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1 of 32© Boardworks Ltd 2009
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3 of 32© Boardworks Ltd 2009 Sensing devices
4 of 32© Boardworks Ltd 2009 Light dependent resistors A Light Dependent Resistor (LDR) is an input transducer, converting light energy to a change in electrical properties. Its resistance decreases as light intensity increases. As photons of light hit a cadmium sulfide track, they give bound electrons enough energy to jump into the conduction band. resistance (Ω) light intensity (lux) LDR symbol cadmium sulfide track The resistance can fall from 1 MΩ in darkness to 500 Ω in light.
5 of 32© Boardworks Ltd 2009 Thermistors Negative temperature coefficient (NTC) thermistors are input transducers that have a decreasing resistance as temperature is increased. As the surrounding temperature increases, the electrons in the metal oxide of the thermistor gain energy. This increases the number of charge carriers, decreasing resistance. resistance (Ω) temperature (°C) thermistor symbol
6 of 32© Boardworks Ltd 2009 Sensors summary
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8 of 32© Boardworks Ltd 2009 Sharing voltage
9 of 32© Boardworks Ltd 2009 Potential dividers Potential dividers reduce voltage. Varying the ratio of a pair of resistors changes the output voltage of a circuit. 0 V0 V V IN R1R1 R2R2 V OUT V OUT will be a fraction of V IN. The magnitude of V OUT is dependent upon the ratio of the two resistors R 1 and R 2. V OUT 0 V0 V = R2R2 V IN R 1 + R 2 ×
10 of 32© Boardworks Ltd 2009 Using the potential divider equation
11 of 32© Boardworks Ltd 2009 Sensors and potential dividers
12 of 32© Boardworks Ltd 2009 Potential divider questions
13 of 32© Boardworks Ltd 2009 Potential dividers summary
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15 of 32© Boardworks Ltd 2009 Electrical power power (W) = voltage (V) × current (A) The power, or rate of energy transfer, of a device is a product of the voltage and current passing through the component. What is the power of a bulb which uses a 230 V mains supply and has a current of 0.44 A passing through it? What is the voltage across a microchip if it has a normal operating power of 0.5 W and draws a current of 0.1 A? V = P ÷ I P = V × I = 230 × 0.44= W = 0.5 ÷ 0.1= 5 V
16 of 32© Boardworks Ltd 2009 Different forms of the power equation Electrical power can also be calculated using resistance. P = V × I P = I 2 × R P = V 2 ÷ R and… V = I × R P = V × I Therefore, using substitution: P = I × R × I and… I = V ÷ R P = V × V ÷ R Therefore, using substitution: The equations linking power to resistance are found by substituting the equation V = I × R into the power equation:
17 of 32© Boardworks Ltd 2009 Energy in circuits
18 of 32© Boardworks Ltd 2009 A light bulb converts electrical energy to useful light and wasted heat. Efficiency Efficiency is a measure of how well a device transforms energy into useful forms. light What is the efficiency of the bulb if it converts 50 J of electrical energy into 45 J of heat energy? efficiency = = electrical heat × 100 useful energy out total energy in × 100= 10 % 5 50
19 of 32© Boardworks Ltd 2009 Efficiency of a motor What is the efficiency of this system, if the motor takes 5 seconds to lift the weight? (take gravity to be 9.81 N/kg) 1.4 kg 1.5 m 6 V6 V2 A2 A motor pulley = efficiency = useful energy out total energy in × 100 energy into system: electrical energy = I × t × V = 2 × 5 × 6= 60.0 J energy used: gravitational potential energy = m × g × h = 1.4 × 9.81 × 1.5= 20.6 J 60.0 = 34.3 % 20.6 × 100
20 of 32© Boardworks Ltd 2009 Electricity in the home
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22 of 32© Boardworks Ltd 2009 Current and drift velocity Current is a flow of charge. Electrical devices activate almost instantly once they are supplied with power, however the electrons actually move around a circuit quite slowly. Their velocity is called drift velocity. Current and drift velocity are linked by the following equation: I = nAve I = current (amps) n = charged particles per unit volume A = cross-sectional area (m 2 ) v = drift velocity (m/s) e = charge on an electron (1.6 x C)
23 of 32© Boardworks Ltd 2009 Understanding I = nAve
24 of 32© Boardworks Ltd 2009 Alternating current and direct current
25 of 32© Boardworks Ltd 2009 RMS voltage The voltage of AC can be viewed using an oscilloscope. There are three common voltage measures, namely peak, peak-to-peak and RMS (root mean squared) voltage. RMS is a measure of the average magnitude of the voltage. V RMS = V PEAK √2 RMS voltage zero volts peak voltage peak-to-peak voltage
26 of 32© Boardworks Ltd 2009 RMS current and RMS power To investigate voltage we use an oscilloscope connected across a resistor. As V I, the equation for calculating RMS current is similar to the equation for RMS voltage: I RMS The equation for RMS power is a little different: P PEAK = I PEAK × V PEAK P RMS = I RMS × V RMS = P RMS = = I PEAK √2 × I PEAK √2 V PEAK √2 P PEAK 2
27 of 32© Boardworks Ltd 2009 AC calculations
28 of 32© Boardworks Ltd 2009 AC/DC summary
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30 of 32© Boardworks Ltd 2009 Glossary
31 of 32© Boardworks Ltd 2009 What’s the keyword?
32 of 32© Boardworks Ltd 2009 Multiple-choice quiz
The moving charges (the microscopic particles) from the electric supply constitute an electric current Conventionally: The direction of the electric.
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