Science in Punggol study guide · Article 0007 of 20
To explain an energy change, identify the starting source or store, follow the conversion, and account for the outputs. Energy does not disappear when a device works or a moving object slows down.
A torch provides light, a fan moves air and a toy car rolls down a ramp. These examples become easier to explain when we follow energy through the whole process.
On this page: Find the starting source or store · Follow all the outputs · Decide which output is useful · Worked question
Find the starting source or store
A battery stores chemical potential energy. In a working circuit, this is converted into electrical energy supplied to the device. In a torch, electrical energy is converted into light and heat energy.
The battery does not create energy from nothing. Its stored energy changes as it operates. A torch also needs a working circuit; putting a cell beside a bulb is not enough to make it light.
A raised toy car has gravitational potential energy. When released down a ramp, this is converted into kinetic energy, which is energy of movement. The person who raised the car supplied energy earlier in the sequence.
Looking for that earlier step prevents an incomplete explanation. The car’s movement has an energy source even if no motor or battery is visible.
Follow all the outputs
A fan converts electrical energy into kinetic energy of its moving blades and air. It can also produce sound and heat. The most noticeable effect is not necessarily the only output.
A moving toy may make sounds as its parts vibrate. Friction can also produce heating. When the toy slows down, energy has been transferred and converted into other forms, including energy that spreads into the surroundings.
Use energy terms appropriate to the question and your current learning. Kinetic energy means energy of movement. “Heat energy” is a familiar primary Science term for an output associated with warming; more detailed thermal descriptions can be added later.
Decide which output is useful
An output is useful when it serves the intended purpose. Light is the main useful output of a reading lamp. Heat is useful in a heater. The same energy form can be wanted in one device and unwanted in another.
Calling an output “wasted” does not mean its energy has been destroyed. It means the output is less useful for the task. The surroundings are still part of the energy story.
A device may stop working when it no longer receives a suitable energy supply. That does not mean energy previously transferred has vanished. Follow where that energy went instead.
For a safe home observation, examine a familiar device from the outside without opening its casing. Draw the energy sequence, then add the effects you notice. Use the intended task to decide which output is useful.
Worked question
Question: In this original practice scenario, a battery-powered toy car moves across the floor. Its motor makes a sound and its casing becomes slightly warm. Describe the main energy conversion and identify the useful output for movement.
Worked explanation: Chemical potential energy in the battery is converted into electrical energy, then into kinetic energy of the moving car. Kinetic energy is useful for making the car travel. Sound and heat are additional outputs that do not serve that particular purpose.
Common mistake: Do not write that the remaining energy was “used up and disappeared”. Account for the other outputs. Also include the battery’s stored energy, rather than starting as though electrical energy appeared without a source.
