40 2.5 V When this voltage Ω 0.016 A reaches 0.7 V, the (a) 3 V transistor switches on 3V (a) (NPN) Transistor and the LED lights. 0.5 V As the temperature increases, the (a) 3.8 V resistance of the 1.0 V thermistor decreases. 0.6 V The voltage across the thermistor decreases (a) V1 = 6 V, V2 = 6 V and the voltage across V1 = 4 V, V2 = 10 V R1 increases. V1 = 7 V, V2 = 3 V When this voltage V1 = 16 V, V2 = 6 V reaches 0.7 V, the V1 = 4.5 V, V2 = 13.5 transistor switches on V and the LED lights. V1 = 5.5 V, V2 = 3 V Swap the positions of the thermistor and R1. (a) MOSFET As brightness of light decreases, the resistance of the LDR increases. The voltage across the LDR increases. When this reaches a high enough voltage, the MOSFET switches on and the lamp lights. Switching Circuits (p42 47) The brightness at Electrical (p48 50) – – which the lamp turns (a) 0V on can be altered. The light bulb uses up 15 V 60 J of electrical energy 15 V (a) 13.3 V every second. 0V 1900 Ω 150 (a) 80 W Ω (a) 4V This circuit could turn 85 W 8 V on a fan when a 45 000 J 0 V computer overheats. 1 305 000 J 12 V 250 s As the temperature 140 s (a) 700 increases, the Ω 3.5 V resistance of the 90 W 2.5 V thermistor decreases. 3 V The voltage across the 62 640 000 J 100 °C thermistor decreases and the voltage across 600 s (a) 6 V R1 increases.
40 Ω (a) 3 V 3V 0.5 V (a) 3.8 V 1.0 V 0.6 V (a) V1 = 6 V, V2 = 6 V V1 = 4 V, V2 = 10 V V1 = 7 V, V2 = 3 V V1 = 16 V, V2 = 6 V V1 = 4.5 V, V2 = 13.5 V V1 = 5.5 V, V2 = 3 V Switching Circuits (p42 – 47) (a) 0V 15 V 15 V 0V (a) 4V 8 V 0 V 12 V (a) 700 Ω 3.5 V 2.5 V 3 V 100 °C (a) 6 V 2.5 V 0.016 A (a) (NPN) Transistor As the temperature increases, the resistance of the thermistor decreases. The voltage across the thermistor decreases and the voltage across R1 increases. When this voltage reaches 0.7 V, the transistor switches on and the LED lights. Swap the positions of the thermistor and R1. (a) MOSFET As brightness of light decreases, the resistance of the LDR increases. The voltage across the LDR increases. When this reaches a high enough voltage, the MOSFET switches on and the lamp lights. The brightness at which the lamp turns on can be altered. (a) 13.3 V 1900 Ω 150 Ω This circuit could turn on a fan when a computer overheats. As the temperature increases, the resistance of the thermistor decreases. The voltage across the thermistor decreases and the voltage across R1 increases. When this voltage reaches 0.7 V, the transistor switches on and the LED lights. Electrical (p48 – 50) The light bulb uses up 60 J of electrical energy every second. (a) 80 W 85 W 45 000 J 1 305 000 J 250 s 140 s 90 W 62 640 000 J 600 s
Electricity-and-Energy-Problem-Booklet-St-Lukes.docx