Introduction to energy
2: Which of the following are properties of energy?
Energy can change matter. Energy can be measured.
A bowl of hot soup is left on a kitchen countertop on a cold day. Over time, the temperature of the soup in the bowl approaches room temperature. What is happening to the average speed of the molecules in the soup during this process?
It decreases.
A cup of hot tea is left on a kitchen countertop on a cold day. Over time, the temperature of the tea in the cup approaches room temperature. What is happening to the average speed of the particles in the tea during this process?
It decreases.
We can regulate our temperature via perspiration. Our skin secretes water as part of sweat. That sweat absorbs thermal energy from the body, increases in temperature, and eventually evaporates. This process cools the body off. What is happening to the water molecules in sweat during this process?
The water molecules gain kinetic energy until they become water vapor.
Students observe their teacher mixing aqueous barium hydroxide and aqueous ammonium nitrate in a container: Ba(OH)2(aq)+2NH4NO3(aq)->Ba(NO3)2+2H2O+2NH3 The students touch the container before and after the reaction. After the reaction, the students notice that the container is cooler afterwards. Which of the following best explains the decrease in thermal energy?
There was an increase in electrical potential energy stored in molecular bonds.
Wong is a professional weightlifter. On his third attempt, he lifts a 200 kilogram weight above his head. Which of the following describe an energy transfer in this scenario?
Wong transfers gravitational potential energy to the weight.
Two roommates are sitting in their living room on a hot summer afternoon. One of them plugs a fan into a wall socket and turns it on. Which forms of energy are most involved in the operation of the fan?
electrical and kinetic
A group of friends are sitting around a campfire. One person notices that as a stationary log burns, it produces a flame and eventually turns to ash. Which forms of energy are most involved in the log burning?
light, chemical, and thermal
A person is quietly reading in a library and hears a thunderstorm starting outside. The person sees a lightning strike that is followed by a thunderclap as rain starts to fall from the sky. Which forms of energy are most involved in a thunderstorm?
light, kinetic, and sound
Which of the following statements are true about elastic energy?
At the microscopic scale, elastic energy can be understood as the energy that atoms have due to their positions. elastic energy is a macroscopic form of energy that we can model and measure.
A wall clock uses a battery to move the hands. How is energy transferred from the battery to the wall clock?
Charged particles change their position in a field and reduce their potential energy.
Which of the following statements are true about chemical energy?
Chemical energy is a macroscopic form of energy that we can model and measure.
Which of the following best explains chemical energy at the macroscopic scale?
Chemical energy is the macroscopic manifestation of electric potential energy stored in chemical bonds.
Which of the following best describes the elastic energy in a compressed spring at the microscopic level?
Elastic energy exists because electric potential energy is stored between the spring's particles.
Which of the following best explains elastic energy in a stretched spring at the macroscopic scale?
Elastic energy is the macroscopic manifestation of electric potential energy between the stretched spring's particles.
Which of the following explains the source of chemical energy at the microscopic scale?
Electric potential energy is stored in chemical bonds between atoms.
A teenager turns on a TV, which is plugged into an electrical wall outlet. The TV screen lights up, and music begins to come from its speakers. Which of the following best describes how energy flows through this system?
Electrical energy is transferred to the TV and converted into light and sound energy.
A woman plugs a hair dryer into an electrical wall outlet and directs the jet of hot air towards her hair. Which of the following best describes how energy flows through this system?
Electrical energy is transferred to the hair dryer and converted into thermal and kinetic energy.
A person turns on a washing machine, which is plugged into a wall electrical outlet. The washing machine fills with water and begins to spin the clothes inside of it. Which of the following best describes how energy flows through this system?
Electrical energy is transferred to the washing machine and converted into kinetic energy
3: Which of the following are properties of energy?
Energy can be measured. Energy can be stored.
Which of the following are properties of energy?
Energy can be stored. Energy can change matter.
A skateboarder is riding toward a ramp. As she reaches the lowest point of the ramp, B, she starts to ride up the ramp and has enough speed to just make it to the top, point A. Assume that there is no friction present. Which of the following is true about the energy of the skateboarder?
Her kinetic energy was converted into gravitational potential energy as she rode up the ramp.
A skateboarder stands at the top of the ramp, and begins to slide down the ramp, passing through the lowest point B, and then up another ramp until reaching the highest point C. Assume that there is no friction present. Which of the following is true about the energy of the skateboarder?
His mechanical energy is the same at point C as it was at point A.
A glass of cold water is left on a kitchen countertop on a hot day. Over time, the temperature of the water in the glass approaches room temperature. What is happening to the average speed of the water molecules during this process?
