Chapter 12 Quiz

Part A: Multiple Choice

Select the best answer for each question.

1. A circuit has a voltage of 120 volts and a resistance of 8 ohms. According to Ohm's Law, what is the current flowing through the circuit?

a) 960 amps b) 15 amps c) 8 amps d) 128 amps

2. In the water analogy for electricity, which physical quantity is best represented by the flow rate of water through a pipe?

a) Voltage b) Resistance c) Power d) Current

3. Why does household electricity use alternating current (AC) rather than direct current (DC)?

a) AC is safer than DC at residential voltages b) AC can be efficiently transformed to higher voltages for long-distance transmission c) DC cannot power motors or appliances effectively d) AC is less expensive to generate

4. In the United States, household AC current completes how many full cycles per second?

a) 50 Hz b) 120 Hz c) 60 Hz d) 240 Hz

5. A standard 240-volt appliance circuit in a home uses:

a) Two separate single-phase services from different utility transformers b) Both hot legs of the split-phase service, measured from Leg A to Leg B c) A higher-voltage transformer installed at the electrical panel d) The neutral conductor charged to 240 volts

6. Your main circuit breaker is rated at 200 amps. You turn it off. Which of the following is true?

a) All power in the home is completely de-energized, including the meter and service entrance conductors b) The service entrance conductors above the main breaker remain live and energized c) The panel is now safe to open and work in d) The utility automatically disconnects power from the transformer

7. What is the primary purpose of a grounding electrode system (ground rods)?

a) To provide a return path for current under normal operating conditions b) To reduce monthly electricity consumption c) To provide a fault current path to earth and enable breakers to trip d) To equalize voltage between the two hot legs

8. What is the difference between grounding and bonding?

a) Grounding applies to metal water pipes; bonding applies to electrical wires b) Grounding connects the electrical system to earth; bonding connects metallic systems together at the same potential c) Bonding is required only in new construction; grounding applies to all homes d) There is no meaningful difference; the terms are interchangeable

9. You receive an electric bill showing 1,200 kWh consumed over 30 days. Your rate is $0.15 per kWh. What is the energy charge portion of your bill?

a) $180 b) $18 c) $1,800 d) $120

10. A device draws 0.5 amps at 120 volts. What is its power consumption?

a) 240 watts b) 60 watts c) 120 watts d) 600 watts

Part B: Short Answer

Answer each question in 3–5 sentences.

11. Explain why a short circuit is more immediately dangerous than an overload, in terms of the physical mechanisms involved. Why does a circuit breaker respond differently to each?

12. You are looking at a home's electrical panel from the outside and see that two wires enter the top of the panel from the utility. You turn off the main 200-amp breaker. Are those two wires now safe to touch? Explain why or why not.

13. A homeowner wants to plug a 1,500-watt electric space heater and a 1,200-watt hair dryer into two outlets in the same bedroom. Both outlets are on a 15-amp circuit. Using P = V × I, explain whether this is safe and what will likely happen if they try it.

14. Describe what a GFCI outlet detects and why it provides protection that a standard circuit breaker cannot. In your answer, explain what types of locations require GFCI protection and why.

15. Your electric bill shows 950 kWh consumed in January and 1,850 kWh in July. You have central air conditioning and gas heating. Explain what accounts for this difference, and describe one strategy available to you to reduce your summer bill without reducing comfort.

Answer Key

Part A

  1. b) 15 amps — V = I × R; I = V ÷ R = 120 ÷ 8 = 15 amps
  2. d) Current — Current is the flow of electrons, analogous to the flow rate (gallons per minute) of water through a pipe
  3. b) AC can be efficiently transformed to higher voltages — Transformers only work with AC; high-voltage transmission dramatically reduces losses over distance
  4. c) 60 Hz — US standard is 60 Hz (Europe and much of the world uses 50 Hz)
  5. b) Both hot legs of the split-phase service — The two hot legs are 180 degrees out of phase; measuring from Leg A to Leg B gives 240 volts
  6. b) The service entrance conductors above the main breaker remain live — The main breaker disconnects branch circuits but cannot de-energize the conductors feeding it from the meter
  7. c) To provide a fault current path to earth and enable breakers to trip — The grounding electrode system is the safety path, not the normal return path (that's the neutral)
  8. b) Grounding connects the electrical system to earth; bonding connects metallic systems together — Grounding and bonding are distinct: grounding provides the earth path; bonding equalizes potential between metal systems
  9. **a) $180** — 1,200 kWh × $0.15 = $180
  10. b) 60 watts — P = V × I = 120 × 0.5 = 60 watts

Part B (Model Answers)

11. A short circuit creates a near-zero resistance path, so by Ohm's Law (V = I × R), with voltage held constant and resistance near zero, current surges to extremely high levels almost instantly. This triggers the breaker's magnetic mechanism, which uses an electromagnet to trip within milliseconds. An overload is a more gradual condition — current slightly exceeds the rated amperage, heating the bimetallic strip in the breaker over time until the strip deforms and trips the mechanism. The thermal response takes seconds to minutes; the magnetic response takes milliseconds.

12. No, those wires are not safe to touch. The main breaker disconnects power from the hot busbars and branch circuits within the panel, but it cannot interrupt the service entrance conductors between the meter and the main breaker. These conductors remain energized at 240 volts (120V on each leg to neutral) regardless of the main breaker position. Only the utility company, by disconnecting at the meter or cutting power at the transformer, can de-energize those conductors. This is why working inside the top of the main panel — even with the main breaker off — requires a utility disconnect.

13. The space heater draws 1,500W ÷ 120V = 12.5 amps. The hair dryer draws 1,200W ÷ 120V = 10 amps. Together they draw 22.5 amps. A 15-amp circuit is rated for continuous loads of up to 15 amps (and by the 80% rule, sustained loads should not exceed 12 amps). Running both devices simultaneously significantly overloads the 15-amp circuit. The circuit breaker will trip — probably within a few minutes, perhaps sooner. This is actually the breaker working correctly: it's preventing the wiring from overheating. The solution is to plug one device into an outlet on a different circuit, or to not run both simultaneously.

14. A GFCI outlet monitors the current balance between the hot and neutral conductors. Under normal conditions, current flowing out on the hot wire returns on the neutral, and they are exactly equal. If even 5 milliamps of current is flowing to ground — such as through a person who has touched a live conductor — there's an imbalance the GFCI detects. It trips in approximately 1/40th of a second, fast enough to prevent electrocution in most circumstances. A standard circuit breaker protects against overcurrent (too many amps), but a person being electrocuted typically only draws a fraction of an amp — far too little to trip a breaker. GFCI protection is required in bathrooms, kitchens near sinks, garages, outdoors, unfinished basements, and other wet or damp locations where shock risks are elevated.

15. The difference is almost entirely attributable to central air conditioning. Gas heating doesn't appear on the electric bill (except the blower motor, which is a much smaller draw). Central air conditioning in summer is typically the single largest residential electrical load in warm climates — a 3-ton unit can draw 3,600–5,000 watts when running, and in a hot July it may run for 8–12 hours per day, consuming 400–600 kWh per month for AC alone. Strategies to reduce summer cooling bills include: raising the thermostat set point by 2–3 degrees (significant savings), using a programmable or smart thermostat to reduce cooling when the house is unoccupied, ensuring the AC system's filter is clean (a dirty filter reduces efficiency), having the refrigerant charge and coils serviced, adding shade to south-facing windows, and sealing air leaks in the building envelope. Some utilities also offer time-of-use rates where shifting AC demand to off-peak hours reduces costs.