09 Electric Current 2009

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ELECTRIC CURRENT How can we profit electric energy? As we have seen in the previous chapter, when charges are separated, they try to get together again. But electric forces are too strong and they collapse in a fraction of a second (provided there is a path along which they can move). If this happens all of a sudden, huge amounts of energy are released and an explosion occurs: lightning are a good example. (In fact, sparks are tiny little lightnings) No benefit can be obtained this way: The problem was solved when Alessandro Volta, following Galvani’s investigations, invented the electrochemical cell (battery). We can define it as a device that changes chemical energy into electrical energy Electrochemical Cells In the electrochemical cell a redox reaction produces a voltage between the output terminals of the cell (called the poles). Redox is used here in the wider sense (that is, electron transfer). If A and B are two substances that react in a redox process and they are mixed in a test tube, the reaction occurs instantaneously and energy is released as heat (thermal energy). In an electrochemical cell the electrons are not transferred directly from particle to particle because both reactants are kept in different chambers! One of them (say A) is craving for the other’s electrons; the other one (say B) is ready to give them to A. If we connect these two chambers by means of a conducting path (a metal wire will be OK), the electrons will move from one chamber to the other one establishing an electric current. As they do so, they go from a more energetic to a less energetic state and the energy released can be changed in different ways: movement energy (an electric fan), light energy (bulbs), sound energy (radio), etc. In the diagram there is a section (shaded grey) “sb” usually called a salt bridge. This is a paste containing salts (ionic compounds). As the electrons move from B to A, cations are formed at B and anions at A. In order to keep the electric balance in both compartments, “sb” provides the necessary ions. As you can appreciate, a battery has no net charge at all. There is not such thing as a positive charge at the (+) pole or a negative charge at the bottom. (Try putting some small papers close to the ends of the battery: are they attracted to them as it happened with a rubbed ruler?). The (+) and (-) symbols stand for higher voltage and lower voltage ends. Voltages generated by cells have historically been referred to as emf (electromotive force) because they provide the force that keeps the electrons moving.

2 Electric Circuits Electric circuits are closed conducting paths to let charges flow from high to low energy levels. The energy lost by the charges along it is changed into another useful form of energy (heat, light, sound, chemical, kinetic, magnetic etc.). The source of electric energy (a battery, cell or electromagnetic generator) provides the emf (electromotive force). Metallic wires (usually copper wires) form the loops or paths along which the charges (electrons most times) move and “loads” will be the apparatuses (bulbs, TV set, hair drier, etc.) that use the energy released by the moving charges. A switch opens and closes the circuit to allow or cut the charges and energy flows. Measuring devices as ammeters and voltmeters are frequently attached to circuits. The figure shows a simple electric circuit charged with two bulbs. To better understand how a circuit is made, we use symbols for the different elements in it. Circuit Diagrams: Symbols

The following circuit represents the drawing at the bottom of page -1-.

Electric Current Electric current is the rate of charge flow past a given point in an electric circuit. It is measured in coulombs/second. This unit has a special name: ampere (A). Current (A) = Charges (C) / Time (s) Ordinary matter is made up of atoms which have positively charged nuclei and negatively charged electrons surrounding them. Now, which of the charges move? For electric current in a copper wire, the charge carriers are the mobile electrons in the conductivity band of the metal: The positively charged copper ions are essentially stationary in the metal lattice. Nevertheless, treatments of electric circuits usually use conventional current, as if positive charges were moving. Debate continues about this practice, but the physical nature of the charge carriers in copper is fairly straightforward. In other applications of electric current however, the identification of the charge carriers is not so simple. In many substances, electric conduction is not just free electron

3 movement: in conducting solutions (electrolytes) the carriers are cations and anions (positively and negatively charged particles) at the same time. Resistance Although conductors let the charges flow there is some reluctance to this movement. As electrons move we can imagine them knocking against the atoms’ lattice. As they bump and bump, atoms move and vibrate faster increasing the temperature of the conductor taking some energy from the moving charges. Electrons are accelerated by the electric forces and stopped by the “friction” against the lattice. Finally they get to a steady state in which the current flows evenly (this happens in less than millionths of a second). The value of this current depends on the voltage (energy) supplied by the source and the resistance (reluctance) of the conductor. The electrical resistance of a circuit component or device is defined as the ratio of the voltage applied to the electric current which flows through it. It is measured in units called Ohms (Ω). 1 Ω = 1V / 1 A. An element has a resistance of one Ohm if a 1 Ampere current flows through it when the voltage applied is just one Volt. Resistance (Ω) = Voltage (V) / Current (A) Ohm's Law For many conductors of electricity, the electric current which flows across them is directly proportional to the voltage applied to them (this means that the resistance does not depend on voltage applied) If this ratio keeps constant over a wide range of voltages, the material is said to be an "ohmic" material and then the current can be predicted from the relationship:

Resistor Combinations The combination rules for any number of resistors in series (one element after the other) or parallel (all elements side by side) can be derived. It is important to notice that in the series case the resistances are added (more and more obstacles appear!) and the combined resistance gets bigger and bigger. In the parallel case you are giving more chances with the same difficulty so it is easier for charges to flow! The more the branches the easier the flow and the combined resistance grows smaller and smaller. Ammeters and Voltmeters An ammeter is an instrument for measuring the electric current in amperes in a branch of an electric circuit. It must

