MAGNETISM AND ELECTRICITY Science 1, Class IX
Objectives • Magnetism • Magnetic effect of electricity • Oersted’s experiment • Right hand rule • Tangent Galvanometer
RECAP • Magnet - material that produces a magnetic field • Magnetic field: • invisible • a force that pulls ferromagnetic materials • attracts or repels other magnets • Types of magnets: • permanent magnets: Creates its own persistent magnetic field • temporary magnets : Act like a magnet only in the presence of another magnetic field • electromagnet - ?
Properties of a magent (contd) • Both poles of a magnet have equal strength • Strength of a magnet near its poles is more • Mutual attraction exists between a magnet and magnetic substance • A magnet has 2 poles – north, south – that cannot be isolated
Earth as a magnet • The earth behaves like a magnet • In what direction does a freely suspended magnet come to rest? • We know that unlike poles of 2 magnets attract, and like poles repel each other • WITH THESE POINTS IN MIND, WHAT CONCLUSION CAN WE DRAW ABOUT THE EARTH AS A MAGNET? • Where do the earth’s magnetic north and south poles lie?
Earth as a magnet (contd) • Think of a giant bar magnet within the earth • the earth's magnetic north pole is in the geographic south pole ( south of Australia) • the earth's magnetic south pole is in the geographic north pole (north of Hudson bay, Canada) • magnetic field lines come out of the Earth near Antarctica and enter near Canada • axis of rotation is not the same as the axis through the earth's magnetic poles
Magnetic lines of force • Plotting the magnetic lines of force around a bar magnet: http://www.youtube.com/watch?v=fghLhJe1JLY • Magnetic lines of force • start at the north pole, towards the south pole of the magnet • do not intersect each other • Neutral points
Magnetic compass • Pivoted magnetic needle mounted in a box • North pole is painted red in colour • What was the compass used for in early days? • Activity: • Move a magnetic needle around a bar magnet • What does the deflection of the needle indicate about the strength of the magnetic field - when the compass is close to the bar magnet - when it is away from the bar magnet
Electromagnetism • magnetic field is present around a current carrying conductor • established by Oersted experiment: • wind insulated copper wire around a box • place a magnetic compass in the center of the box • connect the ends of the wire to a battery and pass the current What do you observe: • When you reverse direction of current? • When you increase no. of turns in the wire?
Magnetic lines of force in a straight current carrying conductor • in the center of a square piece of cardboard, make a hole and pass a wire through it • make the cardboard horizontal and the wire perpendicular to it • spread iron filings on the board • connect the ends of the wire to a battery and pass current through it • tap the board gently What happens? • iron filings get magnetized and arrange themselves along the magnetic lines of force in concentric circles around the wire • If instead of iron filings, we placed a few magnetic compasses?
Maxwell’s right hand grip rule • Used to determine the direction of magnetic field produced by a current carrying conductor • Circular current carrying conductor What do you observe on: • Changing the diameter of the coil • Increasing the number of turns in the circular wire
Quiz • Which of the pictures below shows the magnetic field that produced by a single straight current-carrying wire? • Which shows the field produced by a circular coil carrying current?
Did you know? • MRI – Magnetic Resonance Imaging • Keeping the human body in a very high magnetic field (30,000 times higher than that of the earth) • Helps in accurate of diagnosis of defects in bones, ligaments and tissues • How are electromagnets used in junkyards?
Objectives • Electricity • Electric current • Electro motive force (e.m.f) • Potential difference (voltage) • Electrical resistance – Ohm’s law • Electric power • Heating effect of electricity • Lighting from electricity • Applications at home • Electric wiring at home, plugs, switches, fuses • Meter reading • Calculation of power consumption • Saving electricity
Electricity • The energy acquired by charged particles by virtue of their motion is called electrical energy or electricity Which effect of electricity is used in the following appliances? • electric bulb • electromagnet • fuse • iron box • calling bell
Electric potential • Charge - positive, negative • SI unit of electrical charge = coulomb Potential energy of a charged particle in an electric field • If you hold a stone of mass 'm' at a distance 'h' from the ground, what is the potential energy of the stone? mass = m height = h gravitational force = G • potential energy = m * G * h joules (kg•N•kg^-1•m = N•m)
Electric potential (contd) • Similarly, a positive charge 'q' in a uniform electrical field 'E', at a distance 's' from the negative plate has an electric potential energy.
