Electric Current


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Electric Current

  1. 1. Electric Current Electric current is defined as the rate at which charge flows through a surface (the cross section of a wire, for example). Despite referring to many different things, the word current is often used by itself instead of the longer, more formal "electric current". The adjective "electrical" is implied by the context of the situation being described. The phrase "current through a toaster" surely refers to the flow of electrons through the heating element and not the flow of slices of bread through the slots. As with all quantities defined as a rate, there are two ways to write the definition of electric current — average current for those who claim ignorance of calculus … I= Δq Δt and instantaneous current for those with no fear of calculus … I= lim Δq dq = Δt → 0 Δt dt The unit of current is the ampère [A]
  2. 2. Voltage Of a Cell The output voltage produced by a single cell, dry cell battery is the same for any size [AAA, AA, C, D, etc.], but the voltage does vary depending on the components of which the battery is made. For example a fresh/new Carbon-Zinc dry cell will produce one and a half [1.5] volts, while a Nickel-Cadmium [Ni-Cad] will produce one and a quarter [1.25] volts, and a Nickel-Metal Hydride [Ni-Mh] will produce one and two tenths [1.20] volts. These voltages are for a new, unused battery. As the battery is used, chemical reactions between the components cause the voltage output to decrease. However, "rechargable" batteries such as the Ni-Cad and the Ni-Mh batteries can be "recharged" for a limited number of "cycles" [a cycle is defined as one full discharge followed by one full recharge]. In these rechargable batteries, the discharging process causes chemical reactions between the components which can be reversed by the recharging process. There are limitations however, and eventually the battery components become used up to the point that recharging becomes less and less effective. Measuring Current, Voltage and Resistance
  3. 3. The most commonly used piece of equipment for electrical measurements is the multimeter, which is capable of measuring current (amps), voltage (volts) and resistance (ohms). There are two basic type of multimeter - analogue and digital. Each has advantages and disadvantages, depending upon the type of measurement being taken. The voltage across a component is a measure of the difference in electrical potential from one side of the component to the other and so the meter must be attached as shown: Current is a measure of the rate of flow of electrons through the circuit. To measure the flow of current, the circuit must be broken and the meter must be placed in the circuit such that the current flow goes through it. This is shown below:
  4. 4. In order to measure resistance, the component must first be removed from the circuit. This is to ensure that the other components in the circuit do not affect the reading. The meter probes are then connected either side of the component as shown: Ohm’s Law 1. Ohm's Law deals with the relationship between voltage and current in an ideal conductor. This relationship states that: The potential difference (voltage) across an ideal conductor is proportional to the current through it.
  5. 5. The constant of proportionality is called the "resistance", R. Ohm's Law is given by: V=IR where V is the potential difference between two points which include a resistance R. I is the current flowing through the resistance. For biological work, it is often preferable to use the conductance, g = 1/R; In this form Ohm's Law is: I=gV 2. Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference across it varies linearly with the current. 3. Ohm's Law can be used to solve simple circuits. A complete circuit is one which is a closed loop. It contains at least one source of voltage (thus providing an increase of potential energy), and at least one potential drop i.e., a place where potential energy decreases. The sum of the voltages around a complete circuit is zero. 4. An increase of potential energy in a circuit causes a charge to move from a lower to a higher potential (ie. voltage). Note the difference between potential energy and potential. Because of the electrostatic force, which tries to move a positive charge from a higher to a lower potential, there must be another 'force' to move charge from a lower potential to a higher inside the battery. This so-called force is called the electromotive force, or emf. The SI unit for the emf is a volt (and thus this is not really a force, despite its name). We will use a script E, the symbol , to represent the emf. A decrease of potential energy can occur by various means. For example, heat lost in a circuit due to some electrical resistance could be one source of energy drop. Because energy is conserved, the potential difference across an emf must be equal to the potential difference across the rest of the circuit. That is, Ohm's Law will be satisfied: =IR
  6. 6. Resistance Resistance is the property of a component which restricts the flow of electric current. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component. Resistance is measured in ohms; the symbol for ohm is an omega . 1 is quite small for electronics so resistances are often given in k and M . 1 k = 1000 1 M = 1000000 .  High resistance leads to Small current with low voltage and low voltage leads to large current with same voltage.  The wires connecting the components in a circuit have a low resistance while wires in filaments of the lamps have high resistance.  When they connect in series their resistances combine the same way as the voltages of the cells in series – They add. Lamps and current size  The size of the current flowing can be estimated by looking at the brightness of the lamps.  The lamp will shine brighter with more power and it will shine less when there is less power.  The lamp also shares it’s power with other components in the circuit Light-dependent resistor A light-dependent resistor (LDR) is made from 2 piece of metal which are jointed together by a semiconductor. Semiconductor is a material that has just a few electrons which can move freely. When the LDR receives light energy, more electrons are released in the semiconductor, and the resistance of the LDR becomes lower. When the amount of light shining on it reduced, fewer electrons are released and the resistance increases.
  7. 7. Parallel circuits  Lamps can be arranged in a circuit ‘side by side’ which is called a parallel circuit. The resistance of the lamps do not combine the same current.  It would receive the electrons as if it was the only component in the circuit . Diodes In electronics, a diode is a two-terminal electronic component with an asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p-n junction connected to two electrical terminals. A vacuum tube diode is a vacuum tube with two electrodes, a plate (anode) and heated cathode. Diodes are different and useful electrical components. Diodes are used in many applications like the following. Converting AC power from the 60Hz line into DC power for radios, televisions, telephone answering machines, computers, and many other electronic devices. Converting radio frequency signals into audible signals in radios.
  8. 8. LED • • • An LED is a semiconductor diode, allowing a current to flow in only one direction and it produces light. It can emit red, yellow and green light. In electronic circuits an LED performs the same task more efficiently. Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. They do dozens of different jobs and are found in all kinds of devices. Among other things, they form numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light. Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in a semiconductor material, and they last just as long as a standard transistor. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours. Tiny LEDs are already replacing the tubes that light up LCD HDTVs to make dramatically thinner televisions.