It increases.
The diagrams below represent the motion of molecules of a gas. Each container holds the same type of gas and has the same volume. The length of each arrow is proportional to the speed of each molecule.
It is highest in container A because container A contains a higher amount of molecules at the highest speed.
The diagrams below represent the motion of molecules of a gas. Each container holds the same type of gas and has the same volume. The length of each arrow is proportional to the speed of each molecule. Which of the following correctly describes the thermal energies of the samples?
It is highest in container A, because container A contains the most molecules at the highest speed.
The diagrams below represent the motion of molecules of a gas. Each container holds the same type of gas and has the same volume. The length of each arrow is proportional to the speed of each molecule. Which of the following correctly describes the thermal energies of the samples? 2
It is lowest in container C because container C contains the fewest molecules at the lowest speed.
A roller coaster car goes down from point A to point B. The car continues to roll along the track and rolls up another hill to reach point C, which is the same height as point A. Assume friction is negligible. How does the mechanical energy of the car at point B compare to that at point C?
It is the same because energy is conserved during the motion.
A sledder slides down a hill from point A to point B. Assume friction is negligible. How does the sledder's mechanical energy at point A compare to that at point B?
It is the same because energy is conserved during the motion.
Carl uses energy from a fully charged battery pack to light up a high efficiency light bulb, which converts about 80% of its electrical energy into light. After an hour, the battery pack is no longer full. He also notices that the temperature of air near the light bulb has risen slightly. What happened to the energy from the battery pack?
It mostly transformed into light energy, and a small portion transformed into thermal energy.
Steve uses a battery to power on an old low-efficiency light bulb. This old light bulb converts only 20\%20%20, percent of electrical energy into light. At first, the battery is full, but after two hours of use it is almost exhausted. Steve also notices that the air temperature near the bulb has increased. What happened to the electrical energy from the battery pack?
It mostly transformed into thermal energy, and a small portion transformed into light energy.
Pedro's favorite activity at the aquapark is the waterslide. Assume friction is negligible. How does his mechanical energy change as he slides?
It stays the same because there is no loss of energy.
Ahmad uses an electric pump to raise water to a reservoir on his farm. The pump is very noisy, and Ahmad's uncle, who is an engineer, told him that only 50% of the energy it consumes is delivered as gravitational potential energy to the water. When Ahmad turns off the pump, he notices that when he touches it, it has not warmed up. What happened to the electrical energy provided to the pump?
It transformed into gravitational potential energy and into sound in equal portions
Jane goes for a long run around her local park. She runs a lap around the park, works up a sweat, and then returns to her starting point. How does Jane's energy compare before and after the run?
Jane's energy decreases because some energy is converted into heat.
When Lynne sings in her bedroom, her mother hears her from the living room. How does Lynne's sound travel from her bedroom to the living room?
Molecules in the air vibrate and transfer kinetic energy to the molecules next to them.
In her apartment, Sasha leans her ear against the wall and can hear music coming from a car that is stopped in the street. How does the sound propagate through the wall?
Particles that compose the wall oscillate and disturb their neighbors, transferring kinetic energy.
As the clock is running out in her basketball game, Sarah shoots the ball to attempt a last-second basket. After the ball soars through the basket, the scorekeeper changes the electronic scoreboard's screen to give Sarah's team the lead. Which of the following describe an energy transfer in this scenario?
Sarah transfers kinetic energy to the basketball.
Students observe their teacher mixing calcium oxide and water in a beaker to obtain calcium hydroxide: CaO+H2O->Ca(OH)2 The teacher allows them to touch the beaker before and after the reaction. The students notice that the beaker is warmer afterwards. Why did the thermal energy increase?
The amount of electrical potential energy stored in the molecular bonds decreased.
n his first soccer game of the season, Jorge kicks the ball toward the goal, but the other team's goalie catches it out of the air. Which of the following describe an energy transfer in this scenario?
The ball transfers kinetic energy to the goalie.
A man filling up balloons for a party accidentally charges them by rubbing them against his shirt. When he places two balloons on a table the electric force between the balloons forces them apart. The two balloons slide in opposite directions across the table before stopping. After the movement, the electric potential energy of the balloons has decreased. What is the best explanation for why the potential energy in the electric field between the two balloons decreased?
The balloons moved in the direction of the electric force.
A man filling up balloons for a party accidentally charges them by rubbing them against his shirt. When he places two balloons on a table, the man feels the balloons trying to move apart. He pushes the balloons closer together and ties them to one another. After the balloons were pushed together, the electric potential energy of the balloons increased. What is the best explanation for why the potential energy in the electric field between the two balloons increased?