Electric potential (contd) • What is the work done by the field 'E' on the charge 'q' (assuming 'E' is uniform)? • Force on charge 'q' , • This force will cause the charged particle to accelerate in the direction of the force (here, towards -vely charged plate) • After particle has moved a distance 's', work done on it 'W' will be: • Positive charges will accelerate in the direction of the field • negative charges will accelerate in a direction opposite to that of the field Definition • The work done in bringing a unit positive charge from infinity to a point in an electric field is called potential at that point
Electric potential (contd) • SI unit of electric potential at any point = joule/coulomb = volt • An electric potential of one volt means one joule of work is done in bringing one coulomb of positive charge from infinity to that point
Potential difference • Consider the experiment below Observations • Does water flow from A -> B or B -> A? • What is the change in the level of water in the 2 bottles? • When does the water flow stop? We use the same reasoning to understand the movement of charge
Potential difference (contd) • Charged particles move from one point to another provided there is a potential difference between the points • Electrons move from an electron-rich region to an electron-deficient region • Potential difference (or) Voltage = amount of work done in moving a unit positive charge from point to another = = = one joule of work per coulomb of charge = one volt SI unit of potential difference = volt
Potential difference (contd) • Instrument used to measure potential difference – voltmeter • It is connected in parallel between the 2 points where the potential difference/voltage has to be measured DO NOT CONFUSE A VOLTMETER WITH A VOLTAMETER
Electric current • Consider 2 points A and B A B • If a conductor is connected between A and B, • what is the direction of flow of electrons – electron current • What is the direction of flow of electric current • What happens when the amount of +ve charge in A = amount of -ve charge in B?
Electric current (contd) • Electric current is not just the flow of electrons - it is the flow of +ve or -ve charges under the influence of a potential difference • Conventionally, the direction of flow of electric current - opposite to the direction of flow of electrons • amount of electrons flowing in one direction = amount of positive charge flowing in the opposite direction • Electric current
Electric current (contd) • SI unit of current – ampere • The electric current is said to be one ampere when one coulomb of charge flows through any cross section of a conductor in one sec • Instrument used to measure electric current is ammeter • It is connected in series at the point in the circuit where the current is to be measured
Electrical circuit • Electric circuit - path along which electric current flows Circuit diagram • shows how different components in a circuit are connected using corresponding symbols for the components • Electric circuit corresponding circuit diagram • 1 – cell, 2 – bulb, 3 – switch, 4 - conductor
Electrical circuit • Symbols used in electric circuits
Electromotive force • Consider the flow of water between 2 water tanks • Water flows from tank M -> N in the pipe AB • To maintain a uniform flow of water, the level of water in M has to maintained for which energy is required • We could use a pump to fill water from tank N to M Using this analogy to electric charges: • Consider a metallic conductor – the free electrons are in random motion (delocalized electrons) • When the conductor is connected to an electric cell, the electrons start moving uniformly in a particular direction • To get this uniform flow of electric charge in a conductor, the potential difference between its ends has to be maintained constant - energy required to maintain p.d in a conductor is e.m.f • Sources of e.m.f - electric cell, electric generator, solar cell, thermocouple
Electromotive force (contd) • when there is no current in an electric circuit, the p.d. between the terminals of that source(cell) is its emf • so, if emf is the p.d. between the terminals of the source of emf(cell), unit of both emf and p.d. are the same – volt • S.I unit of emf – volt and instrument for measuring emf – voltmeter Difference between emf and potential difference
Electric resistance • When electrons flow through the conductor, they collide with the constituents present in the body of the conductor – there will be some obstruction to the flow of electrons due to these collisions • "Property of a conductor to obstruct the flow of electrons through it is called electric resistance“ • What is the effect of this resistance? • What happens to the flow of electrons when the electrical resistance offered by the conductor is: 1. high - flow of electrons is lessened, hence, conductivity decreases - insulators 2. low - flow of electrons increases, hence, conductivity increases – conductors • If you connect a voltmeter across the ends of a battery (emf = 12V), will the reading show 12V?
