. is voltage when no current is drawn 5.5 5.4 0 1 2 y = -0.1x + 6 3 4 5 6 Current (A) E 6V (b) Calculate the internal resistance. E IR Ir E Voltage (V ) Ir V E Ir V rI E r 0.1 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 24 cont (c) Calculate the short circuit current. E I (R r) R0 E Ir 6 I 0.1 I 60 A (d) A lamp of resistance 1.5 is connected across the terminals of this supply. Calculate (i) the terminal p.d.and (ii) the power delivered to the lamp. E I (R r) 6 I (1.5 0.1) 6 I 3.75 A 1.6 t. p.d IR 3.75 1.5 5.63 V P IV 3.75 5.63 21.1 W Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 25 Calculate the p.d. across R2 in each case. R2 V2 Vs R1 R2 V2 8k 5 2k 8k 8 V2 5 4 V 10 R2 V2 Vs R1 R2 V2 R2 Vs R1 R2 1k 5 4k 1k V2 750 5 500 750 1 V2 5 1 V 5 V2 750 5 3 V 1250 V2 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 26 Calculate the p.d. across AB (voltmeter reading) in each case. Potential at A = 6 V Potential at A = 2/7 x 5 = 1.4 V Potential at A = 2/5 x 10 = 4 V Potential at B = 3 V Potential at B = 8/18 x 5 = 2.2 V Potential at B = 4/10 x 10 = 4 V p.d. across AB = 3 V p.d. across AB = - 0.8 V p.d. across AB = 0 Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 27 (a) Calculate the reading on the voltmeter. Potential at A = 6/15 x 9 = 3.6 V Potential at B = 3/9 x 9 = 3 V p.d. across AB = 0.6 V (b) What alteration could be made to balance the bridge circuit ? Increase the 9 k resistor to 12 k Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 28 Three pupils are asked to construct balanced Wheatstone bridges. Their attempts are shown. One of the circuits gives a balanced Wheatstone bridge, one gives an off balance Wheatstone bridge and one is not a Wheatstone bridge. (a) Identify each circuit. Unbalanced WheatstoneBalanced WheatstoneNon Wheatstone (b) How would you test that balance had been obtained? The galvo should read zero in a balanced Wheatstone Bridge (c) In the off – balance Wheatstone bridge (i) calculate the potential difference across the galvanometer. Potential at A = 5/15 x 1.5 = 0.5 V Potential at B = 10/15 x 1.5 = 1 V (ii) in which direction will electric p.d. across AB = 0.5 V current flow through the galvanometer? From B to A Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 29 Calculate the value of the unknown resistor X in each case. R1 R3 R2 R4 R1 R3 R2 R4 120 120 X 9 4k 12k 15k X X 9 12k 15k X 45 k 4k R1 R3 R2 R4 10k 3.6k 25k X X 3.6k 25k 9 k 10k Click the mouse to continue Physics Support Materials Higher Electricity and Electronics Electric Fields and Resistors in Circuits 30 The circuit shown opposite is balanced. (a)What is the value of resistance X? R1 R3 R2 R4 10 5 20 X X 10 (b) Will the bridge be unbalanced if (i) a 5 resistor is inserted next to the 10 resistor (ii) a 3 V supply is used.?The bridge will be unbalanced if the resistance is increased to 15 The bridge will still be balanced if the supply voltage is changed (c) What is the function of resistor R and what is the disadvantage of using it as shown? The resistor R protects the sensitive galvo from large currents. In series the resistor reduces the sensitivity of the galvo Click the mouse to continue