Energy Transfers and Systems Flashcards
When energy is transferred by heating, is it transferred from a hotter object to a colder object or from a colder object to a hotter object?
From a hotter object to a colder object
True or false? You can increase the energy of a closed system.
False
There’s never a net change in energy in a closed system
Describe the energy transfers that take place when an electric toothbrush is used
The toothbrush transfers energy electrically from the chemical energy store of its battery to the kinetic energy store of its bristles. It also transfers energy to the surroundings by sound and by heating.
Mindy runs at 6m/s. She has a mass of 40kg. Calculate the amount of energy in her kinetic energy store.
720 J
KE = 1/2 x m x v^2 = 1/2 x 40 x 36 = 20 x 36 = 720J
Mindy climbed 4m up a tree. Calculate the change in her gravitational potential energy.
1600J
g = 10 m/s^2, so ΔGPE = m x g x Δh = 40 x 10 x 4 = 1600J
Kunal rubs his hands together to keep warm. Explain why Kunal’s hands warm up.
By rubbing his hands together Kunal is doing work against friction, which causes energy to be transferred to the thermal energy stores of his hands, increasing their temperature.
Kunal says no energy is wasted when he rubs his hands together because he wants his hands to heat up. Explain why Kunal is wrong.
Energy is still wasted because energy will also be transferred to the thermal energy stores of the surroundings (some energy will also be dissipated by sound).
