Chapter 2 : Electricity

1. 2. 2.1 Electric Field and Charge Flow2.1.1 Electric charge, Q

1. Two types of charge: ___________, ___________2. An electron : a negative charge (_____________________)3. A proton : a positive charge (_____________________)4. SI unit of charge is the coulomb or C.5. A body is :*neutral (equal number of positive and negative charges)*charged positive (an atom loses electrons)*charged negative (an atom gains electrons)6. Like charges repel, unlike charges attract.Total charge, Q =A small body has an initial charge of + 3.0 C. What is its final charge ifa) it acquires 1.875 1019 electrons from another body,b) it loses 1.25 1019 electrons?

2.1.2 Static electricity (Electrostatics charges)*electric charges which do not move*can be produced by rubbing two materials together (rubbing polythene rod with a dry woolen cloth)*electrons are pulled from woolen cloth to the polythene rod*polythene rod gains electrons ___________charged Woolen cloth loses electrons ___________ charged

Detecting and testing small charge (____________________)*When a charged object touches the metal cap at the top of electroscope, some of the charges are transferred to the gold leaf and the metal plate. (giving charge to the electroscope)*Similar charges on the gold leaf and metal plate __________ each other > the leaf rises

*When a positively charged rod is brought near the metal cap of a positively charged electroscope, the gold leaf rises even moreReason: Electrons are attracted to the metal cap by the positive charges on the rod. The gold leaf and the metal plate have more positive charge. The gold leaf is repelled further by a stronger force.Effects of static electricity

1. Rubbing plastic comb on your shirt sleeve and use it to pick up small pieces of tissue paper.

2. When a negatively charged polythene rod is brought near a fine stream of water, the stream of running water is bent towards the rod because of the force of attraction between opposite charges.

3. Carpet attracting dust

*A carpet surface becomes charged by the rubbing of shoes.*Each dust particle carries equal numbers of positive and negative charges (electrons) >> neutral*When dust particles fall from shoes onto the carpet, the electrons of the dust particles are displaced towards the carpet. The positive charges are displaced away from the carpet.*The force of attraction between the negative electrons on the dust particles and the positive charges on the carpet causes the dust particles to stick to the carpet.

4. Earthing of a vehicle

*Vehicles (car, lorry tanker, aircraft) will become charged due to friction with the air.*When a person touches the outside of the vehicle, the highly charged vehicle will discharge through him and the person will receive a brief electric shock.*If the vehicle is a petroleum tanker, the sparks produced during discharge may start a fire.*To avoid this, a long metal chain which touches the ground is usually tied to the back of the vehicle.*Thus, the electric charge produced by friction can flow from the vehicle to the earth through the metal chain.Applications of electrostatic charges1. Electrostatic crop sprayer

*A strongly charged metal wire charges the tiny droplets of pesticide as they leave the nozzle.*The droplets of pesticides have the same charge, so they repel each other and spread out evenly to cover a wide area.*When a droplet is near a leaf, it induces an opposite charge on the leaf and is attracted to it.

2. Electrostatic paint sprayer

*The droplets of paint coming out from a nozzle are charged positively as the nozzle is connected to a positive potential.*As the droplets of paint have same charge, they repel each other to form a wide fine spray.*The body is earthed during spraying and thus the droplets of paint are attracted to the metal body, giving a very even coating.*save cost, even spraying, reach inaccessible parts

3. Electrostatic air filter (to clean air in room or factory)

*When the fan on top spins, dirty air is sucked from below through a filter netting. Bigger dirt particles are filtered off by the netting.*The dirty air then moves up through two sets of metal grids. Grid A is charged positive and Grid B is charged negative.*Dust particles are charged by the grids. Negatively charged particles are attracted to grid A and positively charged dust particles are attracted to grid B.*The foul smell of the air is removed through absorption by charcoal placed on top. Thus what flows out of the filter is fresh clean air.

4. Electrostatic precipitator (remove ash and dust from the waste gases in power stations or factory chimneys before being discharged to the atmosphere)(1) Smoke particles pick up a negative charge.(2) Smoke particles are attracted to the collecting plates.(3) Collecting plates are knocked to remove the smoke particles

5. Photostating machine (Photocopier)*The selenium-coated drum is charged positively as it rotates past a charged rod.

*Drum becomes evenly charged.

*Print image are focused on the drum. Parts which receive light lose all their charges. Dark areas on the drum surface retain their charges.

*Fine powdered ink (toner) which is negatively charged is attracted to the charged areas of the drum.

