Energy & Heat Transfer — Chapter Practice, Grade 8

Trace energy transfers and transformations and investigate how thermal energy moves.

Problem 1

Find the kinetic energy of a 4 kg cart moving at 5 m/s.
Answer and explanation

50 J

KE = ½mv² = ½ × 4 × 5² = ½ × 4 × 25 = 50 J.

Problem 2

A 10 kg object sits on a shelf 2 m high. Find its gravitational potential energy. (g = 9.8 m/s²)
Answer and explanation

196 J

PE = mgh = 10 × 9.8 × 2 = 196 J.

Problem 3

If you double an object's speed, its kinetic energy…
  1. doubles
  2. triples
  3. quadruples (×4)
  4. stays the same
Answer and explanation

quadruples (×4)

Since KE = ½mv², speed is squared, so doubling speed multiplies KE by 2² = 4.

Problem 4

Find the kinetic energy of a 1 kg ball moving at 6 m/s.
Answer and explanation

18 J

KE = ½mv² = ½ × 1 × 6² = ½ × 1 × 36 = 18 J.

Problem 5

Explain why a roller coaster car has the most kinetic energy at the bottom of the first hill and the most potential energy at the top.
Answer and explanation

At the top, the car is high up and barely moving, so it has maximum potential energy and almost no kinetic energy. As it drops, that potential energy converts into kinetic energy, so by the bottom it's moving fastest (maximum kinetic energy) but has almost no height left (minimum potential energy).

Problem 6

A 3 kg object has 96 J of kinetic energy. What is its speed?
Answer and explanation

8 m/s

96 = ½ × 3 × v², so 96 = 1.5v², giving v² = 64, so v = 8 m/s.

Problem 7

Using g=10 N/kg, find the potential-energy gain of a 2 kg book raised 1.5 m. Explain the reasoning in your own words.
Answer and explanation

The book–Earth system gains 30 J of gravitational potential energy.

Use change in GPE=mgh.

Substitute 2×10×1.5=30 J.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

A 3 kg object is raised 2 m. Use g=10 N/kg to find gravitational potential-energy increase. Show the reasoning, not just the conclusion.
Hints

Square the speed before multiplying.

Answer and explanation

ΔPE=mgh=3×10×2=60 J.

Problem 9

Two equal-speed carts have masses 2 kg and 6 kg. Compare their kinetic energies.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The 6 kg cart has three times the KE because energy is proportional to mass at fixed speed.

Problem 10

An energy value needs its system and referenceAn energy value needs its system and referenceObject + EarthGPE reference neededMoving objectKE depends on frameEnergy comparisons require a receiving system, reference level and conditions.A height zero and a motion reference are choices used to organize the account.
Gravitational energy is associated with the interacting object–Earth arrangement and is quoted relative to a chosen zero height. Kinetic energy describes motion relative to a chosen reference frame. The boxes organize which system/reference a number belongs to; they do not imply that energy is a material stored inside each box or that absolute values are meaningful without those choices.
For this unit’s illustrated case, why can shifting the zero-height reference change a quoted GPE without changing a predicted energy gain during the same rise?
Answer and explanation

It changes both absolute values by the same reference offset, leaving the height difference and therefore the physical energy change unchanged.

Reference means the agreed comparison used to quote a quantity. State it so two people calculate the same question. For changes, compare the same system and reference in both states rather than mixing unrelated totals.

It changes both absolute values by the same reference offset, leaving the height difference and therefore the physical energy change unchanged.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 11

A supplied 2 kg object moves at 3 m/s and then at 6 m/s in the same reference frame. Compare kinetic energies and say whether doubling speed doubles kinetic energy.
Answer and explanation

KE values are 9 J and 36 J, so doubling speed quadruples KE in this fixed-mass model.

Use ½mv² with the same mass and units.

Compare the ratio after squaring both speeds.

KE values are 9 J and 36 J, so doubling speed quadruples KE in this fixed-mass model.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 12

A light bulb uses 100 J of electrical energy and produces 20 J of light energy. The rest becomes thermal energy. How many joules become thermal energy?
Answer and explanation

80 J

100 J − 20 J (light) = 80 J of thermal energy.

Problem 13

A car engine converts chemical energy (gasoline) into…
  1. only electrical energy
  2. mechanical energy and thermal energy
  3. only sound energy
  4. matter
Answer and explanation

mechanical energy and thermal energy

Burning gasoline releases chemical energy that becomes mechanical energy (motion) and thermal energy (heat).