Electric resistance (contd) • SI unit of electrical resistance is ohm (Ω) • The resistance offered by a conductor is one ohm , when a current of one ampere flows through a conductor in which the potential difference between any two points is one volt • Can we have a conductor that is perfect, i.e. having zero resistance? YES SUPER CONDUCTORS • materials that offer zero resistance to the flow of current when the materials are brought below a certain temperature. • mercury becomes a superconductor below 4.2K • Think about an application of super conductors
Electrical resistance (contd) • Factors on which resistance of a conductor depends:
Test your knowledge • What is the nature of the filament in a bulb? - we know that copper is a good conductor of electricity – Can it be used to make the filament? - what is the effect of electricity that makes a bulb light up? • Why are conductors used in domestic wiring insulated? What material is commonly used in domestic wiring? • Can tungsten be used in domestic wiring? • Aluminium wires used in the transmission of electricity are not insulated. Why? • Why do electricians wear rubber shoes and gloves while working?
Ohm’s law • Experiment to find out the ratio between potential difference and electric current A – ammeter, V – voltmeter • B – electrical cell • X – resistance • S – plug key • Rh – rheostat (electrical component in which resistance introduced into a circuit is readily variable with a sliding handle that alters the length of the conductor) • Obtain 3 sets of values for I and V – plot a graph
Ohm’s law (contd) • Ohm's law states that: In any conductor, at constant temperature, the current flowing through the conductor is directly proportional to the potential difference across its ends. • Ohm's law helps to find out the electrical resistance of a material
Electrical power • power = rate at which work is done • amount of work done = amount of energy consumed • so, power = rate at which energy is consumed • electrical power = rate at which electrical energy is consumed = rate at which work is done by electrical energy • SI unit of electrical power is watt • When an electrical appliance consumes electrical energy at the rate of one joule per second,its power is said to be one watt • other common units of power: • one kilowatt = 1000 watts • one horsepower(hp) = 746 watts • horsepower is usually used to express the electric power of water pumps
Formulae recap (contd) • According to the law of conservation of energy, - work done by electricity(W) = Electrical energy consumed (E) • W = Pt , E = Pt
Electrical billing • In this bulb, we see 50W and 12V mentioned - what does this mean? Bulb is to be used in a circuit having a pd of 12V, and consumes 50 joules of energy in one second • Electrical energy consumed = electric power * duration • Using the power printed on the bulb, we can calculate the amount of electrical energy consumed. • The usage of electric energy is in units, where • one unit = one kilowatt-hour = 1000 w * (60 * 60) s = 3600000 joules • so, one kilowatt-hour = 3600000 joules
Think it over • what happens when a bulb marked 6V or 9V is used between a potential difference of 220V? • if a bulb marked 40V or 60V is used in a 6V circuit? • A fan regulator is used to control and vary its rotational speed. Would it be possible to minimize the consumption of electricity by setting the regulator to its lowest speed? • How are electrical transmission lines drawn on poles harmful to the environment? • Electrical energy(E) is converted into heat energy(Q), so, Heat liberated(Q) = E = Pt = I^2Rt • when resistance is negligible and tends towards zero, Q = 0, and there is no liberation of heat • when resistance is very high and tends towards infinity, then also Q = 0, since the material becomes and insulator and there is no current passing through it to cause heating • With this is mind - electric boxes produce heat whereas the conducting wires used in the same circuit don't. Why?