*The toner is transferred to the paper and the images are printed on the paper.

*The toner is heated to melt the powdered ink and fix the images permanently on the paper.

2.1.3 Electric current, I

1. Electric current, I = rate of flow of charge

2. SI unit of current, I =1 A is the steady current that flows through a wire when a charge of 1 coulomb flows through the wire in 1 second.Hence, 1 A = 1 C s-1

3. The amount of charge, Q, flowing through any conductor is given by :

4. When free electrons move through the wire from B to A, the electric current, I, flows from A to B.

ExerciseA fuse wire will blow if the current flowing through it exceeds 5.0 A for a time of 80 ms.a) What is the quantity of electric charge that has passed through the fuse wire in 80 ms?b) How many electrons have passed through the fuse wire in that time, given that the charge on 1 electron is 1.6 10-19 C?2.1.4 The electric field

1. Electric field = a region surrounding a charged body where electrostatic force can be experienced.

2. An electric field can be represented by:

3. Arrangement used to observe electric field patterns:

4. Patterns of electric field using different types of electrode:Electric field patterns

Effect of an electric field1. A ping-pong ball coated with a conducting material

*When a high voltage is applied across the two plates, the ball is observed to be stationary.*This is because opposite charges are induced on the surface of the ball, so that the ball is attracted by the positive and negative charge plates with equal force, F.*If the ping-pong ball is slightly displaced to one side, the ball will bounce back between the two plates continuously.

2. Spreading of a candle flame

*When the high d.c. voltage connected to plates P and Q is switched on, it is observed that:a) the candle flame spreads out in two opposite directionsb) the spread of the flame towards the negative (P) plate is biggerExplanationa) Heat energy from the candle flame produces ionization of the air molecules. Positive and negative ions are formed around the flame.b) Positive ions are heavier, and move slower. Movement of the positive ions towards the negative plate P causes a bigger spread of the flame.c) Electrons are lighter and move at a higher speed. When they move towards the positive plate Q, they cause a smaller spread of the flame.

3. Lightning

*When a negatively charged thundercloud passes above a tall building, positive charges are induced on the roof.*A strong electric field between the cloud and the roof produces a strong force of attraction between the opposite charges.*Electrons will suddenly accelerate from the cloud to the roof and this causes the building to be struck by lightning.*A lightning conductor with sharp spikes is fixed on the highest part of the roof to reduce the risk of lightning strikes in two ways:a) The lightning conductor carries some induced charges to the ground and also cancels out some of the charges on the cloud. This reduces the chances of lightning striking.b) If lightning does strike, the lightning conductor provides an easy path for the electrons to pass to the ground without damaging the building.2.2 Relationship between electric current and potential difference2.2.1 Potential difference (Voltage)

Electric potential at X (+ve terminal) ( ) Electric potential at Y (-ve terminal)>> Electric current flows from X to Y, passing through an electric bulb, because of the electric potential difference across the battery.

1. The potential difference between two points in a circuit causes an electric charge to move between the two points.

2. An electron will move from a point of low potential to a point of high potential.Hence, an electric current I will flow from a point of low potential to a point of high potential.

3. Potential difference, V, between two points: work done (or energy produced) when 1C of charge moves between the two points in an electric field.

Or

4. The SI unit of potential difference ( or voltage ) is volt (V).1 Volt is the potential difference between two points if 1 J of energy is produced when 1 C of charge flows between the two points.

Exercise

If 2 1019 electrons pass a point on a wire in 5 seconds, what is the magnitude of the current that flows through the wire?

The energy released between points A and B is 60 J when a charge of 20 C flows from A to B.a) What is the potential difference between A and B?b) If the time taken for the 20 C to flow between A and B is 40s, what is the value of the current flowing through the bulb?

Chapter 2 : Electricity

2.2.2 13

2.2.3 Ohms Law

Ohms law states that the current, I, flowing through a metal conductor is directly proportional to the potential difference, V, across the conductor (if the temperature and other physical conditions remain constant)

1. Graph for a conductor that obeys Ohms law ( )

2. Resistance, R = ratio of the potential difference (or voltage), V, across the conductor to the current, I, flowing through it.

3. 1 is the resistance of a conductor when a potential difference of 1 volt applied across it produces a current of 1A through it.

ExerciseA 9 V battery connected across a resistance wire produces a steady current of 0.3 A through the wire. If the internal resistance of the battery can be neglected, what is the resistance of the wire?

