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CAST Practice Questions (2026) — Free Sample Questions with Answers

19 free CAST questions from our 150-question bank, grouped by content domain. Each answer explains why the right option is right — and why every wrong option is wrong.

Graphic Arithmetic (3)Mechanical Concepts (8)Reading for Comprehension (8)
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Graphic Arithmetic

Question 1
Service Center Site Plan
OfficeFleet GarageWarehouseFuelIsland210'15'60'A90'20'12'60'23'25'B140'12'40'28'45'15'15'C85'
Dimensions are not necessarily drawn to scale.

What is the area of the Fleet Garage in square feet?

Show answer & explanation

Answer: D

  1. 1. Width = 90' (top chain); length = 60' (right-side chain)
  2. 2. Area = 90' × 60' = 5,400 square feet

300 square feet: This is the perimeter, 2 × (90' + 60') = 300', not the area.

2,400 square feet: This is the Office's area (60' × 40'), not the Fleet Garage's.

3,600 square feet: Used the Office's 40' length in place of the Garage's 60' length: 90' × 40'.

5,400 square feet: Correct. The Fleet Garage is 90' wide (top chain) and 60' long (right-side chain), so its area is 90' × 60' = 5,400 square feet.

None of the Above: The correct value, 5,400 square feet, is listed as choice D.

Area of a rectangle = width × length; take both from the dimension lines that touch that building.

Question 2
Service Center Site Plan
OfficeFleet GarageWarehouseFuelIsland210'15'60'A90'20'12'60'23'25'B140'12'40'28'45'15'15'C85'
Dimensions are not necessarily drawn to scale.

What is the distance ("A") between the Office and the Fleet Garage?

Show answer & explanation

Answer: B

  1. 1. Width chain: 15' + 60' + A + 90' + 20' = 210'
  2. 2. 15' + 60' + 90' + 20' = 185'
  3. 3. A = 210' − 185' = 25'

20': Used the printed 20' between the Fleet Garage and the right edge, not the gap A.

25': Correct. Across the top, 15' + 60' + A + 90' + 20' make up the full 210' width, so A = 210' − 185' = 25'.

35': Left out the 20' strip on the right: 210' − (15' + 60' + 90') = 35'.

40': Left out the 15' strip on the left: 210' − (60' + 90' + 20') = 40'.

None of the Above: The correct value, 25', is listed as choice B.

Add every known piece of the chain, then subtract that sum from the overall width.

Question 3

What is the width ("C"), measured from left to right, of the Warehouse on the site plan?

Service Center Site Plan
OfficeFleet GarageWarehouseFuelIsland210'15'60'A90'20'12'60'23'25'B140'12'40'28'45'15'15'C85'
Dimensions are not necessarily drawn to scale.
Show answer & explanation

Answer: C

  1. 1. Bottom chain: 15' + C = 85'
  2. 2. C = 85' − 15' = 70'

45': Used the Warehouse's 45' length (top to bottom) instead of its width.

60': Used the 60' width of the Office, the building above the Warehouse.

70': Correct. The 15' setback plus the width C make up the printed 85' from the left edge to the Warehouse's right side, so C = 85' − 15' = 70'.

85': Used the whole 85' dimension without subtracting the 15' setback from the left edge.

None of the Above: The correct value, 70', is listed as choice C.

When a long dimension covers a gap plus a building, subtract the gap to get the building alone.

Mechanical Concepts

Question 4

A brass strip is bonded on top of a steel strip, clamped at one end, and heated. Brass expands more than steel when heated. Which way will the free end bend?

BrassSteelABClamp
Show answer & explanation

Answer: B

Direction A: Reversed: the longer brass layer ends up on the outside of the curve, which bends the strip away from the brass.

Direction B: Correct. When heated, the brass on top grows longer than the steel below it. The bonded strip must curve with the longer brass on the outside, so the free end bends down toward the steel side, Direction B.

It stays straight: The two metals expand by different amounts, so the bonded strip cannot stay straight.

A heated bimetal strip bends toward the metal that expands less; thermostats use this to open and close contacts.

Question 5

Ball A rolls slowly and Ball B rolls fast off the same level table edge at the same moment. Ignoring air resistance, which ball hits the floor first? (If equal, mark C.)

