NCEA Level 1 AS92047 Flashcards for Energy Concepts

Question 1

A warm bowl of soup cools on a bench. If your system contains only the soup, where should you draw its boundary?

Accepted Answer

Around the soup. The bowl and room are outside that boundary, so energy transferred from the soup to either one leaves the system.

Question 2

A seed pod falls and speeds up. Which energy changes can you infer for the seed pod and Earth together?

Accepted Answer

Gravitational potential energy decreases as height falls. The pod's kinetic energy increases as its speed rises.

Question 3

A solid metal sample goes from 16 °C to 29 °C. What temperature change should you report as evidence?

Accepted Answer

Its temperature rose by 13 °C. Subtract 16 °C from 29 °C.

Question 4

A battery powers a small reading lamp. What energy pathway would you show with arrows from the battery to the room?

Accepted Answer

Chemical energy in the battery → electrical energy transferred to the lamp → light and thermal energy transferred to the surroundings.

Question 5

One machine transfers 60 J in 3 s and another transfers 60 J in 6 s. Which has greater average power?

Accepted Answer

The first machine. It transfers the same energy in half the time, so its average power is twice as large, 20 W compared with 10 W.

Question 6

A hoist raises a 3 kg tool bag by 2 m. Use g = 10 N/kg. How much gravitational potential energy does the bag–Earth system gain?

Use ΔEₚ = mgΔh.

Accepted Answer

60 J. Multiply mass, gravitational field strength and vertical rise. That gives 3 × 10 × 2 = 60 J.

Question 7

One end of a metal rod touches a hot surface. The other end warms up. What mechanism moves energy through the rod?

Accepted Answer

Conduction. Energy passes along the solid from its hotter end towards its cooler end.

Question 8

A battery, switch and lamp are connected in one loop. What switch position would you mark on a circuit diagram to explain why the lamp goes out?

Accepted Answer

An open switch. The break in the loop stops current through the lamp.

Question 9

A 2 kg glider slides at 3 m/s. What is its kinetic energy?

Use Eₖ = ½mv².

Accepted Answer

9 J. Half the mass times speed squared gives ½ × 2 × 3² = 9 J.

Question 10

A solid block has mass 0.40 kg and specific heat capacity 500 J/(kg·°C). Its temperature rises by 12 °C. How much energy does it gain?

Use E = mcΔT.

Accepted Answer

2,400 J. Use E = mcΔT because the block stays solid. Multiply 0.40 × 500 × 12.

Question 11

A motor lifts a bucket. You want to treat the motor's work as an energy input and the gravitational potential energy gain as a change within the system. What goes inside your boundary?

Accepted Answer

The bucket and Earth. Leave the motor outside so its work transfers energy across the boundary.

Question 12

An ohmic resistor has 8 V across it and carries 0.5 A. What is its resistance?

Use V = IR.

Accepted Answer

16 Ω. Rearrange V = IR to get R = V ÷ I, then divide 8 V by 0.5 A.

Question 13

A winch pulls a crate 5 m along a level floor with a steady 24 N force parallel to the motion. How much work does the winch do on the crate?

Use W = Fd.

Accepted Answer

120 J. Use W = Fd because the force points along the displacement. Multiply 24 N by 5 m.

Question 14

A heater warms room-temperature water at the bottom of a tank. What motion should arrows show for the resulting convection current?

Accepted Answer

Warmed water rises and cooler water sinks to replace it. The warmed water is less dense than the cooler water around it.

Question 15

A motor draws 0.30 A from a 12 V supply. How fast does it receive electrical energy?

Use P = VI.

Accepted Answer

3.6 W, or 3.6 J per second. Use P = VI and multiply 12 by 0.30.

Question 16

A mixer receives 150 J from a battery. Its useful mixing action accounts for 90 J. How much goes to other outputs if its stored energy stays unchanged?

Accepted Answer

60 J. The other outputs account for the difference between the 150 J input and the 90 J useful output.

Question 17

You have E = mcΔT and E = mL. Which applies to a pure solid melting at constant temperature?

Accepted Answer

E = mL. The energy goes into the change of state while the temperature stays at the melting point.

Question 18

A climber follows a 7 m route up a wall and gains 4 m in height. Which distance belongs in the change in gravitational potential energy?

Accepted Answer

4 m. The change depends on the vertical rise above the starting point.

Question 19

Where should you connect a voltmeter to measure the voltage across a lamp?

Accepted Answer

In parallel with the lamp. Connect one voltmeter terminal on each side of the lamp.

Question 20

An infrared heater warms your hand across an evacuated gap. Which heat-transfer mechanism crosses the gap?

Accepted Answer

Radiation. Infrared waves can carry energy through a vacuum.

Question 21

A stone drops from rest. Air resistance is negligible. An energy-bar diagram for the stone and Earth shows 30 J of gravitational potential energy at release. How should the total mechanical-energy bar change as the stone falls?

Accepted Answer

It should stay at 30 J. The loss of gravitational potential energy is matched by an equal gain in kinetic energy.