4 be placed in series with the measured branch, and must have very low resistance to avoid significant alteration of the current it is to measure. A voltmeter measures the change in voltage between two points in an electric circuit and therefore must be connected in parallel with the portion of the circuit on which the measurement is made. Electric Power The electric power in watts (joules per second) represents the rate at which energy is converted from the electrical energy of the moving charges to some other form, e.g., heat, mechanical energy, or energy stored in electric fields or magnetic fields. This means that power measures how fast is energy delivered and used up. Power (W) = Energy (J) / Time (s) For a resistor in a circuit it can be easily deduced that the power is given by the product of applied voltage and the electric current: Power = Voltage . Current Thermo-luminescent Effects of a Current As we have already seen when studying resistance the electrons transfer their energy to the wires (or any conductor) as they move through them. Now if the atoms become more energetic and move faster, the temperature of the wire will rise. That is called the thermal effect of a current or the Joule effect. Some elements in a circuit are designed to change as much energy as available into heat: electric heaters for instance. In other cases the heating of the wires needs to be kept as a low as possible to take profit of all the energy for other purposes and to prevent overheating of the circuit. If materials get too hot they will start glowing (as iron at the ironsmith’s). In an electric bulb (lamp) they will get red hot, then white hot and if they shine enough a room will be perfectly illuminated. Chemical Effects of Electric Currents: Electrolysis Electrolysis has been mentioned when describing the properties of ionic compounds. It was said to be the breaking down of substances by means of an electric current. In electrolysis energy from an applied voltage is used to drive an otherwise non spontaneous reaction. Applying a reverse voltage to a Daniel cell, for example, will revert the spontaneous chemical process which we get energy from and electric energy (from another source makes thing work “the other way round” (compare the diagrams).

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Electrolytic processes are very important for the preparation of pure substances like aluminium and chlorine. A Word on Electric Shock The primary variable for determining the severity of electric shock is the electric current which passes through the body that depends upon the voltage and the resistance of the path it follows through the body. An approximate general framework for shock effects is as follows:

Shock Physiological Effects Voltage required to produce the current with assumed body resistance:

Electric Current Physiological Effect (1 second contact)

100,000 ohms

....1,000 ohms

1 mA

Threshold of feeling, tingling sensation.

100 V

1V

5 mA

Accepted as maximum harmless current

500 V

5V

10-20 mA

Beginning of sustained muscular contraction 1000 V ("Can't let go" current.)

10 V

100-300 mA

Ventricular fibrillation, fatal Respiratory function continues.

100 V

6A

Sustained ventricular contraction followed by normal heart rhythm. (defibrillation). Temporary 600000 V respiratory paralysis and possibly burns.

if

continued.

10000 V

6000 V

One difficulty in establishing the conditions for electrical safety is that a voltage which produces only a mild tingling sensation under one circumstance can be a lethal shock hazard under other conditions. Will the 220 volt common household voltage produce a dangerous shock? It depends! If your body resistance is 100,000 ohms, then the current would be 22 mA But if you have just played a couple of sets of tennis, are sweaty and barefoot then, your resistance to ground might be as low as 1000 ohms. Then the current would be 220 mA The severity of shock from a given source will depend upon its path through your body

6 Some further considerations about household electricity Electric energy is a versatile, easy to use and easy to transport form of energy. Nevertheless its use requires of some precautions to avoid hazardous situations. Two parallel circuits are usually wired: one of the circuits for illumination purpose will tolerate a current of 5 Amps maximum. The second one goes to the sockets to connect home appliances and allows for 30 Amps currents maximum. Fuses and circuit breakers should be used to control the current if it rises to dangerous intensities. Most appliances have fuses; these are resistors with a low melting point that will melt (fuse) if the temperature rises to an inconvenient value. Grounding is a must for all appliances. Modern circuitry includes a “ground wire” all along the house. The third pin of the plugs connects the appliance to this wire so that any current leakage will be discharged through this wire and not the user’s body Never ever touch or use electric energy with wet hands or barefoot. These two things lower your resistance to dangerous levels allowing lethal currents to flow through your body. You can find some information at the end of the previous notes (Electric Current)

PROBLEMS ON ELECTRIC CIRCUITS 1- A total charge of 1.24 C flows in a period of 0.63 s. What is the current? 2- A cordless drill operates using a 14.4 V battery pack. The battery is rated at 2 amp hours which means that it can deliver a current of two amps for a period of 1 hour. a- How much charge passes through in 1 hour? b- How much energy do they carry? c- How much charge is held by the battery? 3- Draw two circuits in which three bulbs are connected with a 12 V battery. In one of the circuits the bulbs are connected in series; in the other one, in parallel. Three switches are inserted one before each bulb. a- Explain what happens if any of the switches is open in the first case b- What happens in the second case? c- How do you think the bulbs are connected in a house circuit? 4- An electric torch (flash light) uses two cells 1.5 V each. a- Draw the circuit of the torch b- If the bulb is rated 0.4 W calculate the current when the switch is on. c- Calculate the bulb’s resistance when fully operating.

7 d- Draw the circuit of the torch but add an ammeter and a voltmeter to it 5- Draw a simple circuit with a voltmeter and an ammeter. When you measure any magnitude you should alter the system as least as possible. According to this, discuss whether the resistances of an ammeter and of a voltmeter should be very low or very high. 6- A resistor of value 50 ohms is rated at 1 watt. This means that if it has to give out more power than 1 watt, it will start to get hot. What is the maximum current that it can handle

7- Use the circuit below to complete the table:

V (a) (b) (c)

12 V 14.4

I 0.30 A 0.52 A

8- What current is consumed by a 60 W light bulb operating on the 230 V mains? 9- Can a 12 V car battery be dangerous?

R 18 Ω 88 Ω