Ball A landsBall B landsA: slowB: fast
Show answer & explanation

Answer: C

Ball A: Assumed a shorter path takes less time; both balls fall the same height, starting with no downward speed.

Ball B: Speed across the table carries B farther out, but it does not speed up its fall.

Balls A and B equally: Correct. Sideways speed does not affect falling. Both balls start with no downward speed and fall the same height under the same gravity, so they land together; B simply lands farther away.

Horizontal and vertical motion are independent: objects launched sideways from the same height fall for the same time.

Question 6

Small piston A is pushed down with 20 lb. The two cylinders are joined and filled with oil. Which piston pushes with the greater force? (If equal, mark C.)

20 lb pushLoadPiston APiston B
Show answer & explanation

Answer: B

Piston A: Reversed: the small piston only sets the pressure; the large piston has more area for that pressure to act on.

Piston B: Correct. The oil carries the same pressure everywhere, and force equals pressure times area. Piston B has a much larger area, so it pushes up with far more than 20 lb.

Pistons A and B equally: The pressure is equal, not the force; the larger piston turns that pressure into a larger force.

A hydraulic system passes the same pressure to every piston, so a larger piston delivers a larger force.

Question 7

Tank B holds more water than Tank A, but its water is only 5 ft deep. When the valve is opened, which way will water flow through the pipe?

AB8 ft5 ftTank AValve (closed)Tank B
Show answer & explanation

Answer: B

Direction A: Assumed the larger amount of water pushes harder; the push at the pipe depends on depth, and Tank B is shallower.

Direction B: Correct. Pressure at the pipe depends on depth: 8 ft in Tank A against 5 ft in Tank B. Water flows from the higher pressure in A toward B until the levels match.

Water will not flow: The levels differ by 3 ft, so the pressures at the pipe are unequal and water must flow.

Water flows between connected tanks from the higher level to the lower level, whatever the amount of water in each.

Question 8

A 60 lb weight hangs 4 feet left of the pivot. Ignoring the beam’s weight, at which point must a 40 lb weight hang to balance the beam?

60 lb4 ftA4 ftB6 ftC8 ft
Distances on the right are measured from the pivot.
Show answer & explanation

Answer: B

  1. 1. Left moment: 60 lb × 4 ft = 240 lb·ft
  2. 2. Right moment needed: 40 lb × d = 240 lb·ft
  3. 3. d = 240 ÷ 40 = 6 ft, which is point B

Point A: Matched the 4-foot distance; equal distances balance only equal weights, and 40 × 4 = 160 is less than 240.

Point B: Correct. The moments must match: 60 lb × 4 ft = 240 lb·ft, and 40 lb × 6 ft = 240 lb·ft, so the 40 lb weight hangs at point B.

Point C: At 8 ft the moment is 40 × 8 = 320 lb·ft, more than 240, so the right side would go down.

A beam balances when weight × distance is the same on both sides of the pivot; a lighter weight must hang farther out.

Question 9

Both frictionless pulley systems lift the same 200 lb load. Which pull rope needs less force to raise the load? (If equal, mark C.)

ABPull200 lbPull200 lb
Show answer & explanation

Answer: A

  1. 1. A: 4 segments hold the movable block: 200 lb ÷ 4 = 50 lb
  2. 2. B: 2 segments hold the movable block: 200 lb ÷ 2 = 100 lb

Rope A: Correct. Four rope segments hold the movable block in A, so each carries a quarter of the load: a 50 lb pull, versus 100 lb with the two segments in B.

Rope B: Picked the simpler rigging; fewer segments holding the movable block mean a larger pull.

Ropes A and B equally: The loads are the same, but the number of supporting segments differs (4 versus 2), so the pulls differ.

Ideal pull = load ÷ number of rope segments holding the movable block; do not count the free end you pull on.

Question 10

All three switches are closed and the lamp is lit. Which single switch, if opened, will turn off the lamp?

BatteryLampCAB
All three switches are shown closed.
Show answer & explanation

Answer: C

Switch A: Switch A is in parallel with switch B; current still flows through B.

Switch B: Switch B is in parallel with switch A; current still flows through A.

Switch C: Correct. Switch C is in series with the lamp, so opening it breaks the only path. A and B are in parallel, so if one opens, current still flows through the other.

A switch in series with the lamp controls the whole circuit; a switch in a parallel branch controls only its own branch.

