HIGHER LEVEL Chapter 24 – Applied Electricity (Option 2) ‘Section B’ Short Question: Typical of Question 5 in Leaving Cert. Exam Q1 Answer all of the following parts (a), (b), (c), etc. (a) Give one contribution made to physics by Nicholas Callan. (7) induction coil / batteries / prevent rusting (7) (b) Transformers usually have a soft iron laminated core.What is the purpose of the soft iron core and why is it laminated? (7) make transformer more efficient / reduce eddy currents (in the iron core) / reduce heat or energy loss (in the iron core) (7) (c) Draw the truth table for a NOT gate. (7) table with header ‘input’ and ‘output’ (3) completed table with two states correctly shown (4) (d) How can a galvanometer be converted into an ohmmeter? (7) variable resistance and battery (4) in series (3) (e) Draw the basic structure of a bi-polar transistor. (7) Collector n Base p n Emitter three layers (4) emitter, base and collector (or e, b, c) connections correctly labelled (3) 158 INVESTIGATING PHYSICS HIGHER LEVEL (f) Draw a diagram to show how a galvanometer can be converted into a voltmeter. (7) High resistance galvanometer (symbol / sketch) in series with (4) (large valued) resistor / multiplier (3) (g) Give two uses of transistors. (7) voltage amplifier / voltage inverter / current amplifier / switch / logic gate / (NOT gate (any two 4 ؉ 3 (h) What contribution did George Boole make to modern-day electronics? (7) he developed Boolean algebra / developed the maths (logic) used with logic gates (7) (i) What is the main energy conversion that takes place in an a.c. generator? (7) mechanical / kinetic (energy) (4) to electrical (energy) (3) (j) Give two advantages of the induction motor over a d.c. motor. (7) no brushes (to replace) / not affected by (minor) voltage fluctuations / less / no electrical interference / stabilised / smoother / constant rate of rotation, (etc. / less friction / no sparking (any two 4 ؉ 3 ‘Section B’ Full Questions Q2 A current-carrying conductor experiences a force in a magnetic field. Name the factors that affect the magnitude of the force. (9) (magnetic) flux density, current, length, sinu / u (any three 3 ؋ 3) Describe a laboratory experiment to demonstrate the principle that a current- carrying conductor in a magnetic field experiences a force. (12) Horseshoe Aluminium foil magnet Apparatus: Battery, magnet, conductor, switch (stated / implied) (3) Procedure: Send current through conductor (in a magnetic field) (3) Result: Conductor moves (3) APPLIED ELECTRICITY (OPTION 2) 159 HIGHER LEVEL Conclusion: Any point of detail (e.g. F in opposite direction if current reversed) (3) The operation of a moving-coil galvanometer is based on this principle. List two other devices based on this principle. (6) (loudspeaker / d.c. motor / Barlow’s wheel / etc. (any two 3 ؉ 3 (ammeter, voltmeter, etc. unacceptable) What is the function of a moving-coil galvanometer? Draw a circuit diagram to show how a moving-coil galvanometer can be converted into an ammeter. (9) to measure or detect (very) small currents / to indicate direction of I or V (3) Current Low resistance (diagram showing: galvanometer and resistor / in parallel (3 ؉ 3 A moving-coil galvanometer has a resistance of 100 ⍀ and a full-scale deflection of 5.00 mA. Calculate the size of the resistor required to convert it into an ammeter with a full-scale deflection of 1.00 A. (9) current through resistor ϭ 0.995 A (3) ϭ ϭ VR VG / (0.995)R (0.005)(100) (3) R ϭ 0.50(3) ⍀ (3) What is the effective resistance of the ammeter? (6) 1 1 1 1 1 1 ϭ ϩ ϭ ϩ (3) RT R1 R2 > RT 0.503 100 ϭ (RT )0.500 Æ (3) Why does the magnet in a moving-coil galvanometer have curved pole faces? (5) to give a radial magnetic field / torque on coil is constant / u ϰ I / to give a uniform scale / arm of couple remains constant (5) Q3 Explain the difference between intrinsic and extrinsic conduction in semiconductors. (12) intrinsic conduction is in a pure semiconductor (6) extrinsic conduction is in an impure semiconductor (6) 160 INVESTIGATING PHYSICS HIGHER LEVEL Draw a labelled diagram of a p-n junction. (9) Ϫ ϩ p-type n-type ϪϪ ϩϩ Ϫ ϩ ϪϪ ϩϩ Depletion layer ‘Holes’ are the Electrons are the majority charge majority charge carriers carriers two layers (3) label p and n (3) reference to depletion layer (3) Explain how a p-n diode can be used to convert a.c. to d.c. (9) a.c. to diode (3) conducts in forward bias / voltage (3) no conduction in reverse bias (3) Draw a diagram of a circuit used to get a smooth d.c. output from an a.c. source. (9) pq Input a.c. input Output C A a.c. r s B D d.c. output Capacitor four diodes / bridge rectifier (3) capacitor (3) full circuit including a.c. in and d.c. out (3) Explain why a light-emitting diode (LED) emits light when a suitable forward bias is applied to it. (9) current flows (3) electrical energy to light energy (3) junction near surface (3) APPLIED ELECTRICITY (OPTION 2) 161 HIGHER LEVEL Give two ways in which a photodiode differs from an LED and give a use for a photodiode. (8) photodiode requires light (3) LED is operated in forward bias but photodiode in reverse bias (3) light meter / burglar alarms / optic fibre / counters / checkout / automatic door / bar-coding /oil-burner (2) Q4 The transistor was one of the most important inventions of the twentieth century. Describe the underlined term. (4) (semiconductor device / any use (2 ؉ 2 Name the three currents flowing in a transistor and state the relationship between them. (9) emitter, base, and collector currents / Ie, Ib, Ic (3) Ie ϭ Ic ϩ Ib (6) Fig 24.27 shows the circuit of a voltage amplifier. Explain how the circuit operates. (12) Load Bias resistor resistor Output Input voltage voltage Fig 24.27 varying the input voltage causes a change in the base current (3) the base current controls the size of the collector current (3) the load resistor and transistor (together) act as a potential divider (3) small change in input current / voltage (3) large change in output current / voltage (3) 162 INVESTIGATING PHYSICS HIGHER LEVEL Draw a sketch of the input and output voltages, using the same axes and scales when a small varying voltage is applied to the amplifier. (9) V / V V / V Output Output Voltage Voltage t / s t / s Input voltage Input voltage (a) Output voltage taken across the load (b) Output voltage taken across the transistor resistor remains in phase with input voltage. causes it to be inverted. ‘alternating’ graph of input and output (showing V varying with t) (3) output having larger amplitude (3) output inverted (180° phase difference) (3) A NOT gate is a voltage inverter. Draw a circuit diagram to show how a transistor can be used as a voltage inverter. (12) Ic RL RP Vbattery Output voltage I b Vo Input voltage Ie Vi diagram showing: R in series with transistor across battery (3) limiting / protective resistor at base (3) V in indicated (across base / emitter) (3) V out indicated (across collector / emitter) (3) Give the truth table of an AND gate. (10) table with header ‘input’ and ‘output’ (4) 0-1; 1-0; 0-0 gives 0 (output) (3) 1-1 gives 1 (output) (3) APPLIED ELECTRICITY (OPTION 2) 163 HIGHER LEVEL ‘Section B’ Full ‘STS’ Question: Typical of Question 11 in Leaving Cert. Exam Q5 Read the following passage and answer the following questions. The scientist whose research would unite electricity and magnetism was Michael Faraday. He developed the first electric motor in 1821, showing that a current- carrying conductor could be made to revolve around a magnet. He went on to expand on Oersted’s observation that an electric current produces a magnetic effect. Perhaps, Faraday thought, the opposite was also true: a moving magnetic field could generate an electric current.This was to be called electromagnetic induction. Soon he had created the first electric generator, and everyday life would never be the same again. His experiments with induced currents produced the transformer. (Adapted from Milestones of Science, Curt Supple; 2000) (i) List three factors that affect the force on a current-carrying conductor placed near a magnet. (7) magnetic flux density (B), current (I), length (l), sin u (any three 3 ؉ 2 ؉ 2) (ii) What energy transformation takes place in an electric motor? (7) electrical energy (4) to kinetic energy (3) (iii) What is the function of a commutator in a d.c. motor? (7) allows current to always flow // so that torque (4) into the same end of the coil (and out the other end) // is always in the same direction (3) (iv) Draw a sketch of the output voltage from an a.c. generator. (7) varying (voltage) with respect to time (4) sinusoidal shape (3) (v) How are the slip rings connected to an external circuit in an a.c. generator? (7) brushes (4) springs / carbon / metal (3) 164 INVESTIGATING PHYSICS HIGHER LEVEL (vi) A transformer and an induction coil can both be used to change a small voltage into a larger voltage.What is the basic difference in the operation of these two devices? (7) transformer: a.c. input / a.c. output / current flows through secondary (4) induction coil: d.c. input / (adjusted for) d.c. output / sparks across gap in secondary (3) (vii) Name the Irish physicist who invented the induction coil. (7) (Dr Nicholas) Callan (7) (viii) Give two factors that affect the efficiency of a transformer.