The balloons moved opposite to the direction of the electric force.
While cleaning her car, Linda notices that the car's gas tank is almost empty. She drives her car from her home to the gas station, fills up the tank, and then goes back home. How has the car's total energy changed after this trip?
The car's energy increases because its tank is fuller at the end.
There is an attractive gravitational force between two planets that are each moving through space. The two planets interact through a gravitational field. As the two planets move closer together, how does the energy stored in the gravitational field change?
The potential energy in the field decreases.
Two buckets of sand are in a closed system. The two buckets are at a fixed distance apart and are interacting through a gravitational field. If sand is removed from one of the buckets, how will the energy stored in the gravitational field change?
The potential energy in the field decreases.
Two students are exploring electric charge using two negatively charged foam plates. They remove negative charge from both plates without changing the plates' positions. The plates are interacting through an electric field. How does the energy stored in the field change after the plates become less charged?
The potential energy in the field decreases.
Two students are exploring electric charge using two negatively charged foam plates. They remove some negative charge from one of the plates without changing the plates' positions. The plates are interacting through an electric field. How does the energy stored in the field change after the plate becomes less charged?
The potential energy in the field decreases.
There is an attractive gravitational force between two planets that are each moving through space. The two planets are interacting through a gravitational field. As the two planets move farther apart, how does the energy stored in the gravitational field change?
The potential energy in the field increases
There is an attractive gravitational force between an astronaut named Bettie and the planet Earth. During a space flight, even though Bettie is not touching Earth, she and the planet interact through a gravitational field. As Bettie moves farther away from Earth, how does the energy stored in the gravitational field change?
The potential energy in the field increases.
Two buckets of sand are in a closed system. The two buckets are at a fixed distance apart and are interacting through a gravitational field. If the same amount of sand is added to both buckets, how will the energy stored in the gravitational field change?
The potential energy in the field increases.
Two students are exploring electric charge using two negatively charged foam plates. The students rub the plates with a wool cloth to add additional negative charge to both plates without changing the plates' positions. The plates are interacting through an electric field. How does the energy stored in the field change after the plates become more charged?
The potential energy in the field increases.
A skateboarder rides down a ramp. Assume that there is no friction present. Which of the following is true about the energy of the skateboarder?
The skateboarder's gravitational potential energy will be converted into kinetic energy.
A man filling up balloons for a party accidentally charges them by rubbing them against his shirt. When he places a balloon on his table, a piece of string moves across the table and sticks to the balloon. After the string stuck to the balloon, the electric potential energy of the balloon has decreased. What is the best explanation for why the potential energy in the electric field around the balloon decreased?
The string moved in the direction of the electric force.
On a camping trip, Dejounte turns on a battery-powered flashlight for 20 minutes to get ready for bed, and then turns it off. How does the energy of the flashlight and battery system compare before and after it has been turned on and off?
The system's energy decreased because some energy was converted into light.
A cold compress made with a wet washcloth is used to lower fever. The water in the washcloth absorbs thermal energy from the body, increases in temperature, and eventually evaporates. This process cools the body off. What is happening to the water molecules in the washcloth during this process?
The water molecules change phase after gaining kinetic energy.
Aliya is swimming in the pool under the water, but when her mother calls her name from the deck, Aliya hears her. How does Aliya's mother's voice travel from the deck through the water?
Water molecules vibrate and transfer the energy carried by the sound wave to nearby molecules.
Condensation is the transformation of water vapor into a liquid state. For example, when warm air containing water vapor inside a heated car comes in contact with the cold windows on a cold day, water droplets form on the interior side of the glass. What is happening to the water molecules on the window surface?
Water vapor molecules lose kinetic energy until they become liquid water.
Students observe their teacher mixing nitrogen and hydrogen gases in a jar to obtain ammonia: N2+3H2->2NH3 The teacher allows them to touch the jar before and after the reaction. The students notice that the jar is warmer afterwards. Which of the following caused the increase in thermal energy?
a decrease in electrical potential energy stored in molecular bonds
Two buckets of sand, A and B, are in a closed system. The two buckets are a fixed distance apart and are interacting through a gravitational field. Which of the following changes would increase the energy stored in the gravitational field between the two buckets?
adding sand to both of the buckets adding sand to bucket A
An emergency light that runs on a battery comes on when a house's power fails. How is energy transferred from the battery to the emergency light?
from electric charges that move in a electric field and change its potential energy
A remote control is used to turn on the television and change channels. How is energy transferred from the battery to the remote control?
from particles that have charge and move in a electric field to reduce their potential energy