Question 11

Both drivers have 16 teeth and turn at the same speed. In B, a 32-tooth idler gear sits between the driver and the output gear. Which output gear turns faster? (If equal, mark C.)

ABDriver16 teethOutput16 teethDriver16 teethIdler32 teethOutput16 teeth
Show answer & explanation

Answer: C

  1. 1. A: 16 ÷ 16 = 1, so output A turns at driver speed
  2. 2. B: (16 ÷ 32) × (32 ÷ 16) = 1, so output B also turns at driver speed

Output A: Assumed the extra gear in B slows the train down; an idler passes on the same number of teeth per second that it receives.

Output B: Assumed the large idler speeds up the output; whatever ratio it adds at one mesh it takes back at the next.

Outputs A and B equally: Correct. An idler gear changes only the direction. In both setups a 16-tooth driver ends at a 16-tooth output, so each output turns at the driver’s speed.

An idler gear reverses direction but does not change the speed ratio; only the driver and output tooth counts matter.

Reading for Comprehension

Question 12

Power from Falling Water

1A hydroelectric plant turns the energy of moving water into electricity. It is one of the oldest ways of generating power on a large scale, and unlike a coal or gas plant, it burns no fuel. The river itself does the work, and the plant simply guides the water through a series of steps.

2The first step is the dam. By blocking a river, the dam holds back a large lake called a reservoir. Water stored high behind the dam has energy because of its height, much like a rock resting at the top of a hill. When operators open the intake gates, water rushes into the penstock, a large pipe that slopes steeply downward through or beside the dam. As the water drops, it picks up speed and pressure.

3At the bottom of the penstock, the water strikes the curved blades of a turbine and sets it spinning. A long steel shaft connects the turbine to a generator mounted above it. Inside the generator, a ring of large magnets on the turning part, called the rotor, sweeps past coils of copper wire in the stationary part, called the stator. This motion produces an electric current in the coils. Having given up much of its energy, the water flows out through a channel called the tailrace and rejoins the river downstream.

4Two things decide how much power a plant can produce: the head, which is the vertical distance the water falls, and the flow, which is the amount of water passing through the turbines each second. Doubling either one roughly doubles the power. For this reason, a tall dam on a modest river can produce as much electricity as a low dam on a much larger river. Many people assume that the size of the reservoir sets a plant's output, but the reservoir mainly decides how long the plant can keep running when the river runs low.

5Hydroelectric turbines can be started or stopped in minutes, so these plants are valuable when demand for electricity rises suddenly. Some plants, called pumped-storage plants, use cheap electricity at night to pump water up into a high reservoir and then release it through the turbines during the busiest hours of the day. Pumping takes more energy than the plant later recovers, but it moves that energy to the hours when it is needed most.

According to the fourth paragraph, which change would roughly double a plant's power output?

Show answer & explanation

Answer: B

Doubling the size of the reservoir behind the dam: Misconception: the passage says the reservoir mainly decides how long the plant can run, not its output.

Doubling the vertical distance the water falls: Correct. Paragraph 4 defines head as the vertical fall and states, "Doubling either one roughly doubles the power."

Doubling the length of the tailrace below the plant: The tailrace only returns water to the river; length is never linked to power.

Doubling the number of minutes needed to start the turbines: Start-up time (paragraph 5) has nothing to do with output.

Power depends on head (height of fall) and flow (water per second).

Question 13

Power from Falling Water

1A hydroelectric plant turns the energy of moving water into electricity. It is one of the oldest ways of generating power on a large scale, and unlike a coal or gas plant, it burns no fuel. The river itself does the work, and the plant simply guides the water through a series of steps.

2The first step is the dam. By blocking a river, the dam holds back a large lake called a reservoir. Water stored high behind the dam has energy because of its height, much like a rock resting at the top of a hill. When operators open the intake gates, water rushes into the penstock, a large pipe that slopes steeply downward through or beside the dam. As the water drops, it picks up speed and pressure.

3At the bottom of the penstock, the water strikes the curved blades of a turbine and sets it spinning. A long steel shaft connects the turbine to a generator mounted above it. Inside the generator, a ring of large magnets on the turning part, called the rotor, sweeps past coils of copper wire in the stationary part, called the stator. This motion produces an electric current in the coils. Having given up much of its energy, the water flows out through a channel called the tailrace and rejoins the river downstream.