Problem 14

Which best describes what happens to energy in any transformation?
  1. Some energy is created
  2. Energy disappears
  3. Energy changes form, but the total amount is conserved
  4. Energy always increases
Answer and explanation

Energy changes form, but the total amount is conserved

Energy transformations change the form of energy, but the total amount stays the same.

Problem 15

Trace the energy transformations that occur when you eat food and then go for a run.
Answer and explanation

Chemical energy stored in food is transformed by your body into mechanical (kinetic) energy for movement and into thermal energy that warms your body, with some chemical energy also stored for later use.

Problem 16

A solar panel receives 500 J of light energy and outputs 90 J of electrical energy. How many joules were NOT converted into electrical energy?
Answer and explanation

410 J

500 J − 90 J = 410 J was not converted into electrical energy (mostly lost as thermal energy).

Problem 17

A motor receives 80 J electrically, produces 50 J of useful motion and 5 J of sound. Account for the rest. Explain the reasoning in your own words.
Answer and explanation

25 J must be accounted for, such as warming of the motor and surroundings.

Add named outputs: 50+5=55 J.

Subtract 80−55=25 J for remaining transfers.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 18

A motor lifts a weight while warming. Identify the useful energy change and one other output. Show the reasoning, not just the conclusion.
Hints

Include warming even when light is the desired output.

Answer and explanation

The weight gains gravitational potential energy; the motor and surroundings also gain thermal energy.

Problem 19

A solar panel charges a battery. Describe the main sequence of energy transfer and storage.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Radiant energy arrives from sunlight, electrical energy is transferred, and chemical energy is stored in the battery, with other outputs such as heating.

Problem 20

Use the illustrative service records to propose a conditional source/storage or demand plan. Explain one benefit, one limitation and two additional evidence needs, tracing rather than creating energy.
Hints

Use the illustrated study titled Energy sources and future choices. Keep the observations, model and conditions separate.

Answer and explanation

Supply X alone does not meet the stated 20-unit evening need, while storage with 15 units charged and 80% useful return delivers 12 evening units. The remaining 8 need another supply or a justified demand reduction. A proposal should compare resource/impact/reliability evidence for that equal service and acknowledge omitted costs and timing. No source is universally best from these few records, and lost useful fraction remains accounted energy elsewhere.

Supply X alone does not meet the stated 20-unit evening need, while storage with 15 units charged and 80% useful return delivers 12 evening units. The remaining 8 need another supply or a justified demand reduction. A proposal should compare resource/impact/reliability evidence for that equal service and acknowledge omitted costs and timing. No source is universally best from these few records, and lost useful fraction remains accounted energy elsewhere.

Response criteria

  • Use equal service and the supplied timing.
  • Calculate/trace storage output and demand gap.
  • Evaluate benefits, limits and missing evidence fairly.

Problem 21

Useful energy can decline while the full account balancesUseful energy can decline while the full account balancesDevice / local objectUseful transfer 70 JSurrounding roomThermal increase 30 JInput account: 100 J = useful 70 J + surroundings 30 J.A smaller boundary can miss a real transfer without energy being destroyed.
The illustrated 100 J input becomes 70 J of the defined useful transfer and 30 J of thermal increase in surrounding objects. The red arrow is energy transfer across the chosen boundary, not a gas or an electric-current arrow. A real account must include the actual stores and partners instead of calling all missing useful energy destroyed.
For this unit’s illustrated case, what is missing if the report counts only the useful 70 J? Would enlarging the boundary change the useful-output definition?
Answer and explanation

The 30 J transferred to surroundings is omitted. Enlarging the boundary includes that change, while useful output still depends on the stated job.

A boundary is an accounting choice, not a wall that energy automatically obeys. Name the objects receiving energy and the pathway by which it reaches them. “Less useful” and “less total energy” are different claims.

The 30 J transferred to surroundings is omitted. Enlarging the boundary includes that change, while useful output still depends on the stated job.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 22

A supplied motor system receives 150 J and gives 90 J to the useful job, 40 J to thermal increase and 20 J to sound/other measured changes. Build a complete account and distinguish a transfer from a store.
Answer and explanation

150 = 90+40+20 J. Transfers name energy moving between systems, while stores/contributions name energy accounted for within chosen objects/arrangements. No supplied energy is missing.

Sum all stated outputs and changes.

