© Gunda W. C. – 2017PHYSICS DIRECTORYFORFORM THREE

© Gunda W. C. – 2017SUBJECT MATTERS TO COVER

© Gunda W. C. – 20171. APPLICATIONS OF VECTORS SCALAR AND VECTOR QUANTITIES Scalar Quantity - A quantity which is not associated with a direction Vector Quantity - A quantity which is associated with a direction SPEED VS. VELOCITYSpeed - Refers to the distance covered per unit of time without specifying direction (scalar) Velocity - Refers to the distance covered in a given direction per unit of time (vector) RELATIVE MOTION Relative Velocity - The vector difference between two velocities of two objects

© Gunda W. C. – 2017Continue . . .Resolution of vectors - To resolve vectors into two components (vertical and horizontal) Parallelogram Law of Forces - If two forces are represented in magnitude and directions by an adjacent side of a parallelogram, then their resultant is represented in magnitude and direction by the diagonal of the parallelogram Triangle Law of Forces - (Add vector problems)

© Gunda W. C. – 20172. FRICTION CONCEPT OF FRICTIONFriction - The force resisting motion between surfaces which are sliding against each other by converting kinetic energy into heat (ex a block moving down an inclined plane) Coefficient of friction - A scalar quantity which describes the ratio of the force of friction between two bodies and the force pressing them together Ways of reducing friction - Using a lubricant like oil/water/grease between two solid surfaces, streamlining, polishing surfaces, separating surfaces by air, uses rollers or ball bearingsAdvantages of friction - Allows walking, cars can brake, used to for parachutes

© Gunda W. C. – 2017Continue . . .Limiting Friction - The maximum value of frictional force exerted between two surfaces not moving relative to each other (Add friction problems) Types of friction - Dry, fluid, lubricated, skin, internal Dry - Resists relative lateral motion of two solid surfaces in contact Fluid - Friction between layers within a viscous fluid that are moving relative to each other Lubricated - Friction which occurs when a fluid separates two solid surfaces (ex oil in a motor) Skin - The force resisting the motion of a solid body through a fluid Internal - The force resisting motion between elements of a solid material as it deforms

© Gunda W. C. – 2017Continue . . .LAWS OF FRICTIONAmontons’ 1st Law - The force of friction is directly proportional to the applied load Amontons’ 2nd Law - The force of friction is independent of the apparent area of contact Coulomb’s Law of Friction - Kinetic friction is independent of the sliding velocity

© Gunda W. C. – 20173. LIGHTREFLECTION OF LIGHT FROM CURVED MIRRORSConcave Mirror - Has a reflecting surface that bulges inward (away from the incident light) and reflect light inward to one focal light (they focus light) Convex Mirror - A mirror in which the reflective surface bulges towards the light source, scattering the light Plane Mirror - A flat mirror Principle Focus - The point where light rays originating from a point on an object converge with one another Incident ray of light - The ray of light leaving the mirror after reflection Radius of Curvature - The distance from the vertex to the centre of curvature of the surface

© Gunda W. C. – 2017Continue . . .Principle Axis - The main axis of the lens or mirror Pole - A point that describes the position and orientation of a line with respect to a given circle DIFFERENTIATE BETWEEN REAL AND VIRTUAL IMAGESReal Image - A representation of an object in which the perceived location is actually a point of convergence of the rays of light that make up the image (ex. images on a cinema screen) Virtual Image - An image in which outgoing rays from a point on the object always intersect at a point (Example. an image in a flat mirror)

© Gunda W. C. – 2017Continue . . .Characteristics of an image formed by a convex mirror - The image is virtual, erect and smaller than the object, except if the object is closer than the focal point Characteristics of an image formed by a concave mirror - The image is always real except when the object is between the focal point and the mirror, the image is always inverted, the size depends on the distance from the mirror Position of an image in a concave mirror of a distant object - It is formed at the principle focus