4Two things decide how much power a plant can produce: the head, which is the vertical distance the water falls, and the flow, which is the amount of water passing through the turbines each second. Doubling either one roughly doubles the power. For this reason, a tall dam on a modest river can produce as much electricity as a low dam on a much larger river. Many people assume that the size of the reservoir sets a plant's output, but the reservoir mainly decides how long the plant can keep running when the river runs low.

5Hydroelectric turbines can be started or stopped in minutes, so these plants are valuable when demand for electricity rises suddenly. Some plants, called pumped-storage plants, use cheap electricity at night to pump water up into a high reservoir and then release it through the turbines during the busiest hours of the day. Pumping takes more energy than the plant later recovers, but it moves that energy to the hours when it is needed most.

According to the passage, the penstock is:

Show answer & explanation

Answer: B

the lake of water held back behind the dam: That is the reservoir, also from paragraph 2.

a large pipe that carries water down to the turbine: Correct. Paragraph 2 calls it "a large pipe that slopes steeply downward through or beside the dam."

the channel that returns water to the river downstream: That is the tailrace, described in paragraph 3.

the stationary part of the generator that holds the coils: That is the stator, described in paragraph 3.

Match each named part to the sentence that defines it; several parts are defined close together.

Question 14

Which of the following best states the main idea of the passage?

Power from Falling Water

1A hydroelectric plant turns the energy of moving water into electricity. It is one of the oldest ways of generating power on a large scale, and unlike a coal or gas plant, it burns no fuel. The river itself does the work, and the plant simply guides the water through a series of steps.

2The first step is the dam. By blocking a river, the dam holds back a large lake called a reservoir. Water stored high behind the dam has energy because of its height, much like a rock resting at the top of a hill. When operators open the intake gates, water rushes into the penstock, a large pipe that slopes steeply downward through or beside the dam. As the water drops, it picks up speed and pressure.

3At the bottom of the penstock, the water strikes the curved blades of a turbine and sets it spinning. A long steel shaft connects the turbine to a generator mounted above it. Inside the generator, a ring of large magnets on the turning part, called the rotor, sweeps past coils of copper wire in the stationary part, called the stator. This motion produces an electric current in the coils. Having given up much of its energy, the water flows out through a channel called the tailrace and rejoins the river downstream.

4Two things decide how much power a plant can produce: the head, which is the vertical distance the water falls, and the flow, which is the amount of water passing through the turbines each second. Doubling either one roughly doubles the power. For this reason, a tall dam on a modest river can produce as much electricity as a low dam on a much larger river. Many people assume that the size of the reservoir sets a plant's output, but the reservoir mainly decides how long the plant can keep running when the river runs low.

5Hydroelectric turbines can be started or stopped in minutes, so these plants are valuable when demand for electricity rises suddenly. Some plants, called pumped-storage plants, use cheap electricity at night to pump water up into a high reservoir and then release it through the turbines during the busiest hours of the day. Pumping takes more energy than the plant later recovers, but it moves that energy to the hours when it is needed most.

Show answer & explanation

Answer: B

Pumped-storage plants move energy from night to the busiest hours of the day.: True, but only the fifth paragraph's point, not the whole passage.

A hydro plant turns falling water into electricity in a series of steps.: Correct. The first paragraph says the plant "guides the water through a series of steps," and paragraphs 2–5 describe them.

The size of a reservoir determines how much power a hydroelectric plant makes.: This is the misconception the fourth paragraph corrects.

Hydroelectric plants are cleaner than coal plants because they burn no fuel.: The passage notes they burn no fuel but never compares how clean they are; this is outside knowledge and only one detail.

A main-idea answer must cover the whole passage, not one paragraph's detail.

Question 15

Changing the Voltage: How a Transformer Works

1Electricity leaving a power plant may travel hundreds of miles before it reaches a kitchen outlet. Along the way, its voltage is raised and lowered several times. The device that does this work is the transformer, one of the simplest and most important machines in the electric system. A transformer has no engine and, in most cases, no moving parts at all. It often sits quietly in a gray metal can on a utility pole.

2A basic transformer has two coils of insulated wire wrapped around a shared core of iron. The coil that receives power is the primary winding, and the coil that sends power out is the secondary winding. The two coils are not connected to each other by any wire. Instead, they are linked by magnetism. When current flows in the primary, it creates a magnetic field, and the iron core carries that field through the secondary coil.