Identify actual receiving systems rather than treating arrows as material flowing through every object.

150 = 90+40+20 J. Transfers name energy moving between systems, while stores/contributions name energy accounted for within chosen objects/arrangements. No supplied energy is missing.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 23

A 25°C metal spoon is placed in 80°C soup. They reach thermal equilibrium at 65°C. By how many degrees did the spoon's temperature change?
Answer and explanation

40 °C

Temperature change = 65 − 25 = 40°C.

Problem 24

Which method of heat transfer does NOT require any matter or medium?
  1. conduction
  2. convection
  3. radiation
  4. friction
Answer and explanation

radiation

Radiation travels as electromagnetic waves and can cross empty space with no medium needed.

Problem 25

Stirring a pot of soup and watching it circulate as it heats is an example of…
  1. conduction
  2. convection
  3. radiation
  4. sublimation
Answer and explanation

convection

Convection is heat transfer through the movement of a fluid, like circulating soup.

Problem 26

Explain why a metal spoon in hot soup feels hotter to the touch than a wooden spoon in the same soup.
Answer and explanation

Metal conducts heat much faster than wood, so thermal energy transfers quickly from the soup through the metal spoon to your hand, while wood is a poor conductor and transfers heat much more slowly.

Problem 27

Two objects are placed in contact: one at 90°C, one at 30°C. They reach thermal equilibrium at 60°C. By how many degrees did the cooler object's temperature rise?
Answer and explanation

30 °C

The cooler object rose from 30°C to 60°C, a change of 60 − 30 = 30°C.

Problem 28

Why can a vacuum flask reduce heat transfer even though it cannot stop all radiation? Explain the reasoning in your own words.
Answer and explanation

The gap limits conduction and convection; radiation remains possible.

A vacuum largely removes matter needed for conduction and convection across the gap.

Radiation can still cross the gap, so reflective surfaces and other design features help.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 29

Why does trapped air in insulation reduce some thermal transfer? Show the reasoning, not just the conclusion.
Hints

More than one process can occur in the same scene.

Answer and explanation

Air conducts poorly compared with many solids, and small trapped pockets limit bulk convection. Radiation may still contribute.

Problem 30

A warm mug cools on a table. Name two possible energy-transfer paths.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Conduction to the table, convection to surrounding air, and radiation are possible; select any two and identify the receiving surroundings.

Problem 31

Net heat transfer follows a temperature differenceNet heat transfer follows a temperature differenceWarmer object80 °CCooler object20 °CNet energy transferFor equal masses of the same substance in an ideal isolated comparison:equilibrium is 50 °C, not 100 °C; energy transfers are not temperature addition.
The model compares equal masses of the same substance initially at 80 and 20 °C, with no phase changes or net exchange with other objects. Net energy transfer is from warmer to cooler, reaching 50 °C under the stated ideal assumptions. The red arrow shows energy transfer, not flow of a substance called heat or cold. Different masses or substances would require a different final-temperature account.
For this unit’s illustrated case, would simply adding the initial temperatures give the equilibrium temperature? Explain what is held comparable here.
Answer and explanation

No. For this ideal equal-mass, same-substance case the result is the midpoint 50 °C, not 100 °C. Amount, substance, phase and outside transfers affect the prediction.

Heat names energy transferred because of a temperature difference. Thermal energy names an energy contribution within an account. When temperatures match, the simple model has no net heat transfer between these two objects, though microscopic exchanges can still occur.

No. For this ideal equal-mass, same-substance case the result is the midpoint 50 °C, not 100 °C. Amount, substance, phase and outside transfers affect the prediction.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 32

An object receives energy across a vacuum gap from a warmer source. Which of conduction, convection or radiation can explain the gap transfer? What would a full net-exchange account include?
Answer and explanation

Radiation can cross the vacuum. Convection needs moving material and conduction needs an interacting material pathway. A full net account includes both objects’ emission/absorption and their temperatures.

Use the presence or absence of matter to distinguish mechanisms.

Do not mistake a selected one-way arrow for every transfer in the full exchange.

Radiation can cross the vacuum. Convection needs moving material and conduction needs an interacting material pathway. A full net account includes both objects’ emission/absorption and their temperatures.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 33

A ball has 80 J of potential energy at the top of a hill and 0 J of kinetic energy. Ignoring friction, what is its kinetic energy at the bottom?
Answer and explanation

80 J

By conservation of energy, all 80 J of potential energy converts to kinetic energy at the bottom.