© Gunda W. C. – 2017Continue . . .REFRACTION OF LIGHT Refractive Index - The ratio of the speed of light to the medium Refraction - A change in the direction of a wave due to a change in its speed. For light it is the change of speed of light (and hence its direction) due it entering a different medium Total Internal Reflection - An optical phenomenon which occurs when a ray of light strikes a medium boundary at an angle larger than the critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected Angle of Incidence - The angle formed between the incident ray and the normal a the point of incidence

© Gunda W. C. – 2017Continue . . .Normal (to a flat surface) - Is a vector that is perpendicular to that surface Critical Angle - The angle of incidence above which total internal reflection occurs. It is measured with respect to the normal Conditions necessary for total internal reflection - Light must be passing from a dense medium to one which is less dense, the incident light must be greater than the critical angle of the medium Conditions giving rise to a critical angle - Light travels from a dense medium to one which is less dense and is refracted at 90°

© Gunda W. C. – 2017Continue . . .Cause of refraction of light when passing through transparent media - This is due to the fact that light changes velocity when moving from one medium to another Mirage - A naturally occurring optical phenomenon where light rays are bent to produce a displaced image of distant objects or the skyWhy a swimming pool appears shallower than its depth - It happens because light rays bend as they pass from water to air. When they pass from water to air, they are reflected with an angle of refraction greater than the angle of incidence

© Gunda W. C. – 2017Continue . . .1. Three slabs of different types of glasses are placed on a table one on top of the other in the following order from below: Where will the mark on the table appear to be? Given: μA = 1.4 μB = 1.5 μC = 1.6 DA = 1.2cm DB = 1.8cm DC = 0.8cm D represents real depth and d represents apparent depth

© Gunda W. C. – 2017Continue . . .

© Gunda W. C. – 2017Continue . . .Refraction of light by a rectangular prism - (Not found in form four exams) Refraction of light by a triangular prism - (Not found in form four exams) COLOURS OF LIGHT Primary Colours - Sets of colours which can be combined to make a useful range of colours Secondary Colours - A colour made by mixing two primary colours Complimentary Colours - Colours which when added together form white light

© Gunda W. C. – 2017Continue . . .Additive Colour - The use of red, green or blue light to produce other colours by combining them together. This is used in projectors Subtractive Colour - The colour that the surface displays are the colours which are reflected by the material. This is used in the mixing of paints, dyes and inks. The colours used are generally cyan, magenta and yellow Chromatic Aberration - A type of distortion in which there is a failure in the lens to focus all colours to the same convergence point due to lenses having different refractive indexes for different wavelengths of light

© Gunda W. C. – 2017Continue . . .Fluorescence - The emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelengthWhy objects appear coloured - An object with colour tends to reflect light of its colour and absorbs the rest. The colour seen by a person is the colour which was reflected back

© Gunda W. C. – 2017Continue . . .Why mixing two paint colours is different than mixing two of the same colours of light (Example blue and yellow) - Pigments in paint absorb light, so the yellow pigment will absorb blue light and reflects yellow/red/green, while the blue pigment absorbs yellow and red light and reflects blue/green. Since the only common colour between the two that is being reflected is green, the colour reflected will be green (this is mixing by subtraction). Blue and yellow light are complementary colours so they add to form white light (this is mixing by addition)

© Gunda W. C. – 2017REFRACTION OF LIGHT BY LENSES Principle focus of a convex lens - The point on the principle axis where all rays originally parallel and close to the axis converge Cause of a blurred image in a concave mirror or convex lens - This is caused by light rays not coming together at the same focus 1. (a) In the following figure are two convex lenses correctly set up as a telescope to view a distant object. One lens has a focal length of 5cm and the other has a focal length of 100cm

© Gunda W. C. – 2017Continue . . .(i) What is A called and what is its focal length? A is called the objective lens and has a focal length of 100cm. The objective lens has a longer wavelength than the eyepiece lens (ii) How far from A is the first image from the distant object? The image of the distant object is formed at the focus which is 100cm (iii) What is the name of B? Eyepiece lens (iv) What acts as the object for B and how far must B be from it if someone is looking through the telescope wants to see the final image at the same distance as the distant object? The image of A acts as the object for B and therefore must be at the focus of B which is 5cm away

© Gunda W. C. – 2017Continue . . .(v) What is the distance between A and B with the telescope set up in part iv? The distance between A and be is the sum of their focal lengths, 100cm + 5cm = 105cm (b) Show by a ray diagram how a suitable placed eye sees an image of a point object which is placed 10cm in front of a plane mirror. Show clearly the position of the image and give two reasons why it is a virtual image.

