Power and Energy Transfers Practice Quiz (14 questions)

1. What does power measure?

A. The rate at which energy is transferred or work is done

B. The total amount of energy transferred during a process

C. The time required for a given amount of work to be done

D. The force applied to an object over a given distance

Answer: (A)

Explanation

Power describes how quickly energy is transferred or work is done. The total amount of energy is not itself a measure of the rate of transfer.

2. A device transfers 1 joule of energy each second. What is its power?

A. 1 joule

B. 1 watt

C. 1 newton

D. 1 second

Answer: (B)

Explanation

One joule transferred per second corresponds to a power of one watt. A joule measures energy, while a second measures time.

3. In a closed system, energy moves between stores during a change. What happens to the system's total energy?

A. It remains constant, even though energy can move between stores

B. It remains constant only when energy stays in its original store

C. It decreases because transferred energy leaves the system's stores

D. It increases because each transfer creates additional energy

Answer: (A)

Explanation

Energy cannot be created or destroyed in a closed system, so its total remains constant while energy may transfer between stores. The idea that energy must stay in its original store confuses conservation with the absence of transfers.

4. What commonly happens to energy during changes in a system?

A. It is dissipated into less useful stores, often the thermal store of the surroundings

B. It is converted into a more useful store, often the kinetic store of the surroundings

C. It is removed from the system, leaving no energy in any store

D. It is concentrated in the original store, making that energy more useful

Answer: (A)

Explanation

During system changes, energy is dissipated and stored in less useful ways, commonly as thermal energy in the surroundings. Energy may be transferred to the surroundings, but it is not thereby destroyed.

5. How is efficiency calculated from energy transfers?

A. Total energy input divided by energy dissipated

B. Total energy input divided by useful energy output

C. Useful energy output divided by energy wasted

D. Useful energy output divided by total energy input

Answer: (D)

Explanation

Efficiency is the useful energy output divided by the total energy input. Reversing the input and output gives the reciprocal, not the efficiency.

6. What unit is used to express efficiency?

A. Efficiency is measured in seconds

B. Efficiency is measured in joules

C. Efficiency has no unit

D. Efficiency is measured in watts

Answer: (C)

Explanation

Efficiency is a ratio of useful output to total input, so it has no unit. Watts and joules measure power and energy, respectively.

7. A constant force acts in the direction of motion over a distance. Which equation gives the work done?

A. $W = F \times s$

B. $W = F + s$

C. $W = \frac{1}{2}Fs^2$

D. $W = \frac{F}{s}$

Answer: (A)

Explanation

Work done is calculated by multiplying the force by the distance moved in the force’s direction. Dividing force by distance gives a different relationship, not the work equation.

8. Which equation gives the kinetic energy of an object with mass $m$ moving at speed $v$?

A. $E_k = \frac{1}{2}mv^2$

B. $E_k = mv^2$

C. $E_k = m^2v$

D. $E_k = \frac{1}{2}mv$

Answer: (A)

Explanation

Kinetic energy is half the mass multiplied by the square of the speed. The expression $mv^2$ omits the factor of one half.

9. Which quantities must be multiplied to calculate an object's gravitational potential energy near Earth's surface?

A. Mass, speed, and height

B. Mass, gravitational field strength, and height

C. Power, gravitational field strength, and distance

D. Force, distance, and time

Answer: (B)

Explanation

Gravitational potential energy is calculated as mass times gravitational field strength times height. Speed is not part of this equation; it appears in the kinetic energy equation.

10. A device operates at constant power $P$ for time $t$. Which equation gives the energy transferred?

A. $E = \frac{P}{t}$

B. $E = P + t$

C. $E = \frac{t}{P}$

D. $E = Pt$

Answer: (D)

Explanation

Energy transferred equals power multiplied by the time for which the device operates. Dividing power by time does not give the energy relationship.

11. As a tennis ball is projected upwards, which energy-store changes occur?

A. Its kinetic store decreases while its gravitational potential store increases.

B. Its kinetic and thermal stores both increase.

C. Its gravitational potential store decreases while its kinetic store increases.

D. Its chemical store decreases while its gravitational potential store increases.

Answer: (A)

Explanation

As the ball rises, its kinetic energy store decreases and its gravitational potential energy store increases. The reverse exchange describes a ball moving downward, not upward.

12. A vehicle slows down as its brakes are applied. Which energy-store change occurs?

A. Its thermal store decreases while its kinetic store increases.

B. Its chemical store decreases while its kinetic store increases.

C. Its kinetic store decreases while its thermal store increases.

D. Its gravitational potential store decreases while its thermal store increases.

Answer: (C)

Explanation

When a vehicle slows down, its kinetic energy store decreases and its thermal energy store increases. A change in gravitational potential energy is not the characteristic store change described for braking.

13. When a battery-powered drill is switched on, which stores change?

A. The gravitational potential store decreases as the kinetic and chemical stores increase.

B. The kinetic store decreases as the chemical and thermal stores increase.

C. The chemical store decreases as the kinetic and thermal stores increase.

D. The thermal store decreases as the chemical and kinetic stores increase.

Answer: (C)

Explanation

Operating the drill decreases the battery's chemical energy store while increasing the drill's kinetic and thermal energy stores. The kinetic store does not decrease as the drill starts operating.

14. As a person slides down a zip-wire, how do the person's energy stores change?

A. The gravitational potential store decreases while the kinetic and thermal stores increase.

B. The chemical store decreases while the gravitational potential and kinetic stores increase.

C. The kinetic store decreases while the gravitational potential and thermal stores increase.

D. The thermal store decreases while the kinetic and gravitational potential stores increase.

Answer: (A)

Explanation

During the descent, the person's gravitational potential energy store decreases while the kinetic and thermal stores increase. The alternative in which gravitational potential energy increases describes motion upward rather than down the zip-wire.