Electricity: Magnetic and Heating Effects · Lesson 4 of 7
Heating Effect of Electric Current
“Follow the path from resistance in a wire to useful heating in appliances and unwanted overheating in circuits.”
• Explain why a current-carrying conductor can become warm. • Define resistance and the heating effect of electric current. • Identify the factors that affect how much heat is produced in a wire. • Explain how heating elements are used in common electrical appliances. • Distinguish useful electrical heating from unwanted overheating and related hazards.
When Electric Current Produces Heat
The same current that creates a magnetic field can also warm a conductor. This can be tested with a short length of nichrome wire connected to a cell through a switch. Before the circuit is closed, the wire is at its ordinary temperature. After current flows for a short time, the wire feels warmer.
The activity uses nichrome because it offers more resistance to current than a similar copper wire. The warming is evidence that some electrical energy is being converted into heat energy while current flows.
Resistance and the Heating Effect
Electric current does not flow equally easily through every conductor. A conductor offers some opposition to the flow of current. Different materials can offer different amounts of this opposition, which is why wires made from different materials can behave differently in the same kind of circuit.
Resistance is the opposition a conductor offers to the flow of electric current.
The production of heat in a conductor because electric current flows through it is called the heating effect of electric current.
In the nichrome experiment, resistance causes some electrical energy to be converted into heat. That is why the wire warms when current passes through it.
What Affects the Heating?
The amount of heat produced is not always the same. The chapter identifies several factors: the material of the wire, its thickness, its length, the magnitude of the current, and the duration for which current flows. In the activity, using two cells instead of one produces more heating over the same duration because the current is larger.
| Factor investigated | How to investigate it fairly |
|---|---|
| Amount of current | Use the same wire for the same time, but compare one cell with more cells |
| Material | Compare conductors while keeping the other conditions as similar as possible |
| Thickness | Compare nichrome wires of equal length but different thickness |
| Length | Compare nichrome wires of the same thickness but different lengths |
| Duration | Use the same setup but allow current to flow for different lengths of time |
The source activity says the wire should be touched only momentarily after the current is switched OFF and should not be held. Such activities should be carried out under teacher supervision.
Heating Elements in Appliances
Electrical heating is useful when an appliance is designed to make heat. Room heaters, electric stoves, kettles, irons, immersion rods and hair dryers all use the heating effect of current. These devices contain a rod or coil of wire called a heating element. In some appliances, the element can become hot enough to glow red.
A heating element is the part of an electrical appliance designed to become hot when electric current flows through it.
Problem
The same nichrome wire is connected first to one cell and then to two cells, with current flowing for the same time in each case. The wire becomes warmer with two cells. What does this show?
- 1.The wire material, length and thickness have not changed.
- 2.The source with more cells provides a larger current in the chapter's comparison.
- 3.The greater heating shows that the heat produced depends on the magnitude of the electric current.
Useful Heating and Unwanted Overheating
Heating is useful inside appliances designed to produce heat, but it can be undesirable elsewhere. Electrical energy can be lost as heat in wires during transmission. Excessive heating can damage plugs and sockets, melt plastic parts, and contribute to fires.
To reduce unnecessary heating in household connections, wires, plugs and sockets should be suitable for the current they are expected to carry. The heating effect also has industrial uses: high-temperature electric furnaces can use electric current to melt scrap steel so that it can be recycled into usable steel.
Quiz
What is resistance?
Why does nichrome wire become warm when current flows?
Which change produced more heating in the source comparison?
Which item is an example of a device that intentionally uses the heating effect?
Which is an example of unwanted heating?
Practice Problems
- Explain the chain from resistance to heating in a current-carrying nichrome wire.
- List the five factors identified in the chapter that can affect heat production in a wire.
- Why must a fair test of wire thickness keep other conditions as similar as possible?
- Choose two household appliances from the lesson and identify the role of their heating element.
- Explain why suitable current-rated wires, plugs and sockets help reduce the risk of unwanted heating.
- Compare an electric heater or stove with heating by burning firewood or charcoal. Give reasons why electrical heating may be more convenient, and consider possible effects on people or surroundings.
Key Takeaways
• Electric current can produce heat in a conductor. • Resistance is opposition to the flow of current, and electrical energy can be converted into heat because of it. • Heating depends on current as well as the material, thickness, length and heating time of the wire. • Heating elements use this effect deliberately in many appliances. • Unwanted overheating can waste energy, damage equipment and create safety hazards.