Conversion of energy • Electricity can be produced from various sources • water (dams) - the first ever hydel power station in Asia was set in 1902 in Shivanasamudra • thermal energy • nuclear energy • wind energy • solar energy • chemical energy • What is the electric energy converted to in the following: 1. bulb 2. iron box, geyser, heater 3. electric train, tram, machines • Electricity in the human body? • messages to and from brain and organs are sent in the form of electric signals • these electric signals are produced by nerve cells or neurons
Measures to save electricity • Electricity is the most convenient form of energy - why? • easily converted to other forms of energy for use in devices • easily transported from place of generation to required place • can produce three main effects - heating, magnetic and chemical effects • eco-friendly when compared to other forms of energy • Saving electricity • Buildings with good natural ventilation and lighting • Use separate circuits for heating and lighting • Adapt thermostats and timers in electrical equipment • Maintain machine parts well – reduce friction in machine parts by regular lubrication • Use transformers in various stages of power transmission • Underground transmission of electricity leads to less wastage than in overhead transmission wires • At home what can you do to save electricity? • Switching off electrical appliances after use – fan, light etc • Using Compact Fluorescent Tubes(CFT) instead of bulbs/tubelights as the consume less energy • Keep bulbs and tubes clean to obtain maximum light • Use flat bottomed utensils on an electric stove
Domestic wiring • Electricity generated in power stations is brought to our homes two thick copper or aluminum wires Colour coding convention: Single phase connection : live wire – red, neutral wire – black, earth wire – green Three phase connection (industries) – there are 3 live wires – red, yellow, blue Why are appliances connected in parallel across the live wire?
Domestic wiring – electric fuse • device which is used to limit the current in an electric circuit. The fuse safeguards the circuit and the electrical appliances from being damaged • made of tin-lead alloy and has a low melting point breaks the circuit if the current exceeds a safe value. • the thickness and length of the fuse wire depends on the maximum current allowed through the circuit • It is connected in series in the beginning of the electric circuits • Electric fuses are always connected in series in an electric circuit. Why? • Nowadays there are many circuit breakers like MCB (Miniature Circuit Breaker) and ELCB (Earth Leakage Circuit Breaker)
Earthing Earthing • To avoid the risk of electric shocks, the metal of an electrical appliance is 'earthed‘ • The metal case of the appliance is connected to the earth (at zero potential) by means of a metal wire • One end of the earth wire is buried deep in the earth • The other end is connected to the three pin socket • When the appliance is switched on the appliance’s metal case will remain at zero potential and prevent electric shocks Electric shock • If we touch any part of an electric circuit that is not insulated, or, the metal case of an appliance that is touching the live wire, our body becomes part of the circuit • Our body offers less resistance to the current, and a large current passes through our body to the earth • Cells lose water and is dangerous
Electrical hazards • We cannot decide if current is passing through a wire just by looking at it • A person or animal can die if they come in contact with a live wire and earth • A fire could break out in defective electric appliances, due to excessive current, loose contact, defective conductors • Wrong usage not only spoils electrical appliances, but can also cause fire accidents • Precautions • If a person touches a live wire or there is an electric fire, immediately switch off the mains • When handling a live circuit to detect defects in electrical appliances • Use rubber gloves and shoes • Stand on a dry wooden plank (with no nails) • Use a tester with properly insulated handles • Use insulated conductors, plugs, switches of good quality and proper electrical rating • If a fuse melts, switch off the mains • Each electrical circuit should have proper earthing • For any problems in the meter, contact the electricity department
Precautions (contd) • Take care to see that transmission lines • Do not touch trees • Are sufficiently away from buildings • Do not touch wires with long metal bars or pipes you may be carrying • Use a dry wooden or plastic stick to move a piece of conducting wire • It is dangerous and a crime to tap connections from the electric pole without permission from the electricity board • Trippers – safety devices which break the electric circuit when there is a leakage of current • Three pin plugs and sockets – connected to neutral, live and earth wires. Ensures better safety due to earth connection • Always use ISI marked electric appliances and parts
Think it over How can a bird stand harmlessly on a high potential electric transmission line? Clue : Why is it dangerous for us to touch a transmission line with a metal bar? How does the current flow? There are some circumstances when birds do get electrocuted. When and why? Clue: Think about a complete circuit
War of currents - a.c. and d.c. Nikola Tesla George Westinghouse Thomas Edison d.c. (direct current) – has a constant direction a.c.(alternating current) – changes direction constantly The electricity that is supplied by the electric board is a.c – it is possible to change its potential difference using transformers The current obtained from batteries(used in our experiments) is direct current