Problem 34

A skater has 300 J of potential energy at the top of a ramp. By the bottom, 45 J converts to thermal energy from friction. What is the skater's kinetic energy at the bottom?
Answer and explanation

255 J

300 J − 45 J (lost to friction) = 255 J of kinetic energy.

Problem 35

A roller-coaster car slows as it climbs a hill. According to conservation of energy, its kinetic energy is mainly being
  1. converted into gravitational potential energy
  2. destroyed by the climb
  3. created out of nothing
  4. turned into extra mass
Answer and explanation

converted into gravitational potential energy

Energy is conserved: as the car rises and slows, kinetic energy transforms into gravitational potential energy rather than being destroyed.

Problem 36

A Newton's cradle eventually stops swinging. Does this violate the law of conservation of energy? Explain.
Answer and explanation

No — the energy isn't destroyed. It transforms into small amounts of sound and thermal energy from friction and air resistance at each collision, until almost none of the original energy is left as visible swinging motion.

Problem 37

A 500 J system has 120 J converted to sound and 80 J converted to thermal energy as an object moves. How much energy remains as kinetic energy?
Answer and explanation

300 J

500 − 120 − 80 = 300 J remains as kinetic energy.

Problem 38

An object starts with 100 J of gravitational potential energy. At the bottom it has 75 J of kinetic energy. What happened to 25 J? Explain the reasoning in your own words.
Answer and explanation

25 J transferred into other energy stores or surroundings; it was not destroyed.

Compare starting and final mechanical energies: 100−75=25 J.

Account for transfers through friction, air resistance or deformation.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 39

A system starts with 120 J. It retains 45 J and transfers the rest out. Find transferred energy. Show the reasoning, not just the conclusion.
Hints

State where the 55 J went instead of calling it destroyed.

Answer and explanation

120−45=75 J.

Problem 40

A rolling ball loses 10 J of kinetic energy but gains only 7 J of potential energy. Identify a plausible missing transfer.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

About 3 J can transfer to thermal energy, sound or other accounted outputs, assuming these are the only other changes.

Problem 41

Mechanical energy can decrease while total energy stays accounted forMechanical energy can decrease while total energy stays accounted forFull system includes object, Earth and relevant surroundings.InitialGPE 100 JLaterGPE 30 JKE 50 JThermal 20 JMechanical energy: 100 → 80 J. Total represented energy: 100 → 100 J.
The supplied initial account has 100 J gravitational energy. The later account has 30 J gravitational, 50 J kinetic and 20 J thermal increase, giving the same 100 J total. Mechanical energy, the sum of kinetic and gravitational contributions here, has fallen to 80 J. The bar widths use one common energy scale and the particular values are illustrative.
For this unit’s illustrated case, how much mechanical energy has changed into the represented thermal contribution? Why is conservation of mechanical energy the wrong assumption for this case?
Answer and explanation

20 J. The account explicitly includes a thermal increase from interactions, so mechanical energy alone does not stay constant even though the full stated total does.

Mechanical energy is a subset of an energy account, not another name for every store. Check whether friction and other interactions transfer energy out of that subset before applying a mechanical-conservation shortcut.

20 J. The account explicitly includes a thermal increase from interactions, so mechanical energy alone does not stay constant even though the full stated total does.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 42

A full supplied energy account starts with 80 J GPE. Later it has 15 J GPE, 45 J KE and 20 J thermal increase. Compare mechanical and total energy before and after.
Answer and explanation

Mechanical energy falls from 80 to 15+45 = 60 J. The stated total stays 80 J once the 20 J thermal increase is included.

Distinguish the mechanical subset from the full account.

Use a common boundary and reference for both states.

Mechanical energy falls from 80 to 15+45 = 60 J. The stated total stays 80 J once the 20 J thermal increase is included.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 43

A circuit has 9 volts and 3 ohms of resistance. Find the current.
Answer and explanation

3 A

I = V ÷ R = 9 ÷ 3 = 3 A.

Problem 44

A current of 4 amps flows through a 6-ohm resistor. Find the voltage.
Answer and explanation

24 V

V = IR = 4 × 6 = 24 V.

Problem 45

A 15-volt circuit has a current of 5 amps. Find the resistance.
Answer and explanation

3 Ω

R = V ÷ I = 15 ÷ 5 = 3 Ω.

