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Heating Effect of an Electric Current - Physics Form 3 Coursework e-Content CDs

Background

In our day to day life we interact with electrical devices such as iron boxes, bulbs, immersion heater, electric cookers and many more. These devices produce heat when supplied with an electric current. In this topic, we will discuss how the heating effect is caused by an electric current.

Objectives

By the end of the topic, you should be able to:

  • Perform and describe experiments illustrating heating effect of an electric current
  • State the factors affecting heating effect of an electric current
  • Derive equations for electrical energy and electrical power
  • Identify devices in which heating effect of an electric current is applied








Introduction

Energy exists in various forms such as mechanical energy, heat energy, chemical energy, electrical energy, light energy and nuclear energy. According to the law of conservation of energy, energy can neither be created nor destroyed. It is only transformed from one form to another. Identify energy transformations in the animation. The transformations are:


  • Gravitational Potential Energy transforms to Mechanical Energy when rushing water turns the turbine, and
  • Mechanical Energy is transformed to Electrical Energy by the excitation of electrons within magnets in the turbine shaft.

Demonstration on the heating effect of an electric current

The following activity is a demonstration on the heating effect of an electric current.

Activity: A demonstration using steel wool

Click the play button and observe what happens.


Observation

When current flows through the steel wool it burns off after a short while. When an electric current is passed through a metallic wire the wire gets heated up and in this case, electrical energy is converted into heat energy. This is known as the 'heating effect of current'. When an electric current is passed through the steel wool, it gets heated up. In this case, electrical energy is converted into heat energy. This is known as the heating effect of an electric current.

Conclusion

When current flows through a conductor it causes a heating effect.

Factors affecting the amount of heat produced

In this sub-topic we will investigate on the factors affecting the amount of heat produced. To study the factors we are going to use two coils (a short one and a long one), an ammeter, and water in a container, a power supply, a rheostat, connecting leads, thermometer, stop clock and a switch.

Amount of current

In this set up, we are going to investigate the heating effect with different amounts of current. Note the initial temperatures in both set ups. Click on the play button and observe temperature changes.

Observation

The final temperature in the set up where we have higher current is higher than in the set up with lower current.

Time taken

In this set up, we shall investigate how the time taken while heating affects the heating effect of an electric current. Click the play button and observe what happens as the current flows for 5 minutes in the first set up and 10 minutes in the second set up.

Observation

The final temperature is higher when it takes a longer time.

Explanation

The heating effect of an electric current depends on the time taken while heating.

Discussion

The higher the current, the higher the rise in temperature and therefore the higher the heating effect.

Conclusion

The heating effect of an electric current depends on the amount of current.

Resistance

In this set up, we shall investigate the effect of resistance on the heating effect of an electric current. To do this, we are going to use two coils of different lengths but equal thickness. Click on the play button and make your observations.

Observation

The thermometer reading in the set up with higher turns is higher than in the coil with less turns

Discussion

Resistance depends on the length of the conductor. The coil with more turns has higher resistance hence causing a higher heating effect.

Applications of heating effect of an electric current

The heating effect of an electric current is made use of in many applications such as electric heater, electric toaster, electric kettle, soldering iron, electric iron among others. In this sub-topic we will discuss the working of filament bulb, electric kettle and electric iron box.

Bulbs

A light bulb consists of an evacuated glass container, with conducting supports to hold a coil of fine tungsten wire. As the current passes through the filament, it reaches very high temperatures and emits energy in the form of light. Tungsten is chosen as the metal for the filament because it has a melting point of about 34000C. The filament is in a vacuum in order to prevent oxidation of the metal, which would simply burn at the high operating temperature, if air were to be present in the bulb. These bulbs are less efficient compared to energy saving bulbs as most of the electrical energy is converted into heat.

Electric kettles

Electric kettles are normally constructed out of durable plastic or steel with an insulated handle and powered by means of mains electricity. Click on the play button to see the photographs of different electric kettles.

 

In modern designs, once the water has reached boiling, the kettle automatically switches off to prevent the water boiling away and damaging the heating element. "Cordless" kettles, sometimes called "jettles", became popular in the late 1980s and 1990s. These kettles consist of a plastic base that connects to the mains outlet and a separate kettle which is portable.

Electric iron

An electric iron used for ironing clothes consists of a coil of high resistance covered by insulating mica sheets. When an electric current flows through the coil, it gets heated. Click on the play button to see different types of electric iron boxes.

Fuse

An important application of heating effect of electric current is a safety device known as "fuse". A fuse is usually made up of alloys of lead and tin. It has very low melting point. It melts with small rise in temperature. It melts when a current passing through it exceeds the rated value.

Explanation

In case of excess current, the fuse wire gets heated up and melts breaking the circuit. This protects the appliance connected via it from getting spoilt.

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