Work and heat as energy transfers
You squeeze the brake on your bike and the brake pad heats up without anyone touching it with a flame. You rest your cold hands on a cup of hot chocolate and they soon feel better. In both cases something has gained energy, but not in the same way: once through a force, once through a difference in temperature. These are the only two doors through which energy can enter or leave a system.
Two ways of transferring energy: work and heat
You rub your hands together vigorously for ten seconds and notice that their temperature rises; then you touch a hot radiator and get the same warm sensation. If the end result in your hands is the same, in which two different ways can an object exchange energy with its surroundings?
Irene goes down a long hill on her bike and squeezes the brake almost all the time. On reaching the bottom, she carefully touches the brake pad and notices that it is…
Irene goes down a long hill on her bike and squeezes the brake almost all the time. On reaching the bottom, she carefully touches the brake pad and notices that it is hot.
- a) Has the energy reached the brake pad through work or through heat?
- b) What force carried out this energy transfer?
Marcos has cold hands and rests them on his sister's mug of hot chocolate. Shortly after, his hands feel much better. a) Have Marcos's hands received energy by work or…
Marcos has cold hands and rests them on his sister's mug of hot chocolate. Shortly after, his hands feel much better.
- a) Have Marcos's hands received energy by work or by heat?
- b) Which body has given off energy and which has gained it?
Classify each situation according to whether energy is transferred by work (a force acts) or by heat (there is a temperature difference): a) You stretch a rubber band…
Classify each situation according to whether energy is transferred by work (a force acts) or by heat (there is a temperature difference):
- a) You stretch a rubber band with your fingers.
- b) A cold frying pan on an electric ceramic hob that is switched on.
- c) The blender stirs the juice forcefully.
- d) You warm your hands by putting them near a campfire.
- e) You rub your hands together to warm up.
The common unit: the joule
In the International System of Units, both work and heat are measured in joules ($\text{J}$), the same unit as energy. Explain in your own words why it makes sense for…
In the International System of Units, both work and heat are measured in joules ($\text{J}$), the same unit as energy. Explain in your own words why it makes sense for all three to share the same unit.
In a classic experiment such as Joule's, a weight of $4\ \text{kg}$ falls and turns paddles that stir the water in a container, as shown in the figure. Take…
In a classic experiment such as Joule's, a weight of $4\ \text{kg}$ falls and turns paddles that stir the water in a container, as shown in the figure. Take $g = 10\ \text{m/s}^2$, so that the weight pulls the string with a force of $40\ \text{N}$.
- a) If the weight descends $1,5\ \text{m}$, calculate the work it does on the paddles in one fall.
- b) The weight is lifted and released again $25$ times. How much total work has been transferred to the water?
- c) What has that work been converted into inside the container? Explain what equivalence between work and heat this experiment illustrates.
The first law: energy never disappears
You flick a coin so that it slides across a smooth table: it slows down until it stops and, if you measure with a very precise infrared thermometer, both the table and the coin have warmed up by a few hundredths of a degree. Where exactly has the coin's movement energy gone?
A car travelling at $90\ \text{km/h}$ brakes to a complete stop. Has the energy carried by the car disappeared? Explain where it has gone according to the first law of…
A car travelling at $90\ \text{km/h}$ brakes to a complete stop. Has the energy carried by the car disappeared? Explain where it has gone according to the first law of thermodynamics.
State whether the sentence is true or false and correct the idea: "When I put an ice cube into a soft drink, the cold from the ice passes into the drink, which is why it…
State whether the sentence is true or false and correct the idea: "When I put an ice cube into a soft drink, the cold from the ice passes into the drink, which is why it cools down."
Energy balances
A system receives $500\ \text{J}$ of work from its surroundings and, at the same time, transfers $200\ \text{J}$ in the form of heat to the environment, as shown in the…
A system receives $500\ \text{J}$ of work from its surroundings and, at the same time, transfers $200\ \text{J}$ in the form of heat to the environment, as shown in the figure.
- a) Calculate by how much the energy of the system has changed.
- b) Explain what the sign of the result means: does the system end up with more or less energy?
The combustion of petrol in a tank releases $20\,000\ \text{J}$ of chemical energy. Of this, the car's wheels receive $6000\ \text{J}$ of useful work, as shown in the…
The combustion of petrol in a tank releases $20\,000\ \text{J}$ of chemical energy. Of this, the car's wheels receive $6000\ \text{J}$ of useful work, as shown in the figure.
- a) Calculate how much energy is lost in the form of heat.
- b) What percentage of the fuel energy is used as work?
- c) State where heat leaves the car and explain why this energy balance does not contradict the first law, even though energy is "lost".