© Gunda W. C. – 2017Continue . . .his image is virtual because it cannot be formed on the screen and there is no actual intersection of light rays when the image is formed (c) Calculate the critical angle for light emerging from glass of refracting index 1.55 into air Given: Refractive index, gηa = 1.55 r = 90° Critical angle, c = ?

© Gunda W. C. – 2017Continue . . .2. (a) Explain with ray diagrams the use of the following lenses (i) As a magnifying glass Convex lens The position of the object (O) is between the principle focus (F) and the lens

© Gunda W. C. – 2017Continue . . .(ii) In a camera Convex lens The position of the object is beyond 2F (b) State the characteristics of the images formed in (a) above (i) The image is virtual, larger than the object, on the same side as the object and is erect (ii) The image is real, smaller than the object, between F and 2F and is inverted

© Gunda W. C. – 2017Continue . . .(ii) Focal length of the lens u = 20cm v = 3u (from above) = 3*20 = 60cmf = ?

© Gunda W. C. – 2017Continue . . .3. A screen is placed 80 cm from an object. A lens is used to produce on the screen an image with a magnification of 3. Calculate the (i) Distance between the object and the lens Given: Magnification = 3 u is the object distance, v is the image distance from the lens u + v = 80cm

© Gunda W. C. – 2017Continue . . .4. A telescope of 5m diameter reflector of focal length 18.0m is used to focus the image of the sun. Using the distance of the sun from Earth and the diameter of the sun as 1.5x1011m and 1.4x109m respectively, calculate the diameter of the image of the sun Given: Object distance, u = 1.5x1011m Object diameter, d = 1.4x109m Focal length, f = 18m v = f = 18m

© Gunda W. C. – 2017Continue . . ..

© Gunda W. C. – 20174. OPTICAL INSTRUMENTS Simple microscope - (Not found in form four exams) Compound microscope - (Not found in form four exams) ASTRONOMICAL TELESCOPE Physics principles used to make telescopes - Reflection, refraction Projection lantern - (Not found in form four exams)

© Gunda W. C. – 2017Continue . . .THE LENS CAMERA Differentiate between images formed in plane mirrors and a pinhole camera –The human eye - (Not found in form four exams)

© Gunda W. C. – 20175. THERMAL EXPANSIONTHERMAL ENERGY Heat - The energy transferred from one body to another due to contact when they are at different temperatures Sources of heat in daily life - The sun, electric circuits (in appliances), engines, our bodies DIFFERENTIATE BETWEEN HEAT AND TEMPERATURE

© Gunda W. C. – 2017THERMAL EXPANSION OF SOLIDSCoefficient of Linear Expansion - Fractional change in linear dimensions (length/radius) per unit temperature change (°C or °K). Its SI unit is length / °C THERMAL EXPANSION OF LIQUIDS Apparent Expansivity of Water - The fractional increase in volume of water as it expands due to a temperature rise in a heated vessel Anomalous Expansion of Water - The tendency of water to expand as it is cooled below 4°C

© Gunda W. C. – 2017THERMAL EXPANSION OF GASES Ideal Gas - A theoretical gas composed of a set of randomly moving particles which obeys the ideal gas law Ideal Gas Law - The equation of a state of a hypothetical ideal gas which approximates the behaviour of many gases under varying conditions combining Boyle’s Law and Charles’s LawBoyle’s Law - Describes the inversely proportional relationship between pressure and volume of a gas (as volume increases, pressure decreases)

© Gunda W. C. – 2017Continue . . .Charles’s Law - A law which describes how gases tend to expand when heated, showing the direct relationship between temperature and volume (as temperature increases, volume increases) Kinetic Theory of Gases - Explains the behaviour of gases based on the movement of their molecules Avagadro’s Hypothesis - Requires that equal volumes of all ideal gases have the same number of molecules at STP