Question 22

A fan draws a steady 9 W of electrical power for 2 min. How much electrical energy does it receive?

Use ΔE = Pt.

Accepted Answer

1,080 J. Convert 2 min to 120 s, then use ΔE = Pt = 9 × 120.

Question 23

On a graph of cumulative energy transferred against time, line A has twice the slope of line B. What does that tell you about power?

Accepted Answer

A transfers energy at twice the rate of B. The slope gives energy per unit time, which is power.

Question 24

Two blocks of equal mass each gain 800 J without changing state. Block A warms by 4 °C and B by 8 °C. Which has greater specific heat capacity?

Accepted Answer

Block A. With the same mass and energy gain, its smaller temperature rise means it takes more energy to warm each kilogram by 1 °C.

Question 25

A loaded trolley travels at 2 m/s on one run and 6 m/s on another. Its mass stays the same. How should the two kinetic-energy bars compare?

Accepted Answer

The bar for 6 m/s should be nine times as tall. Tripling the speed multiplies kinetic energy by three squared when the mass is fixed.

Question 26

A resistor's voltage–current graph has voltage on the vertical axis and current on the horizontal axis. What does the slope of the straight line through the origin give?

Accepted Answer

The resistance. The slope is voltage change divided by current change, equal to V ÷ I for this line through the origin.

Question 27

A 0.15 kg frozen water sample at 0 °C melts completely. Use a latent heat of fusion of 334,000 J/kg. How much energy is absorbed during melting?

Choose from E = mcΔT and E = mL.

Accepted Answer

50,100 J. Use E = mL for the change of state. Multiply 0.15 kg by 334,000 J/kg.

Question 28

A sled slows on level rough ground. Its height stays the same. How would you account for the drop in kinetic energy?

Accepted Answer

The sled and ground gain internal energy through friction. Some energy can also spread to the surroundings through sound.

Question 29

A battery-driven hoist raises a planter at constant speed. Apart from sound and heating, which energy change belongs at the output end of the hoist's energy diagram?

Accepted Answer

An increase in the planter–Earth system's gravitational potential energy. The rising planter keeps the same kinetic energy because its speed is constant.

Question 30

A lamp receives 80 J and emits 16 J as useful light. What fraction of the input is useful, expressed as a percentage?

Accepted Answer

20%. Divide the useful light energy by the electrical input, then multiply by 100.

Question 31

A lid is fitted to a dish of warm water. Evaporation is negligible in both cases. How can the lid reduce cooling through the air above the water?

Accepted Answer

It reduces heat transfer by convection. Less warm air near the surface can move away and be replaced by cooler room air.

Question 32

A cart moves 4 m while a 6 N friction force acts opposite its motion. How much energy does friction remove from the cart's mechanical energy?

Accepted Answer

24 J. Multiply force by distance to get the amount removed. Friction does −24 J of work on the cart because it points against the motion.

Question 33

A resistor is swapped for one with twice the resistance. An ideal supply keeps the voltage across it fixed. How does the current change?

Accepted Answer

It halves. At fixed voltage, I = V ÷ R, so doubling the resistance halves the current.

Question 34

A pure substance is heated at constant power and fixed pressure. On its temperature–time graph, a rising line becomes flat as the solid starts to melt. Why can the heater still transfer energy on that flat section?

Accepted Answer

The energy changes the state from solid to liquid. The temperature stays at the melting point while the solid and liquid coexist.

Question 35

Two lifts each raise the same load through the same height. Lift A takes 5 s and lift B takes 15 s. How do their average useful powers compare?

Accepted Answer

Lift A provides three times the useful power. The lifts give the load the same gravitational potential energy gain, but A does it in one-third of the time.

Question 36

An energy-flow diagram shows 100 J entering a pump, 65 J as useful output and 35 J heating the surroundings. What should the two output arrow widths show if widths represent energy?

Accepted Answer

The useful arrow should be wider. Use widths in the ratio 65:35 so the two outputs together have the input arrow's width.

Question 37

At a fixed 6 V, a 12 Ω heating element is replaced by a 6 Ω element. Both are ohmic. How does electrical power change?

Accepted Answer

It doubles. The current rises from 0.5 A to 1 A, so P = VI rises from 3 W to 6 W.

Question 38

A liquid sample of mass 0.20 kg receives 800 J. Its specific heat capacity is 2,000 J/(kg·°C). Assume no phase change or energy loss. How much does its temperature rise?

Choose from E = mcΔT and E = mL.

Accepted Answer

2 °C. Rearrange E = mcΔT to get ΔT = 800 ÷ (0.20 × 2,000).

Question 39

Two ramps of different lengths bring the same crate to the same raised platform. The crate starts from the same floor each time. How do the gravitational potential energy gains compare?

Accepted Answer

They are equal. The mass, gravitational field strength and vertical rise are the same for both routes.

Question 40

A pump receives 360 J from a supply while drawing a steady 12 W. How long was it running?

