Energy

Heating Effect

The heating effect of a current happens in all electrical devices. The current as it passes through the material of the various components generates some friction, this like any form of friction results in energy being transferred into heat energy.

The electrons that are moving in the conductor are doing work against the resistance to make their way through the material. This work creates the heating of the conductor.

In some situations the heating effect is an advantage, by using materials with high resistances and applying a large enough potential difference to produce a current it is possible to make the material heat up. This is used in conventional filament light bulbs and in electric heaters and cookers. The need for high voltages and high currents also means that these devices use a lot of energy to produce the heat.

However, in many situation the heat produced is a waste energy and has to be allowed to escape into the surroundings to keep the device cool.

The higher the resistance of the material and the higher the current flowing through the material the more heat will be produced.

Explaining the Heating Effect

Energy, figure 1

All conductors are made of a latticework of nuclei forming their molecular structure. For a current to flow the electrons at point A in the diagram must move through the lattice passed the nuclei to get to B and complete the circuit. The nuclei act as obstacles to this flow in two ways. Firstly they are a physical obstacle and secondly they are positively charged and the electrons are negative, therefore, there is an electrostatic force of attraction pulling the light electrons towards the larger nuclei. This results in the collision of the electrons with the nuclei of the lattice as they pass through it. Each collision results in some of the kinetic energy of the electrons being transferred to the nuclei, making the nuclei vibrate faster. The amount of vibration of the atoms or ions in a material is a measure of its heat. So as the level of vibration increases with each collision the overall temperature of the material will increase. This lose of energy as the electrons do work to move pasted the nuclei and through the lattice is the cause of the heating effect of an electrical current.

Reducing the Heating Effect

Due to the fact that the heating effect is a product of the collision between the material of the conductor and the passing electrons the way to reduce unwanted heat being created is to reduce these collisions. This can be achieved by:

  1. Using a low resistant material for the wires and components of the circuit.

This reduces the work done by the electrons as they flow through the material. As there are less collision, less energy is transferred to heat energy. This also makes the device more efficient and less prone to overheating.

  1. Reduce the current flowing in the circuit. This is not always possible, but if this can be achieved then the flow of electrons is reduced and so less work is done overall, thus limiting the heating effect of the current.

A good example of this is to use LEDs not bulbs for lighting, an LED has a very low resistance and requires very little current to work, this significantly reduces the heating effect in the circuits.

Energy, figure 1

Advantages and Disadvantages

There are numerous application for the heating effect of a current, situations in which the effect is an advantage include:

Cookers

Storage Heaters

Hot water immersion heaters

Electric radiators

Irons

Curling tongs

In all of these examples a high current is sent through a high resistance material to maximise the heating effect of the current.

Disadvantages

The production of excessive amounts of heat in a circuit is in most cases a disadvantage and can be dangerous.

Poorly maintained devices with the incorrect fuse can allow a high current to flow, this can lead to excessive heating and result in the device catching fire.

Most electrical components are small and easily damaged by excessive heat, they can melt, and even if they do not catch fire they will stop working, often this will mean the whole device will need replacing.

Computer chips are especially sensitive to being overheated, unfortunately they also generate a great deal of heat, as does the motor on the hard disk. Computer chips need active heat extraction by a fan to keep them cool enough to function. On a large scale, such as the servers needed to power the many social networks and cloud storage services, the heat is so extreme that companies have had to come up with ways to cool the hundreds of serves involved. One company has moved all their serves to a town above the arctic circle, and another is testing a system that places the serves at the bottom of the sea to cool the chips.

Energy, figure 1

Microsoft’s serves about to be lowered to the ocean floor to help keep them cool and deal with one of the disadvantages of the heating effect of a current.

Calculating Energy Transfer

Whenever an electrical circuit is in use energy is being transferred from the power sources, which might be a battery, a cell, a generator or the mains supply. The transfer might be to light and heat in a bulb or sound and light in a TV or movement of a motor. In all cases some heat will also be produced by the current and by any moving parts in the various devices in the circuit.

The energy is supplied to the electrons moving in the circuit to form the current by the potential difference (voltage) generated by the power supply. So the greater the voltage, the greater the current and thus more energy is transferred by the circuit, this combined with the length of time the current is flowing determines how much energy will be transferred.

This can be summarized in the equation;

Energy = Current x time x Voltage or E = ItV

It is important to recall that this is the energy supplied to the circuit not all of this will be used for a useful purpose, some will be wasted a heat energy by the heating effect of the current and in heat loss due to friction from any moving parts.

Example

How much energy is supplied to a computer over 30 minutes with a current of 2.3 A from a 9V supply?

Change time into seconds first: 30 minutes x 60s per minute = 1,800 s

E = ItV = 2.3 A x 1,800s x 9 V = 37,260 J

Explain why a wire will get warm if a current of 2A is flowing through it for 5 minutes?
Your answer should include: electrons / collision / nuclei / metal
Explanation: As the current flows in the wire the electrons are in collision with the nuclei of the metal in the wire. This results in the electrons transferring some of their energy to the nuclei and making the nuclei vibrate at a higher rate. This increases the heat energy of the wire, which is then emitted to the air, so over time the wire will gradually warm up. An effect known as the heating effect of a current.
State why the heating effect can be a disadvantage in a TV.
Your answer should include: energy / converted / waste / heat
Explanation: The heating effect in a TV causes energy to be converted into waste heat energy, this makes the TV less efficient and if the heat is not allowed to escape the TV could overheat and potential catch fire.
What voltage is required to supply 300 KJ of energy to a 5 A lamp over 20 minutes?
Your answer should include: 50V / 50
Explanation: Time in seconds = 20 minutes x 60s per minute = 1,200s E = ItVso V = E (It) = 300,000 (5 x 1.200) = 50 V