© Gunda W. C. – 2017Continue . . .Why gases have pressure - When gases travel in a container they hit the walls which exert a force on the walls (pressure) As the temperature rises, the molecules move faster, thereby hitting the wall more often which increases the pressure. As the temperature decreases, the molecules move slower and hit the wall less often which decreases the pressure Why diffusion happens in gases - Molecules in gas move randomly and when they collide with each other, they bounce in different directions. This causes molecules to move from areas where there are a lot of collisions to areas where there are very little collisions (moving from an area of high concentration to an area of low concentration)

© Gunda W. C. – 20176. TRANSFER OF THERMAL ENERGY CONDUCTION OF HEAT Thermal Conduction - The transfer of thermal energy between neighbouring molecules in a substance due to differences in temperature Thermopile - An electronic device that converts thermal energy into electrical energyGood conductors of heat - Metals like copper, aluminum, iron, silver, led Bad conductors of heat - Nonmetals like diamonds, rubber, glass, cork, paper Convection - The movement of molecules within liquids or gasses Kinetic Energy - The energy possessed by a body due to its motion

© Gunda W. C. – 2017Continue . . .How kinetic energy is related to temperature of gases - The kinetic energy of gas molecules is proportional to the temperature of the gasRADIATION Thermal Radiation - Electromagnetic radiation emitted from a material due to its temperature Good emitters/absorbers of radiant heat - Things with dark colour, metals like copper, iron, silver, led HOW HEAT LOSS IN A THERMOS FLASK IS PREVENTED By Conduction - The flask is made of glass which is a poor conductor of heat, the stopper is made of wood/rubber/cork which are bad conductors of heat, the supporting pad is made of rubber which is a poor conductor of heat By Convection - There is a vacuum between the walls of the flask. Also by closing the flask at the top by using a stopper By Radiation - Using silvered walls to reflect infrared radiation back into the thermos flask

© Gunda W. C. – 20177. MEASUREMENT OF THERMAL ENERGYHEAT CAPACITY Specific Heat Capacity - The measurable physical quantity for the amount of heat required to change a body’s temperature by a given amount. Its SI unit is joules per Kelvin (J/K) DIFFERENTIATE BETWEEN HEAT CAPACITY AND SPECIFIC HEAT CAPACITYHeat Capacity (Thermal Capacity) - The amount of heat required to raise its temperature by 1K Specific Heat Capacity - Heat required to raise the temperature of a unit mass of the substance by 1K EXPLAIN THE FOLLOWING OBSERVATIONS Gas thermometers are more sensitive and accurate than liquid thermometers - This is due to gases having a lower specific heat capacity than liquids

© Gunda W. C. – 2017Continue . . .Alcohol is used in glass thermometers in Arctic regions - This is due to alcohol having a lower freezing point than mercury A house with thick walls is likely to be cooler during the hot season - This is because a thick wall will conduct less heat from the outside into the house Level of liquid being heated in a vessel first falls before starting to rise - This is because the vessel expands first, which increases the internal volume, causing the liquid to fall Linear Expansivity - The faction of its original length by which a rod of the substance expands per Kelvin rise in temperature Coefficient of Linear Expansivity - The fractional increase in length per degree centigrade rise in temperature

© Gunda W. C. – 2017Continue . . .Applications of Bimetallic Strips - Making thermostats, bimetallic thermometers, indicators 1. 200g of a liquid at 21°C is heated to 51°C by a current of 5A at 6V for 5 minutes. What is the specific heat capacity of the liquid?

