If you are passing under a high voltage line you may notice a slight hiss. This is because the high voltages are ionising the air around them.
Electricity and Death The hazards of electricity see table below depend on the amount of electrical current, its frequency, its duration, its path through the body, and the physical condition of the person.
Alternating current at 60 Hz is slightly more dangerous than direct current, but high frequency currents greater than a few kHz are safer because they tend to flow on the surface of the skin and away from the heart and lungs. The resistance of the body varies from about ohms to about , ohms, and thus even very low voltages can produce lethal shocks. Fatalities have occurred at voltages as low as 24 volts. For currents that exceed the "let-go" current mA the person becomes frozen to the circuit, and the current typically rises to a level of about 25 mA where muscular contractions onset.
Then the person is either thrown clear of the circuit or the current continues to rise until ventricular fibrillation or cardiac arrest occurs at mA , assuming the path of the current is through the heart.
Pulsed currents such as one might encounter with the discharge of a Van de Graaff generator or other charged capacitance present special considerations. One can endure currents that would otherwise be lethal if the duration is short enough. For pulses of less than a few seconds duration, the relevant quantity is the square of the current integrated over the time of the pulse. Values of I2t greater than about 0.
For a reasonably low body resistance of ohms, this translates into an energy of about 10 joules. Severe shocks can occur at levels times lower and can startle one into an accident, since it is natural to jerk away from such a shock. Good practice entails standing on an insulated surface and keeping one hand in a pocket or behind one's back while working around high voltage.
In performing electrical demonstrations, one should ideally have a knowledgeable assistant trained in cardiopulmonary resuscitation CPR always present. The probability of resuscitating someone is good if CPR is begun within three minutes but becomes poor after about six minutes. Ans: The voltage is a trade off issue; V is more likely to electrocute, but the lower current means that fewer people die from fires caused by overheated cables.
Frequency issue is more technical as far as I can remember. However they should also be using heavy-duty cable i. Before carrying out the experiment you should try Question 2, page of the workbook part a only. You should also be in a position to answer the questions at the bottom of page of the textbook. Mnemonics :. Hans Oersted discovered the magnetic effect of an electric current in while demonstrating electricity to his students. Describe how you would demonstrate the magnetic effect of an electric current.
What is the resistance of the filament of a light bulb, rated 40 W, when it is connected to the mains? What is the colour of the wire that should be connected to the fuse in a plug? Give the standard colour of the insulation on the wires connected to each of the terminals L, N and E on the plug in the diagram. Name and give the colour of the wire that should be connected to the fuse in a standard three-pin plug.
Plugs are used to connect electrical appliances in the home to the volt ESB supply. A coffee maker has a power rating of W. What is the most suitable fuse to use in the plug of the coffee maker? Calculate the current that flows when the kettle is first plugged in. The fuse in the plug of the kettle in the previous question was replaced with a 5 A fuse This current will only flow for a very short time.
Explain why. Some electrical appliances are supplied with two-pin plugs. Why is an earth wire not required in these devices? Bonding is a safety precaution used in domestic electric circuits.
How does bonding improve safety in the home? Draw a graph to show the relationship between current and voltage for a metal at constant temperature. Electric current. Electric circuit diagram. Types of current.
Safe handling of electrical energy. Magnetism and Electromagnetism. Magnetic field B. Magnetic Field Lines. Devices which exploit the magnetic effects of current can cause interference to sensitive electronic equipment such as television receivers, radio receivers, and electronic heart pacemakers.
This interference may occurs due to the magnetic field itself, the high transient currents that occur when these devices start or stop, and by the electromagnetic radiation caused by these abrupt changes. The chemical effect of current occurs when passing a current through an electrolyte. An electrolyte is a fluid containing ions, which may be water with other chemicals dissolved in it, or perhaps a molten salt.
Current flow in an electrolyte consists of a transfer of charge by ions, and results in chemical change. These chemical changes are utilised in:.
When two metal plates or rods are inserted into an electrolyte and a battery or dc supply connected across the metals as shown in Figure 1 then the process of electrolysis takes place. The positive metallic ions are attracted to the negative plate and negative non-metallic ions are attracted to the positive plate.
Various chemical processes will occur at the positive and negative plates. In electroplating or aluminium refining, the positive metal ions in solution are attracted to the negative plate the cathode , and combine with electrons to become solid metal.
In the electrolysis of water, hydrogen ions combine with electrons to become hydrogen gas, and meanwhile oxygen is produced at the positive electrode the anode. Figure 1: The process of electrolysis The process of electrolysis is put to beneficial use in a number of ways discussed next. Electroplating is the process of coating a metal with a thin layer of a different metal. It is used for:. Refining copper ore is mainly done using the electrolytic process.
Impure copper is connected to the positive electrode and the negative electrode consists of an initially thin piece of pure copper. Copper ions move through the solution and deposit on the negative plate as pure copper. Any impurities fall to the bottom of the tank. Aluminium is also refined in a similar manner, but in a molten electrolyte called cryolite. The process of electrolysis can cause corrosion in a number of ways discussed below. The processes that take place in the corrosion of metals are similar to those in an electrolytic cell.
The electrolyte may be salt water or even rain water with small amounts of dissolved acids or salts. The electrodes may be different metals, or even different crystals in the same piece of metal. Where two dissimilar metals are joined and a drop of moisture is in contact with both metals, a miniature battery is formed which causes a small localised circulating current which in turn causes corrosion. Examples of this are:. Corrosion can be made worse by electric current flowing in the structure.
Electric trams and trains use their rails as one part of the circuit that supplies them with current to drive their motors. Any joints in the rail that also allow ingress of air can form an electrolytic cell, and so a point of corrosion. Another example is the terminals batteries. You may have noticed the corrosion forming at the terminals of batteries in cars, flashlights, and in fact any piece of equipment that uses batteries.
Corrosion can also be prevented by applying an electric current in the opposite direction to that which would cause corrosion. The corrosion is less likely to happen inside a building. Explain why. There are three common ways in which an electric current passing through a material can cause the emission of visible light. When a solid conductor is heated to a high temperature by the passage of an electric current it produces light as well as heat.
An example of this is the common household light bulb. This type of lamp has a thin tungsten filament through which a current is passed, causing it to glow and emit a white-yellow light. This type of lighting is called incandescent lighting. When gases are exposed to a large electric field, electrons are stripped from some of the atoms, and ions are formed.
The electrons and ions are accelerated in the field, bumping into and exciting more atoms. The gas becomes conductive. In this state the electrons orbiting the atoms of gas are excited into higher energy levels. When the electron returns to its normal state, it emits a photon of light energy with energy specific to the gas being used. In some gases this wavelength corresponds to visible light.
For example, neon lights are red, and sodium lights are yellow. The two effects of electric current are: Heating effect of electric current. Magnetic effect of electric current. The electric circuit is a closed loop in which electrical components form a network for flowing the electrons. It has a point of source such as a battery from where the electrons start to flow whereas the point where electrons leave the electrical circuit is called the return.
When an electric current passes through a conductor like a high resistance wire the conductor becomes hot after some time and produces heat. This is called heating effect of Electric Current. This is because of heating effect of electric current. A fuse is a safety device which limits current in circuit as a result it prevents damages to electrical circuits and possible fires.
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