3The key to the process is a discovery made by the English scientist Michael Faraday in 1831: a magnetic field that is changing can produce a voltage in a nearby coil. This effect is called electromagnetic induction. A steady field does nothing to the secondary coil, so a transformer works only with alternating current, which reverses direction many times each second and keeps the magnetic field constantly rising and falling. Fed with direct current, which flows steadily in one direction, a transformer would deliver no output once the current settled.

4The amount by which a transformer changes the voltage depends on the number of turns of wire in each coil. If the secondary has more turns than the primary, the voltage goes up; this is a step-up transformer. If the secondary has fewer turns, the voltage goes down; this is a step-down transformer. A transformer with ten times as many turns on its primary as on its secondary, for example, lowers the voltage to one-tenth. A transformer cannot create energy, however. When it raises the voltage, the current drops in about the same proportion, so the power coming out is never more than the power going in.

5Power companies use this trade between voltage and current to their advantage. Current flowing through a wire heats it, and that heat is wasted energy. By stepping the voltage up at the power plant, utilities can send the same power over long lines with much less current, and therefore much less loss. Near the customer, step-down transformers bring the voltage back down to a safe level for homes. Large transformers are filled with a special oil that both cools and insulates the coils, and a typical distribution transformer delivers about 98 to 99 percent of the energy it receives.

Based on the passage, what would most likely happen if a transformer were connected to a battery that gives a steady current?

Show answer & explanation

Answer: B

It would raise the voltage and lower the current.: Describes a working step-up transformer; with a steady current the magnetic field stops changing, so there is no output at all.

It would give no output once the current settled.: Correct. Paragraph 3 says "A steady field does nothing to the secondary coil"; fed with a steady current, a transformer "would deliver no output once the current settled."

It would overheat because its oil could not cool it.: Mixes in the oil from paragraph 5; the passage links a steady current to no output, not to overheating.

It would send the same power with much less loss.: A detail from paragraph 5 about high-voltage lines, not what happens with a steady current.

Connect the new case to the passage’s rule: a transformer needs a changing magnetic field, so a steady (direct) current gives no output.

Question 16

Changing the Voltage: How a Transformer Works

1Electricity leaving a power plant may travel hundreds of miles before it reaches a kitchen outlet. Along the way, its voltage is raised and lowered several times. The device that does this work is the transformer, one of the simplest and most important machines in the electric system. A transformer has no engine and, in most cases, no moving parts at all. It often sits quietly in a gray metal can on a utility pole.

2A basic transformer has two coils of insulated wire wrapped around a shared core of iron. The coil that receives power is the primary winding, and the coil that sends power out is the secondary winding. The two coils are not connected to each other by any wire. Instead, they are linked by magnetism. When current flows in the primary, it creates a magnetic field, and the iron core carries that field through the secondary coil.

3The key to the process is a discovery made by the English scientist Michael Faraday in 1831: a magnetic field that is changing can produce a voltage in a nearby coil. This effect is called electromagnetic induction. A steady field does nothing to the secondary coil, so a transformer works only with alternating current, which reverses direction many times each second and keeps the magnetic field constantly rising and falling. Fed with direct current, which flows steadily in one direction, a transformer would deliver no output once the current settled.

4The amount by which a transformer changes the voltage depends on the number of turns of wire in each coil. If the secondary has more turns than the primary, the voltage goes up; this is a step-up transformer. If the secondary has fewer turns, the voltage goes down; this is a step-down transformer. A transformer with ten times as many turns on its primary as on its secondary, for example, lowers the voltage to one-tenth. A transformer cannot create energy, however. When it raises the voltage, the current drops in about the same proportion, so the power coming out is never more than the power going in.

5Power companies use this trade between voltage and current to their advantage. Current flowing through a wire heats it, and that heat is wasted energy. By stepping the voltage up at the power plant, utilities can send the same power over long lines with much less current, and therefore much less loss. Near the customer, step-down transformers bring the voltage back down to a safe level for homes. Large transformers are filled with a special oil that both cools and insulates the coils, and a typical distribution transformer delivers about 98 to 99 percent of the energy it receives.

What does the passage mean by "this trade between voltage and current" in the fifth paragraph?