© Gunda W. C. – 2017Continue . . .First we calculate the heat gained, then heat supplied and then we can solve for the heat capacity2. A tin contains water at 290K and is heated at a constant rate. It is observed that the water reaches boiling point after 2 minutes and after another 12 minutes it is completely boiled away. Calculate the specific latent heat of the steam

© Gunda W. C. – 2017Continue . . .Note that the mass of the water (m) will cancel out, since mass is not important in finding the specific latent heat of a substance. The specific latent heat will be the same no matter what the mass is

© Gunda W. C. – 2017Continue . . .3. (a) A compound strip of brass and iron is straight at room temperature. Draw a labeled diagram to show its appearance when it has been: Heated to a high temperature and cooled below 0°C(b) A compound strip of brass and iron 10cm long at 20°C is held horizontally with iron on top. When heated from below by a Bunsen burner, the temperature of the brass is 820°C and the iron is 770°C. Calculate the difference in lengths of the iron and brass

© Gunda W. C. – 2017Continue . . .CHANGE OF STATE Latent Heat - Refers to the amount of energy released or absorbed by a chemical substance during a change of state that occurs without changing its temperature (ex. phase change from ice to water or water to steam) Specific Latent Heat of Fusion - The amount of heat energy absorbed when a unit mass of a substance changes from a solid state to a liquid state at a constant temperature

© Gunda W. C. – 2017Continue . . .1. A block of aluminum, 500g at 20°C was heated in a furnace until just when it melted (i) Find the total quantity of heat required Given: Mass of block, m = 500g or 0.5 kg Initial temperature, To = 20°C = 293K Final temp, T = 660°C = 933K (This is the melting point of aluminum) Latent heat of fusion of aluminum, LAl = 3.2 x 105JKg Specific heat capacity of aluminum, Cal = 920 K/JKg

© Gunda W. C. – 2017Continue . . .(ii) If in this process the furnace consumes 100 litres of gas of calorific value 16800J/litres. Find its efficiency. Given: Volume of the gas, 100 litres Calorific value of the gas = 16800J/litre Energy Absorbed = 454400J

© Gunda W. C. – 20178. VAPOUR AND HUMIDITY VAPOUR Evaporation - Causes the vapourization of a liquid, but occurs only on the surface of a liquid Condensation - The change in the phase of matter from gaseous to liquid droplets Saturated Vapour - A vapour which is in equilibrium with its liquid or solid Unsaturated Vapour - A vapour which has not reached the state of dynamic equilibrium with its own liquid or solid Factors effecting evaporation - Humidity, temperature, barometric pressure, surface area Triple Point of Water - The temperature where all three states of water (liquid, gas (vapour), solid (ice)) exist in equilibrium

© Gunda W. C. – 2017Continue . . . - When a person perspires on a hot day, evaporation occurs and helps to cool the body - Warm air can hold more water vapour than cold air Behaviour of a molecule in a liquid undergoing evaporation and then condensation - The molecule gains enough energy to escape the surface of the liquid through (evaporation). After it has escaped it eventually loses energy and slows down, falling back into the liquid or forming droplets of the liquid elsewhere (condensation) DIFFERENTIATE BETWEEN EVAPORATION AND BOILING

© Gunda W. C. – 2017HUMIDITY Hygrometer - An instrument used to measure relative humidity Dew Point - Temperature at which water vapour present in the air is sufficient enough to saturate it How dew is formed - As the surface of something cools by radiating its heat, atmospheric moisture condenses at a greater rate than it evaporates, resulting in the formation of water droplets Relative Humidity - The measure of the amount of water vapour in the atmosphere

© Gunda W. C. – 20179. CURRENT AND ELECTRICITY ELECTROMOTIVE FORCE (EMF) AND POTENTIAL DIFFERENCE (PD) Voltmeter - An instrument used for measuring the electric potential difference between two points in an electric circuit Electric Potential - A point in space where the electrical potential energy divided by the charge that is associated with an electric field. It is a scalar quantity measured in volts or joule/coulomb Electromotive Force (e.m.f) - The force which tends to cause current to flow Potential Difference (p.d) - Is the potential difference between two terminals of a cell when the cell delivers current to the external circuit. Potential difference is always smaller than the electromotive due to resistance of the cell

