Science expression workbook answers chp 7
- 1. SCIENCE EXPRESSION WORKBOOK ANSWERS CHAPTER 7 – THERMAL PROPERTIES AND THERMOSTAT Pg 57 1. D 2. B 3. D 4. B 5. C 6. The three effects of heat are : change of state, change of temperature, thermal expansion and contraction. 7. a) melting b) boiling/ evaporation c) condensation d) freezing/solidifying Pg 58 8. A hot rivet is inserted through the holes in between two metal plates. One end of the rivet is hammered to form a new rivet head. When the rivet cools, it contracts and pulls the metal plates together tightly. 9. (a) A bimetallic strip is made up of different metals welded or riveted together. At room temperature, the strip is flat. As the surrounding heats up, the bimetallic strip in a thermostat bends because one metal expands more than the other. This bending breaks the electrical contact and switches off the current flowing to the electrical appliance. As the surrounding cools down, the bimetallic strip contracts and straightens back to its original position. The contacts touch and the electricity again flows through the electrical appliance. (b) Electric iron, rice cooker, electric oven, electric kettle 10. (a) When the iron becomes hot, the bimetallic strip expands due to uneven expansion of the different metal strips. The bimetallic strip bends away from the contact point. This causes the circuit to break. Pg 59 10. (b) The temperature knob should be adjusted to move the screw towards the contact points for hot ironing. This adjustment allows the bimetallic strip to expand more at higher temperatures before it bends away from the contact point causing the circuit to break. As a result, the average temperature of the electric iron is increased for hot ironing. 11. When there is a fire, the bimetallic strip is heated. The bimetallic strip curves to close up the contact point due to uneven expansion of the two metals. The electrical circuit is closed and an electrical current flows through the electric bell. The electric bell rings to sound the fire alarm.