Show answer & explanation

Answer: C

Power companies buy and sell electricity with each other.: Takes "trade" in its business sense; the passage never discusses buying or selling power.

The voltage and the current always rise together.: The reverse of the passage: one rises as the other falls.

When the voltage goes up, the current goes down.: Correct. The phrase refers back to paragraph 4: "When it raises the voltage, the current drops in about the same proportion."

Some of the current is turned into heat in the wire.: A detail from the next sentence of paragraph 5, about lost energy, not the meaning of the phrase.

To interpret a phrase like "this trade," find what "this" points back to in the sentences just before it.

Question 17

Changing the Voltage: How a Transformer Works

1Electricity leaving a power plant may travel hundreds of miles before it reaches a kitchen outlet. Along the way, its voltage is raised and lowered several times. The device that does this work is the transformer, one of the simplest and most important machines in the electric system. A transformer has no engine and, in most cases, no moving parts at all. It often sits quietly in a gray metal can on a utility pole.

2A basic transformer has two coils of insulated wire wrapped around a shared core of iron. The coil that receives power is the primary winding, and the coil that sends power out is the secondary winding. The two coils are not connected to each other by any wire. Instead, they are linked by magnetism. When current flows in the primary, it creates a magnetic field, and the iron core carries that field through the secondary coil.

3The key to the process is a discovery made by the English scientist Michael Faraday in 1831: a magnetic field that is changing can produce a voltage in a nearby coil. This effect is called electromagnetic induction. A steady field does nothing to the secondary coil, so a transformer works only with alternating current, which reverses direction many times each second and keeps the magnetic field constantly rising and falling. Fed with direct current, which flows steadily in one direction, a transformer would deliver no output once the current settled.

4The amount by which a transformer changes the voltage depends on the number of turns of wire in each coil. If the secondary has more turns than the primary, the voltage goes up; this is a step-up transformer. If the secondary has fewer turns, the voltage goes down; this is a step-down transformer. A transformer with ten times as many turns on its primary as on its secondary, for example, lowers the voltage to one-tenth. A transformer cannot create energy, however. When it raises the voltage, the current drops in about the same proportion, so the power coming out is never more than the power going in.

5Power companies use this trade between voltage and current to their advantage. Current flowing through a wire heats it, and that heat is wasted energy. By stepping the voltage up at the power plant, utilities can send the same power over long lines with much less current, and therefore much less loss. Near the customer, step-down transformers bring the voltage back down to a safe level for homes. Large transformers are filled with a special oil that both cools and insulates the coils, and a typical distribution transformer delivers about 98 to 99 percent of the energy it receives.

Which of the following is not true of a transformer, according to the passage?

Show answer & explanation

Answer: C

It usually has no moving parts.: True: paragraph 1 says a transformer has "in most cases, no moving parts at all."

It works only with alternating current.: True: paragraph 3 says "a transformer works only with alternating current."

It raises the voltage and current together.: Correct. Paragraph 4 contradicts this: "When it raises the voltage, the current drops in about the same proportion."

It may be filled with oil that keeps the coils cool.: True: paragraph 5 says large transformers are filled with oil that "both cools and insulates the coils."

On a "not true" item, check each option against the passage; the key is the one statement the passage contradicts.

Question 18

Changing the Voltage: How a Transformer Works

1Electricity leaving a power plant may travel hundreds of miles before it reaches a kitchen outlet. Along the way, its voltage is raised and lowered several times. The device that does this work is the transformer, one of the simplest and most important machines in the electric system. A transformer has no engine and, in most cases, no moving parts at all. It often sits quietly in a gray metal can on a utility pole.

2A basic transformer has two coils of insulated wire wrapped around a shared core of iron. The coil that receives power is the primary winding, and the coil that sends power out is the secondary winding. The two coils are not connected to each other by any wire. Instead, they are linked by magnetism. When current flows in the primary, it creates a magnetic field, and the iron core carries that field through the secondary coil.

3The key to the process is a discovery made by the English scientist Michael Faraday in 1831: a magnetic field that is changing can produce a voltage in a nearby coil. This effect is called electromagnetic induction. A steady field does nothing to the secondary coil, so a transformer works only with alternating current, which reverses direction many times each second and keeps the magnetic field constantly rising and falling. Fed with direct current, which flows steadily in one direction, a transformer would deliver no output once the current settled.