© Gunda W. C. – 2017Continue . . .Volt - The SI unit of the electromotive (e.m.f) force and the electric potential difference Types of electric circuits - Open circuits, closed circuits Open Circuits - A circuit which lacks a complete path between the positive and negative terminals of its power source Closed Circuits - A circuit which has a complete path between the positive and negative terminals of its power source Galvanometer - An instrument used for detecting and measuring electric current Shunt - A device which allows electric current to pass around a point in a circuit Motor - A machine which converts electrical energy into mechanical energy

© Gunda W. C. – 2017Continue . . .Dynamo - A generator that produces direct current with the use of a commutator Commutator - A rotary electrical switch in certain types of motors or generators which periodically reverse the current direction between the rotor and external circuit Generators - A device which converts mechanical energy into electrical energy Accumulator - An apparatus used to store energy Examples of accumulators - Rechargeable batteries, capacitors, hydraulic accumulators How you know it’s necessary to recharge an accumulator - The stored charge has been depleted

© Gunda W. C. – 2017Continue . . .1. A moving coil galvanometer of 30Ω resistance which carries a maximum current of 15mA can be converted into an ammeter (i) How can the galvanometer be made to give ampere readings? A galvanometer can be made to give ampere readings by connecting it in parallel to a low resistance called a shunt (ii) If the device is to give 1.5A full scale deflection (f.s.d), what value resistance will be required?

© Gunda W. C. – 2017Continue . . .Given: Current of device, I = 1.5A Current in section g, Ig = 15mA or 0.015A Current in section s, Is = ? Resistance of galvanometer, Rg = 30Ω Resistance of resistor s, Rs = ?

© Gunda W. C. – 2017Continue . . .2. The figure below shows two coils X and Y. X is connected to a battery and Y is connected to a center zero galvanometer G.(i) State and explain the deflection of the galvanometer needle when the switch K is closed for a few seconds and then opened.

© Gunda W. C. – 2017Continue . . .If switch K is closed the galvanometer will deflect and then return to zero. When switch K is opened the galvanometer will deflect in the opposite direction and then return to zero. Deflection happens when K is opened and closed because this is when the flux changes in X and Y since the induced e.m.f depends on the rate of change of flux (ii) Why must the galvanometer be a center zero type? This is so that it can read deflections on either side (iii) What would be done in X to increase the current induced in Y? To increase the induced current in Y you need to increase the number of turns of X

© Gunda W. C. – 2017RESISTANCE TO ELECTRIC CURRENTAmmeter - A measuring instrument which measures the electric current in a circuit Resistor - A two terminal electronic component that produces a voltage across its terminals that is proportional to the electric current in accordance with Ohm’s law (V = IR) Thermistor - A type of resistor whose resistance varies significantly with temperature Rheostat - A two terminal variable resistor used to vary resistance in a circuit Resistivity - Is a measure of how strongly a material opposes the flow of an electric current Ohm’s Law - Current flowing through a conductor is directly proportional to the potential difference (p.d) across the conductor provided that the physical state of the conductor remains unchanged

© Gunda W. C. – 2017Continue . . .Limitations of Ohm’s Law - Does not apply to some electrolytes (Ex dilute H2SO4), does not apply to conduction in gases, does not apply to semiconductors (diodes and transistors) - Ohm’s law is not applicable when physical conditions of the wire are altered Factors affecting resistance of a wire - Length, resistivity, cross sectional area Length of wire (resistance increases with increasing length ( R ∝ l) Resistivity/nature of the wire (resistance increases as resistivity increases (R = Pr / A) Cross section area of the wire (resistance decreases with increasing cross section area of a wire ( R ∝ 1 / A) Resistance of a wire: R = pl / A

© Gunda W. C. – 2017Continue . . .1. In the circuit shown in figure 1, the battery and ammeter have negligible internal resistance. What will be the ammeter reading?

