Model Energy Transfer Between Objects - Physics

1. What is the law of conservation of energy for energy transfer between objects?

   Total energy is constant; it only transfers or transforms. Energy cannot be created or destroyed in isolated systems.

2. What is the formula for kinetic energy of an object of mass $m$ and speed $v$?

   $K=\frac{1}{2}mv^2$. Energy of motion depends on mass and square of velocity.

3. Find the heat gained: $m=2\,\text{kg}$, c=500\,\text{J/(kg\cdot ^\circ C)}, $\Delta T=5\,^\circ\text{C}$; what is $Q$?

   $5000\,\text{J}$. Use $Q=mc\Delta T$: $(2)(500)(5)=5000\,\text{J}$.

4. What is the formula for kinetic energy of an object of mass $m$ moving at speed $v$?

   $K=\frac{1}{2}mv^2$. Energy of motion depends on mass and speed squared.

5. What is the calorimetry condition for an isolated system of objects exchanging heat?

   $\sum Q=0$. In isolation, total heat gained equals total heat lost.

6. Identify the correct energy-conservation model for an isolated system with no external transfer.

   $\Delta E_{\text{system}}=0$. Total energy remains constant when no energy enters or leaves.

7. What is the formula for energy transferred during a phase change at constant temperature?

   $Q=mL$. Latent heat $L$ is energy per unit mass for phase transitions.

8. Find $\eta$ if $E_{in}=200\,\text{J}$ and $E_{useful}=50\,\text{J}$ for a device.

   $\eta=0.25$. Using $\eta=\frac{E_{useful}}{E_{in}}$: $\eta=\frac{50}{200}=0.25$.

9. What is the formula for gravitational potential energy change near Earth's surface?

   $\Delta U_g=mg\Delta h$. Near Earth, $g$ is constant at $9.8\,\text{m/s}^2$.

10. Identify the correct unit for energy transfer (work or heat): $\text{W}$, $\text{J}$, or $\text{N}$.

    $\text{J}$. Joule is the SI unit for all forms of energy.

11. What is the definition of energy transfer between objects in a physical system?

    Energy moving from one object to another via interactions. Objects exchange energy through forces, collisions, or thermal processes.

12. What is the formula for gravitational potential energy change near Earth for height change $\Delta h$?

    $\Delta U_g=mg\Delta h$. Work done against gravity equals weight times vertical displacement.

13. What is the formula for power as energy transferred per unit time?

    $P=\frac{\Delta E}{\Delta t}$. Power measures rate of energy transfer or transformation.

14. Which energy transfer mechanism requires direct contact: conduction, convection, or radiation?

    Conduction. Heat flows through materials via molecular collisions.

15. Which energy-transfer mechanism can occur through empty space with no medium?

    Radiation. Electromagnetic waves carry energy without needing matter to propagate.

16. State the first-law energy model relating internal energy change, heat, and work.

    $\Delta U=Q+W$. First law states internal energy change equals heat plus work done on system.

17. What is the formula for heat transferred due to temperature change (no phase change)?

    $Q=mc\Delta T$. Specific heat formula: $m$ is mass, $c$ is specific heat, $\Delta T$ is temperature change.

18. What is the formula for thermal energy transferred during a phase change at constant temperature?

    $Q=mL$. $m$ is mass and $L$ is latent heat of fusion or vaporization.

19. Identify the energy transfer: a moving box slows on a rough floor; where does its kinetic energy go?

    Into thermal energy of the box and floor (via friction). Friction converts ordered kinetic energy into random molecular motion.

20. Find $Q$ for $m=2\,\text{kg}$, $c=500\,\text{J/(kg\,}^\circ\text{C)}$, $\Delta T=3^\circ\text{C}$.

    $Q=3000\,\text{J}$. Using $Q=mc\Delta T$: $Q=2\times500\times^3=3000\,\text{J}$.

21. Identify the direction of net heat transfer when two objects at $80^\circ\text{C}$ and $20^\circ\text{C}$ touch.

    From $80^\circ\text{C}$ object to $20^\circ\text{C}$ object. Heat flows from hot to cold until thermal equilibrium.

22. Which energy transfer mechanism can occur through a vacuum: conduction, convection, or radiation?

    Radiation. Electromagnetic waves carry energy without a medium.

23. What is the sign of $Q$ for an object that gains thermal energy from its surroundings?

    $Q>0$. Positive $Q$ means heat flows into the system.

24. What is the sign convention for heat $Q$ when energy enters an object as thermal energy?

    $Q>0$. Positive $Q$ means the system gains thermal energy from surroundings.

25. Which energy transfer mechanism is heat transfer by bulk fluid motion: conduction, convection, or radiation?

    Convection. Heated fluid rises, cooler fluid sinks, creating circulation.

26. Find the work done: a $10\,\text{N}$ force acts $3\,\text{m}$ in the same direction as motion.

    $W=30\,\text{J}$. Using $W=Fd$: $W=10\times^3=30\,\text{J}$.

27. What is the sign convention for work $W$ when work is done on an object by the surroundings?

    $W>0$. Positive $W$ means external forces add energy to the system.

28. Choose the correct transfer pathway: the Sun warming your skin through space uses which mechanism?

    Radiation. Only electromagnetic waves can travel through vacuum of space.

29. Which energy-transfer mechanism occurs by bulk motion of a fluid (liquid or gas)?

    Convection. Heated fluid rises, cooler fluid sinks, creating circulation that transfers energy.

30. What is the formula for thermal energy change of a substance with no phase change?

    $Q=mc\Delta T$. Heat capacity $c$ relates temperature change to thermal energy gained.

31. What is the efficiency formula in terms of useful output energy and total input energy?

    $\eta=\frac{E_{useful}}{E_{in}}$. Efficiency is always less than 1 due to energy losses.

32. Which energy-transfer mechanism requires direct contact between objects?

    Conduction. Heat flows through materials when molecules collide and transfer kinetic energy.

33. What is the power formula in terms of energy transferred $\Delta E$ over time interval $\Delta t$?

    $P=\frac{\Delta E}{\Delta t}$. Rate of energy transfer measured in watts (joules per second).

34. Identify the sign of $Q$: a hot metal block cools while in contact with cooler water; what is $Q$ for the block?

    $Q<0$. Negative $Q$ means system loses thermal energy to surroundings.

35. What is the formula for work done by a constant force parallel to displacement?

    $W=Fd$. Force and displacement must be in same direction.

36. What is the sign of $Q$ for an object that loses thermal energy to its surroundings?

    $Q<0$. Negative $Q$ means heat flows out of the system.

37. Find the work done: a $10\,\text{N}$ force pushes $3\,\text{m}$ in the same direction; what is $W$?

    $30\,\text{J}$. Apply $W=Fd$: $(10\,\text{N})(3\,\text{m})=30\,\text{J}$.

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