4The amount by which a transformer changes the voltage depends on the number of turns of wire in each coil. If the secondary has more turns than the primary, the voltage goes up; this is a step-up transformer. If the secondary has fewer turns, the voltage goes down; this is a step-down transformer. A transformer with ten times as many turns on its primary as on its secondary, for example, lowers the voltage to one-tenth. A transformer cannot create energy, however. When it raises the voltage, the current drops in about the same proportion, so the power coming out is never more than the power going in.

5Power companies use this trade between voltage and current to their advantage. Current flowing through a wire heats it, and that heat is wasted energy. By stepping the voltage up at the power plant, utilities can send the same power over long lines with much less current, and therefore much less loss. Near the customer, step-down transformers bring the voltage back down to a safe level for homes. Large transformers are filled with a special oil that both cools and insulates the coils, and a typical distribution transformer delivers about 98 to 99 percent of the energy it receives.

According to the passage, how are the primary and secondary windings connected?

Show answer & explanation

Answer: A

By a magnetic field in the iron core: Correct. Paragraph 2 says the coils "are linked by magnetism" and "the iron core carries that field through the secondary coil."

By a short wire that joins the two coils: The misconception the passage corrects: paragraph 2 says the coils "are not connected to each other by any wire."

By the oil that surrounds the coils: A detail from paragraph 5: the oil cools and insulates the coils; it does not link them.

By the outgoing line attached to both coils: The outgoing line is attached to the secondary winding only; it does not join the two coils.

For a stated-detail question, find the sentence that answers it directly; here, paragraph 2 says the coils are linked by magnetism, not by wire.

Question 19

The main purpose of this passage is to:

Changing the Voltage: How a Transformer Works

1Electricity leaving a power plant may travel hundreds of miles before it reaches a kitchen outlet. Along the way, its voltage is raised and lowered several times. The device that does this work is the transformer, one of the simplest and most important machines in the electric system. A transformer has no engine and, in most cases, no moving parts at all. It often sits quietly in a gray metal can on a utility pole.

2A basic transformer has two coils of insulated wire wrapped around a shared core of iron. The coil that receives power is the primary winding, and the coil that sends power out is the secondary winding. The two coils are not connected to each other by any wire. Instead, they are linked by magnetism. When current flows in the primary, it creates a magnetic field, and the iron core carries that field through the secondary coil.

3The key to the process is a discovery made by the English scientist Michael Faraday in 1831: a magnetic field that is changing can produce a voltage in a nearby coil. This effect is called electromagnetic induction. A steady field does nothing to the secondary coil, so a transformer works only with alternating current, which reverses direction many times each second and keeps the magnetic field constantly rising and falling. Fed with direct current, which flows steadily in one direction, a transformer would deliver no output once the current settled.

4The amount by which a transformer changes the voltage depends on the number of turns of wire in each coil. If the secondary has more turns than the primary, the voltage goes up; this is a step-up transformer. If the secondary has fewer turns, the voltage goes down; this is a step-down transformer. A transformer with ten times as many turns on its primary as on its secondary, for example, lowers the voltage to one-tenth. A transformer cannot create energy, however. When it raises the voltage, the current drops in about the same proportion, so the power coming out is never more than the power going in.

5Power companies use this trade between voltage and current to their advantage. Current flowing through a wire heats it, and that heat is wasted energy. By stepping the voltage up at the power plant, utilities can send the same power over long lines with much less current, and therefore much less loss. Near the customer, step-down transformers bring the voltage back down to a safe level for homes. Large transformers are filled with a special oil that both cools and insulates the coils, and a typical distribution transformer delivers about 98 to 99 percent of the energy it receives.

Show answer & explanation

Answer: C

describe how Faraday discovered electromagnetic induction in 1831: True but partial: Faraday appears only in the third paragraph, as background to the main topic.

warn readers about the dangers of using direct current in the home: Outside knowledge: the passage never discusses the dangers of direct current; it only says a transformer cannot use it.

explain how a transformer works and why utilities use it: Correct. Paragraphs 2–4 explain how a transformer works ("depends on the number of turns of wire in each coil"), and paragraph 5 shows why utilities rely on it.

compare the sizes of step-up and step-down transformers on poles: The passage defines both types in the fourth paragraph but never compares their sizes.

A main-purpose answer must cover the whole passage; an option that matches only one paragraph is true but partial.

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