Science 7–10 · Year 9
Heating water with an immersion heater: electrical energy in against thermal energy gained
Physical sciences — Energy, content groups Electrical energy and Law of conservation of energy (NSW Stage 5 focus area)
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.
The idea
The electrical energy supplied to a heater (V x I x t) sets the thermal energy the water can gain (m x c x change in T), and the shortfall measures energy transferred to the surroundings.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- heater element burns if switched on out of water
- hot water
- 6 A supply leads warming
Controls
- heater under water before switching on and until cool after
- element checked as fully covered at every water mass before switching on
- no glass thermometers
- supply off before handling
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
What you need
- 12 V 50 W immersion heater, 1
- 12 V DC power supply rated 6 A, 1
- polystyrene cup inside a 250 mL beaker, with lid, 1
- digital thermometer or probe, 1
- digital multimeters as ammeter and voltmeter, 2
- electronic balance, 1
- stopwatch, 1
- stirrer, 1
How to do it
- Weigh the empty cup, add about 200 g of room-temperature water and weigh again to find the mass of water.
- Fit the heater and thermometer through the lid so the heater element is fully covered. Record the starting temperature.
- Switch on, start the stopwatch, and record voltage and current every 30 seconds while stirring gently.
- After exactly 300 s switch off, keep stirring, and record the highest temperature reached.
- Calculate energy in (V x I x t) and energy gained by the water (m x 4186 x change in T). Express the gain as a percentage of the input.
- Repeat with 100 g of water and compare the temperature rise. Before switching on, check with the supply off that 100 g still covers the heater element completely; if it does not, use a narrower polystyrene cup in which it does, because the element must never run with any part out of the water.
What you should see
With 12.0 V and 4.0 A for 300 s the heater supplies 14400 J. For 200 g of water that would raise the temperature 17.2 degrees C if none escaped; the measured rise is smaller because some energy warms the heater body, the cup, the thermometer and the air, and the percentage calculated shows how much reached the water. Halving the water mass roughly doubles the rise. The learner knows it worked when the calculated gain is less than the input.
What changes
- What you change
- mass of water (kg) or heating time (s)
- What you measure
- temperature rise (degrees C)
- What you keep the same
- same heater, voltage and current
- lid on and stirring
- starting temperature near room temperature
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Temperature and thermal energy are the same thing.
- The heater makes energy rather than transferring it.
- Doubling the water halves the energy supplied.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-EGY-01SC5-WS-01SC5-WS-05SC5-WS-06
- Australian Curriculum v9AC9S9U05AC9S9I03AC9S9I04AC9S9I06
Sources
The pages the author read to write this activity.