© Gunda W. C. – 2017Continue . . .Since part A and B are in parallel, you will add their inverse. In part B there are two resistors in series, so their resistances will be added Part A: 1 Ω Part B: 1 Ω + 3 Ω = 4Ω 2. In the circuit shown below, the total resistance between X and Y is 2.0 Ω. Calculate the unknown resistance Q

© Gunda W. C. – 2017Continue . . .Note the denominator is 10 because the 6Ω and 4Ω resistors are in series so they are added 6 + 4 = 10

© Gunda W. C. – 2017Continue . . .(2b) A 2.0m long resistance wire of cross section 0.5mm2 has a resistance of 2.2Ω. Find the: Resistivity of the material(ii) Length of the wire that would give a total resistance of 1.0 Ω when placed in parallel

© Gunda W. C. – 2017Continue . . .First we need to find the resistance of the wire, then we can find the length

© Gunda W. C. – 2017Continue . . .Now we need to solve for the length l3. A 5Ω resistor and a 1Ω resistor are connected in parallel to a cell of e.m.f 6V and have an internal resistance of 0.5Ω. Calculate the current flowing around the circuit. Given: E = 6V R = ? r = 0.5Ω I = ? Energy in a circuit: E = I(R + r)First we must solve for R

© Gunda W. C. – 2017Continue . . .Now we can solve for I

© Gunda W. C. – 2017Continue . . .4. A wire of uniform cross sectional area has a length of 10m, a resistance of 2Ω and a resistivity of 2x10-7 Ωm. What is the cross sectional area in m2? Given: l = 10m R = 2Ω ρ = 2x10-7Ωm Resistance of a wire: R = ρl / A

© Gunda W. C. – 2017Continue . . .5. (a) If you are provided with resistors of 5Ω, 10Ω and 20Ω. What are the maximum and minimum resistances which can be obtained by connecting these resistors? Note that maximum resistance occurs when all three resistors are connected in series and minimum resistance occurs when all three resistors are connected in parallel

© Gunda W. C. – 2017Continue . . .(b) Answer the following questions related to the circuit drawn belowCalculate the current passing through the circuit when: (i) Switch K1 is closed If K1 is closed, it will put both resistors in series, so their resistances are added (5 Ω + 3 Ω = 8Ω) Given: V = 2V R = 5 Ω + 3 Ω = 8 Ω

© Gunda W. C. – 2017Continue . . .(ii) Switches K1 and K2 are both closed When both K1 and K2 are closed, the 5 Ω resistor is short circuited and will not affect the current (iii) Switch K1 is open and K2 is closed - This will create an open circuit, therefore no current will flow (there is no path for the current to flow around the circuit) Effects of an electric current - (Not found in form four exams)

© Gunda W. C. – 2017ELECTRIC INSTALLATION Circuit Breaker - An automatically operated electrical switch designed to protect and electrical circuit from damage caused by overload or short circuits Earthing (E) - A wire that is grounded to the earth Live (L) - A wire that has a current running through it. They can kill you if you touch them Neutral (N) - Fuse - A protective device used to control electric current flowing in a circuit by using an alloy with a very low melting point. It breaks the current when the current is too high

© Gunda W. C. – 2017Continue . . .1. Select the best fuse for the following:

© Gunda W. C. – 2017CELLS Simple Cell - Any kind of battery in which the electrochemical reaction is not reversible (Ex. disposable battery)Defects of a simple cell - Polarization, local action Polarization - The process of formation of hydrogen gas around the positive plate of an electric cell. Minimized by using an oxidizing agent called a depolarizer (ex. K2CrO4) Local Action - When a cell is used up when no external current is flowing as a result of impurities in the zinc plate. It is minimized by amalgamating zinc plate with mercury

© Gunda W. C. – 2017HOW ELECTROMOTIVE FORCE (E.M.F) DIFFERS FROM THE POTENTIAL DIFFERENCE (P.D) OF A CELLElectromotive Force (e.m.f) - Is the potential difference between two terminals of a cell when the cell does not deliver current to an external circuit. The total work done in joules per coulomb of electricity in a circuit where the cell is connected. Measured in volts (V) Factors determining the size of an induced e.m.f - Number of turns in the coil, strength of the magnet (magnetic field), rate of change of flux (speed of rotation or movement) Potential Difference (p.d) - Is the potential difference between two terminals of a cell when the cell delivers current to the external circuit. Potential difference is always smaller than the electromotive due to resistance of the cell