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| Source of Electricity |
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| Electrons in different metals can have different energies. If two of these metals are placed in a conducting liquid (electrolyte) a difference in electrical potential is set up between them. A pair of different metals arranged in this way is an electrochemical cell. It converts chemical energy to electrical energy. These cells are known as Primary Cells. |
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| Figure shows different sizes of dry cells which are widely used. They produce a potential difference of 1.5 V. |
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| Alkaline cells and lead acid cells are called secondary cells. These cells can be recharged and have a longer life than primary cells. To increase the potential difference cells are connected in series. Such an arrangement is called a battery. |
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| The current passing through a conductor for a particular voltage depends on the property of the conductor called "resistance". It is measured in ohms. A conductor with a high resistance 'resists' the current, and only a small current flows. A low resistance allows a higher current for a particular voltage. |
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| Figure above shows a lamp L in three different circuits. In figure (a) single bulb L is included in the circuit, in figure (b) two bulbs L1 and L2 are in series and in figure (c), the two bulbs L1 and L2 are in parallel. Let us compare the brightness of the bulbs L1 and L2 with L - (a) when they are in series, and (b) when they are in parallel. You will notice that the bulbs L1 and L2 in figure(c) will have equal brightness as the bulb L. When two or more resistors are arranged in parallel the total resistance is reduced, more current, therefore, more power and more brighter will be the bulb. |
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| When two or more resistors are connected in series total resistance increases because the resistors obstruct the flow of electrons. Hence less current, less power and less brightness. |
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| When the two same current flows through the two lamps L1 and L2, they are said to be connected in series. |
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| When the main current branches into two paths, one through L1 and another through L2, the lamps are connected in parallel. |
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| Potential difference is the condition which governs the direction of flow of electricity from one point to another. We can compare it with the heat flow from a body at a higher temperature to lower temperature. |
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| When a hot body comes in contact with a cold body there is a heat exchange. This flow of heat continues till both the bodies are at the same temperature. The difference between the temperatures of the two bodies determines the direction of the heat flow. |
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| We can also compare the flow of electricity with the flow of water from a higher to a lower level. Consider the two vessels A and B connected by a pipe. The water from the vessel A flows to B till the water levels in both the vessels are equal. Here the difference in the water levels is the condition which governs the direction of the flow of water. |
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| As said earlier, in the metals some of the electrons are not so tightly bound to the atoms. They drift in all directions. The free electrons in a wire XY will not flow in a particular direction unless they are forced to do so. |
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| An external force is required to convert this irregular drift into a flow in one direction. A battery connected across the wire XY provides this force. This force causes the electron to flow in one direction and thus perform work. The work per unit charge in moving an electron once around the circuit is called the electromotive force (emf). A circuit is a closed path in which an electrical current flows. The emf creates an electrical pressure difference across XY. An electric current will then flow from the positive terminal to the battery of the negative terminal (higher potential to lower potential). Note that the free electrons flow in the direction opposite to the conventional current. |
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| For electrons to be driven through a resistance, there has to be a potential difference across its ends. Work is done by the electrical force due to this potential difference. |
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| So potential difference (p.d.) across a resistance is the work done in moving a coulomb of charge through it. |
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| where V = p.d. |
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| W = work done against the resistance |
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| q = charge |
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| If you increase the potential difference across your source of electrical energy by adding an extra cell, then the electrical force pushing the electrons around the circuit also increases. This will increase the rate of flow of charge i.e. current in the circuit. Georg Ohm investigated this relationship between current and voltage, and found that under constant physical conditions the current flowing through a metallic conductor is proportional to the potential difference. |
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| The ratio of V to I is a measure of the electrical resistance of the conductor. |
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| Coulomb |
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| A coulomb is the quantity of electricity which passes a given section of a conductor in one second which carries a current of one ampere (ampere is the unit of electric current). |
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| Ampere |
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| It is the amount of charge passing a given point in one second and is measured by an instrument called ammeter. |
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| Potential difference |
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| Two points are at a potential difference of one volt if one joule of work is done per coulomb of electricity passing between the points. The unit of potential difference is volt. |
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| Electromotive force |
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| It is the work done per coulomb in moving a test charge once around the circuit or it is the potential difference between the two poles of a cell in an open circuit. |
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| The unit of emf is volt. |
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| Resistance |
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It is the ratio of potential difference between the ends of a conductor to the current flowing through it. It is measured in ohms (symbol:  ). |
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| Circuit |
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| An electric circuit is a compete or closed path through which the charge can flow. When the switch is open or the circuit is broken, it is called the open circuit. Electric current will not flow through the open circuit. |
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