5.0 V is applied across a 2.0 resistor. What current will flow through the resistor?

2.2.4 Factors that affect resistance1. Four factors that affect resistance of a conductor:

Resistance

Length of conductor(wire or filament)

Cross-sectional area of conductor(diameter or number of wires)(Highway)

Temperature of the conductoratoms in metallic latticevibrate stronger > block the movement of electrons >flow slower

Type of materialSilverCopperConstantanNichrome

A resistance wire of resistance 3 has length 0.2 m and cross-sectional area 0.02 mm2.What is the resistance of another wire of the same material, but with length 0.80 m and cross sectional area 0.04 mm2?

2.2.5 Superconductors

1. Resistance of a metal increases with temperature.Resistance of a semiconductor decreases with temperature.

2. A superconductor = a material whose resistance become ZERO when its temperature drops to a certain value called the critical temperature, Tc.E.g. mercury becomes a superconductor when its temperature reaches 4.2 K.

3. Other examples of superconductors:MaterialCritical temperature, Tc (K)

Zinc0.88

Aluminium1.19

Mercury4.15

Lead7.18

YBa2Cu3O790

Ti-Ba-Ca-Cu-O125

4. When a small permanent magnet is moved above the surface of a superconductor, a current is induced inside the superconductor.5. As the resistance of the superconductor is zero, the induced current will continue to flow for years, even after the permanent magnet is removed.

6. As the magnetic field produced by a superconductor is opposite in direction to that of the permanent magnet, the permanent magnet will be acted by an opposing magnetic force. Thus, it will float above the surface of the superconductor.

Advantages of superconductors*At critical temperature, a very large current can flow through the superconductor without overheating, as its resistance is zero.*The magnetic field produced by a superconductor is many times more powerful than that produced by any permanent magnet.

Uses of superconductorsa) As superconducting coils (in electromagnets)*produce very powerful magnetic fields*used in scanning devices in hospitals (CT-Scan and MRI)

b) As high temperature superconducting cables (in power transmissions)*Efficient! Low cost! Low power loss!c) Make strong superconducting electromagnets (in MAGLEV train magnetic levitation train) *the train floats about 1 cm above the tracks (due to strong magnetic repulsion between the base of the train and the railway tracks)*the train can slide at high speed (500km/h) without friction

2.3 Series and parallel circuits1. Electrical circuit = complete path along which electric current can flow2. A circuit made of electrical components2.3.1 Resistors in a series circuit

Series circuit = circuit components are connected end to end consecutively to provide a single path for current flow through all the components

*All components in series circuit have the __________ current*If one component fails to allow current to flow through, the whole circuit breaks off.

1. Current, I = 2. Potential difference, V = = = 3. Resistance, R =

2.3.2 Resistors in a parallel circuit

Parallel circuit = components are placed side by side and their corresponding ends are joined together

*Failure of one component does not affect the other components.1. Potential difference, V = = 2. Current, I = =

= 3. Since,

Thus,

Advantages of parallel cicuits1. If one of the electrical appliances does not function, the other appliances can still be in use.2. Additional appliances can be connected in parallel with the existing appliance without the need for more voltage.3. Therfore,most of the household applian ces are connected in parallel.

Effective resistance (Combined resistance)Effective resistor = a single resistor that replaces 2 or more resistors in seires or parallel= has the same effect on the circuit= allow same amount of current to flow in the circuitCombined circuitCombined circuit = a combination of resistors connected in series and in parallel

Calculate the effective resistance

2

3 5 3 2

5

2 4 2 Calculate the effective resistance

3

6 4

2.4 Analysing electromotive force and internal resistanceInternal resistance1. A circuit is a closed loop through which current can continuously flow.a) _____________ = path taken by the current outside the cell b) _____________ = path taken by the current within the cell

2. When current flows in a circuit (internal or external), resistance is produced.a) For external circuit = ______________b) For internal circuit = ______________ or source resistance

3. What is internal resistnace or source resistance?The resistance within the cell or battery itself , (due to its electrolyte or electrodes).(A cell consists of electrodes in a chemical electrolyte.When the cell is connected in a circuit, the current flowing in the electrolyte through the electrodes experience internal resistance.)

To show the exsistence of internal resistance1. Turn on a torch for 20 minutes. The dry cells in the torch becomes hot (due to resistance)2. Hot (resistance) happens when current flows in resistor.3. Curent flows through the cell and external circuit.4. Therefore, resistors in the external circuit become hot too.Electromotive force (e.m.f.)1. A cell = an electrical source which uses chemical reactions to produce a current (changes chemical energy into electrical energy)

2. A battery = combination of two or more cells connected in series

3. _________________________________________________ has to be in a circuit to make a current flow through the circuit.

