Energy transfers：练习题

A Sankey diagram is used to represent the energy transfers in an electric motor. The diagram shows an input of 500 J of electrical energy. The useful energy output is 350 J, and the remaining 150 J is dissipated as thermal energy. What is the efficiency of the motor?

A power station is 35% efficient. If it produces 700 MW of useful electrical power, how much thermal energy is dissipated to the surroundings every second?

Thermal conduction in metals is much faster than in non-metals. Which of the following best explains the role of "free" electrons in this process?

A student investigates thermal energy transfer using a vacuum flask. The flask has a double-walled glass container with a vacuum between the walls. Which statement correctly explains why the vacuum reduces the rate of thermal energy transfer?

A ball of mass \( m \) is released from rest at height \( H \). Neglecting air resistance, what is the ratio of its kinetic energy to its gravitational potential energy when it has fallen to a height of \( \frac{1}{3}H \) above the ground?

A light bulb is supplied with \( 60 \text{ J} \) of electrical energy. It transfers \( 15 \text{ J} \) of energy as light and the rest is dissipated to the surroundings as thermal energy. Calculate the efficiency of the light bulb as a percentage.

A roller coaster car of mass 600 kg is pulled to the top of a track at a height of 45 m above the ground. It then starts from rest and descends the track.

(a) Calculate the gravitational potential energy (GPE) of the car at the top of the track. (Take \( g = 10 \text{ N/kg} \))

(b) At the bottom of the first drop, the car is 5 m above the ground. If 20% of the initial GPE is lost due to friction and air resistance, calculate the kinetic energy (KE) of the car at this point.

(c) Calculate the speed of the car at this 5 m height.

(d) Explain how the principle of conservation of energy applies to this motion.