Problem 46

In a series circuit, if one bulb burns out…
  1. only that bulb goes out; the others stay lit
  2. all the bulbs go out because the circuit is broken
  3. the other bulbs get brighter
  4. nothing changes
Answer and explanation

all the bulbs go out because the circuit is broken

A series circuit has only one path, so a break anywhere stops current everywhere on that path.

Problem 47

Which material is the BEST electrical conductor?
  1. rubber
  2. glass
  3. copper
  4. wood
Answer and explanation

copper

Copper is a metal that conducts electricity very well, which is why it's used in wiring.

Problem 48

Explain one advantage of parallel circuits over series circuits, using your home's wiring as an example.
Answer and explanation

In a parallel circuit, each device, like a lamp or TV, has its own path, so if one device is switched off or stops working, the others keep getting power. That's why turning off one light at home doesn't turn off the whole house.

Problem 49

Two bulbs are in series and one connection is removed. What happens? Compare with two independent parallel branches. Explain the reasoning in your own words.
Answer and explanation

Both series bulbs go out; a bulb in the other intact parallel branch can stay lit.

Breaking the series path stops current through both bulbs.

Opening one parallel branch can leave the other branch's complete path intact.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 50

A 9 V source connects across an 18 Ω resistor. Find current. Show the reasoning, not just the conclusion.
Hints

Check 0.5×12=6 V.

Answer and explanation

I=V/R=9/18=0.5 A in the ideal model.

Problem 51

Why does a gap in a simple series circuit stop steady current throughout the loop?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The gap breaks the continuous conducting path; charge cannot circulate steadily through that loop.

Problem 52

Complete source/return diagrams, compare the three supplied records and explain evidence for energy transfer by current. Identify which observations would be needed to measure energy quantitatively.
Hints

Use the illustrated study titled Observe energy transfer in circuit paths. Keep the observations, model and conditions separate.

Answer and explanation

With the supplied series loop closed, both lamps respond; opening the single path makes both go out. In the parallel model, opening one branch leaves the other complete branch operating. Light/thermal responses provide qualitative energy-transfer evidence, not charge consumption. Quantitative energy needs voltage, current and duration or another justified method, not brightness alone.

With the supplied series loop closed, both lamps respond; opening the single path makes both go out. In the parallel model, opening one branch leaves the other complete branch operating. Light/thermal responses provide qualitative energy-transfer evidence, not charge consumption. Quantitative energy needs voltage, current and duration or another justified method, not brightness alone.

Response criteria

  • Complete conducting paths and distinguish arrangements.
  • Use observed device behavior as qualitative evidence.
  • Separate charge, energy and measurement limits.

Problem 53

Current is charge per time; voltage is energy per chargeCurrent is charge per time; voltage is energy per chargeCharge flow: 4 C in 2 s → current 2 AEnergy per charge: 12 J / 4 C → potential difference 3 VBattery energy transferLamp / receiver
This conceptual closed circuit separates a supplied charge-flow record, 4 coulombs in 2 seconds, from a supplied energy-per-charge record, 12 joules for 4 coulombs across the receiver. They give 2 A and 3 V respectively. The arrow follows conventional current rather than a drawn individual electron path. Charges circulate; a lamp transfers energy rather than using up the charge itself.
For this unit’s illustrated case, what current and potential difference follow from the two records, and why are they different quantities?
Answer and explanation

Current = 4 C/2 s = 2 A; potential difference = 12 J/4 C = 3 V. One is charge-flow rate and the other energy transferred per charge.

A coulomb (C) is a unit of electric charge; an ampere (A) is one coulomb per second, and a volt (V) is one joule per coulomb. Introduce these units before using their ratios. The diagram uses a safe supplied-data circuit, not mains wiring.

Current = 4 C/2 s = 2 A; potential difference = 12 J/4 C = 3 V. One is charge-flow rate and the other energy transferred per charge.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 54

A supplied receiver transfers 18 J for 6 C of charge while 6 C passes in 3 s. Find potential difference and current, then explain why the receiver does not use up the charge.
Answer and explanation

Potential difference 18/6 = 3 V; current 6/3 = 2 A. Charge circulates through the complete circuit while energy is transferred to the receiver.

Use joules per coulomb for voltage and coulombs per second for current.

Keep conserved charge flow distinct from energy transfer.

Potential difference 18/6 = 3 V; current 6/3 = 2 A. Charge circulates through the complete circuit while energy is transferred to the receiver.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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