Energy consumption in daily life: Practice Questions

An LED bulb is rated at \(10 \text{ W}\) and has a luminous flux of \(850 \text{ lm}\). Calculate the efficacy of this light bulb.

A point light source is placed directly above a desk. If the illuminance at a point on the desk \(2 \text{ m}\) below the source is \(400 \text{ lux}\), what will the illuminance be if the source is moved to a distance of \(4 \text{ m}\) from the same point?

A country relies on a mix of energy sources for electricity generation: $$60\%$$ from coal-fired power plants (CO2 emission factor: $$0.9 \text{ kg CO}_2 \text{/kWh}$$), $$30\%$$ from natural gas power plants (CO2 emission factor: $$0.4 \text{ kg CO}_2 \text{/kWh}$$), and $$10\%$$ from hydropower (negligible CO2 emissions). If the country's annual electricity consumption is $$1000 \text{ GWh}$$, what is the total annual CO2 emissions from electricity generation?

An electric hotplate with a power of \(2000 \text{ W}\) is used to heat soup for \(5\) minutes. If the end-use energy efficiency of the hotplate is \(65\%\), how much useful thermal energy is transferred to the soup?

Which of the following is the SI unit for luminous flux?

An induction cooker with a power rating of $$2.0 \text{ kW}$$ takes $$8 \text{ minutes}$$ to boil a certain amount of water. An electric hotplate with a power rating of $$1.5 \text{ kW}$$ takes $$15 \text{ minutes}$$ to perform the same task. Which device consumes less electrical energy for this task, and by how much?

A $$10 \text{ W}$$ LED light bulb provides the same brightness as a $$60 \text{ W}$$ incandescent bulb. If a household replaces ten $$60 \text{ W}$$ incandescent bulbs with ten $$10 \text{ W}$$ LED bulbs, and each bulb is used for $$4$$ hours a day, how much energy is saved daily in total?

An electric heater with a power rating of $$1.2 \text{ kW}$$ is used to heat a room. If the heater has an efficiency of $$90\%$$ and operates for 1 hour, what is the increase in the internal energy of the air in the room? Assume all useful energy goes into heating the air.

A household uses an electric fan with a power rating of \(60 \text{ W}\). If the fan is operated for \(8\) hours a day, calculate the electrical energy consumed by the fan in one week in kilowatt-hours (kWh).

A commercial building in Hong Kong is undergoing an energy audit. The building's air conditioning system consumes an average of $$500\text{ kWh}$$ per day, and its lighting system consumes $$150\text{ kWh}$$ per day.

(a) Calculate the total annual electricity consumption of these two systems in MWh, assuming the building operates for $$365$$ days a year.

(b) If the electricity for the building is generated by a power plant with an overall efficiency of $$40\%$$ using natural gas (calorific value $$= 50\text{ MJ kg}^{-1}$$), what is the total mass of natural gas consumed annually to power these two systems?

(c) The building management decides to upgrade to more efficient LED lighting, reducing lighting consumption by $$60\%$$. Calculate the new total annual electricity consumption (air conditioning + new lighting) in MWh.

(d) Briefly explain one environmental benefit of reducing electricity consumption in buildings.