1. With a kettle, Peter warms up 1,2 litres of water (20°C) till it starts to boil. The power of the kettle is 2000 W. The heating up takes four minutes. The formula for specific heat of the water is 4,2 J/g · °C. How high is the efficiency of the kettle?
2. A kettle can contain 1,4 litres of water. The device has a power of 2000 W. The formula for specific heat of the water is 4,2 J/g · °C and the density is 1,0 g/mL. Calculate how long it would take before the kettle filled to the brim with water (20°C) would be boiling.
Q = E
A semi-circle of radius 8 cm, rocks back and forth along a line. The distance between the line on which the semi-circle sits and the line above is 12 cm. As it rocks without slipping, the semi-circle touches the line above at two points. (When the semi-circle hits the line above, it immediately rocks back in the other direction.) What is the distance between these two points, in millimetres, rounded off to the nearest whole number?