Accepted Answer

30 s. Divide the energy transferred by the power. That gives 360 J ÷ 12 W = 30 s.

Question 41

A flask has an evacuated gap between its walls. Why might adding a shiny low-emissivity surface still reduce heat loss?

Accepted Answer

Radiation can cross the vacuum. The low-emissivity surface emits less infrared at the same temperature, reducing net radiative transfer across the gap.

Question 42

A box sliding down a rough ramp loses 240 J of gravitational potential energy and gains 180 J of kinetic energy. Assume sound is negligible. What happened to the other 60 J?

Accepted Answer

It mainly increased the internal energy of the box and ramp. Mechanical energy fell by 60 J, and friction warmed the surfaces as they rubbed together.

Question 43

Two ohmic heating wires are tested separately at the same current. Wire A has twice the resistance of wire B. Which receives more power?

Accepted Answer

Wire A receives twice as much power. At fixed current, twice the resistance needs twice the voltage, and P = VI doubles.

Question 44

Pump A uses 200 J to give water 120 J of gravitational potential energy. Pump B uses 300 J to give water 150 J. Which uses its input more efficiently?

Accepted Answer

Pump A. It transfers 60% of its input to the water as useful energy. Pump B transfers 50%.

Question 45

Two mugs hold equal masses of water at 80 °C. Assume negligible evaporation. After 5 min, one is at 68 °C and the other at 74 °C. What does this show about their average net energy loss per second?

Accepted Answer

The water that reached 68 °C lost energy at twice the average rate of the other sample. For equal masses and specific heat capacities, its 12 °C drop means twice the energy loss of the 6 °C drop over the same time.

Question 46

A bead starts from rest and slides down a smooth wire. Air resistance is negligible. It has lost 6 J of gravitational potential energy at point A and 10 J at point B. What would you draw for its kinetic-energy bars?

Accepted Answer

A 6 J bar at A and a 10 J bar at B. Each bar equals the gravitational potential energy lost by that point.

Question 47

Two lamps are connected to an ideal battery on separate parallel branches. One lamp's branch opens. Why can the other lamp stay on?

Accepted Answer

Its branch still makes a complete loop through the battery, so current can still flow through that lamp.

Question 48

A 0.10 kg sample of ice at 0 °C melts, then the water warms. Use a latent heat of fusion of 334,000 J/kg, a liquid specific heat capacity of 4,200 J/(kg·°C) and a 10 °C rise. How much energy does the sample gain in total?

The available formulas are E = mcΔT and E = mL.

Accepted Answer

37,600 J. Melting takes 33,400 J from mL. Warming the resulting water takes another 4,200 J from mcΔT.

Question 49

A winch pulls two identical crates one at a time with the same force along the direction of travel. Crate A moves twice as far as crate B. How does the work done by the winch compare?

Accepted Answer

The winch does twice as much work on A. With the same force parallel to the motion, W = Fd scales with distance.

Question 50

At 2 V a component carries 0.20 A. At 4 V it carries 0.30 A. What evidence rules out one constant resistance over these readings?

Accepted Answer

The V ÷ I ratio changes. It is 10 Ω at the first reading and about 13.3 Ω at the second, so the data do not fit one straight line through the origin.

Question 51

A bare metal tray and a foam-lined tray each hold an identical warm sample. Energy leaves mainly through the base. Why does the foam-lined tray keep its sample warm longer?

Accepted Answer

The foam slows conduction through the base. Less energy leaves per second, so the sample's temperature falls more slowly.

Question 52

A 1 kg toy car and a 2 kg toy car have equal kinetic energies. How does the lighter car's speed compare?

Accepted Answer

The lighter car is about 1.41 times as fast. With half the mass, its speed squared must be twice as large to give the same kinetic energy.

Question 53

An element takes 2 A at 9 V for 40 s. How much electrical energy enters the element?

The available formulas are ΔE = Pt and P = VI.

Accepted Answer

720 J. Find power from P = VI = 18 W, then multiply by 40 s.

Question 54

Two solid blocks have the same specific heat capacity and absorb energy at the same constant rate with negligible losses. Block B has twice the mass of block A. How do the slopes of their temperature–time graphs compare?

Accepted Answer

Block B's slope is half as steep. Each block gains the same energy per second, so doubling the mass halves the temperature rise per second.

Question 55

A crane adds 600 J of gravitational potential energy in 3 s. The motor receives 300 W of electrical power throughout. What fraction of the input power becomes useful lifting power?

Accepted Answer

Two-thirds, about 67%. The useful output is 600 J ÷ 3 s = 200 W, and 200 W ÷ 300 W gives the fraction.

Question 56

A room heater sits near floor level. Warm air gathers near the ceiling. How can a small fan reduce the temperature difference between floor and ceiling?

Accepted Answer

It mixes the warm upper air with the cooler air below. Moving the air redistributes thermal energy around the room.

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NCEA Level 1 AS92047 Flashcards for Energy Concepts

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