4. Electromotive force, E, of a dry cell = total electrical energy given to one coulomb of charge flowing through the CELL

If a dry cell has E = 1.5 V, the dry cell will provide 1.5 J of electrical energy to every 1 C of charge passing through the cell.This 1.5 J of energy is transferred to all parts of the circuit.

Relationship between e.m.f. & terminal potential difference

Electromotive force*Work done by a source in driving 1 coulomb of charge around a COMPLETE CIRCUIT (external and internal circuit)*can be measured by voltmeter connected to the terminals of a cell on open circuit

Terminal potential difference , Vt*Work done by a source in driving 1 coulomb of charge through the EXTERNAL RESISTOR*can be measured by voltmeter connected to the terminals of a cell on closed circuit (when the cell sends current through the external resistor)

Since Vt < E ; E Vt = Vd

Lost Volts, Vd*Work done by a source in driving 1 coulomb of charge through the CELL*OR, potential difference required to drive the current through the internal resistance*cant be measured directly

# Work done = Energy# E.m.f = energy supplied by a cell per coulomb of charge to the whole circuit

1. 2. 2.1 2.2 2.3 2.4 2.5 Analysing electrical energy and powerElectrical power

Power,P = amount of energy transferred in one second

1 watt = power of an electrical appliance which can produce a totoal energy of 1 J in 1 sec

Cost of using electrical energy1. Unit for cost of using electrical energy = kilowatt-hour (kWh)2. 1 kWh = total energy consumed by an electrical appliance of power 1 kW in 1 hour.1 kWh =

3. If a television set of power 700 W is switched on for 6 hours a day, then the total electrical energy used in a day is

Energy = Power x Time

Exercise1. The lamp of a motorcycle is labelled 12 V, 15 Wa) Explain the meaning of 12 V, 15 Wb) What is the value of the current flowing through the lamp when it is connected to a 12 V supply?How much is the resistance of the filament of the lamp?

2. The usage of electrical appliances in Alis household in one day is as shown in the table below:ApplianceNumber of unitsPowerTime duration used

Lamp8100 W12 h

Television1700 W 8 h

Kettle12 kW1 h

Fridge1400 W12 h

Fan370 W10 h

a) Determine the total electrical energy (in kWh) used in Alis hosue in 1 day.b) Calculate the cost of electrical energy usage in Alis house in one week if the cost per unit is as follows:First 100 units: 22 cent per unitEvery additional unit: 26 cent per unit

Efficiency of electrical energy1. Efficiency =

2. If power output = power in put, then the electrical appliance has 100% efficiency.

3. For most electrical appliances, efficiency < 100% due to loss of useful input power as heat.

a) Filament lamp and fluorescent lamp Function is to produce light Both types produce heat as side products Fraction of electrical energy that is converted to heat = wastage Fluorescent lamp produce less heat, higher efficiency Fluorescent lamp - 4 to 6 times more efficient Efficiency of lamp =

A tungsten-filament lamp and a fluorescent lamp, each with a power rating of 40 W, produce 8 W and 38 W of light energy respectively. What is the efficiency of each lamp?

b) Hairdryer

Consists of fan and heating coils made from nichrome To increase efficiency, users can choose cool air, warm air or very hot air by switching the hairdryer

c) Comparing the efficiency of electric kettle and imersion heater

Both appliances consist of wires made from nichrome Efficiency of electric kettle is higher*has cover that prevents heat loss to surroundings Efficiency of immersion heater is lower*has long connecting wire that has resistance (waste)

Importance of using energy efficiently Depletion of major fuel supplies (natural gas, coal, oil) Only renewable energy (nuclear, hydroelectric, wind) can be used to generate energy constantly

Ways to improve efficiency of energy usage1. Better insulation or ventilation of buildings Reduce heat loss during cold weather Less fuels needed to keep interior temperature at comfortable level2. Combined heat and power Waste heat produced in the colling water can be channeled back to household building

3. Better use of electricity Use fossil fuels instead of electricity to heat homes and offices during cold weather(avoid loss of energy) Use electricity to run machinery, computer, domestic appliances and so on.

4. Change of lifestyle Use less electrical energy Use public transport Switch on fans and air-conditioners when neccessary Switch off unused electrical appliances Carry out maintenance service on electrical appliances from time to time (fridge, air-conditioners, water heaters) Clean air filters of air-conditioners every month Clean the cooling fins at the back of fridge every two months Wipe the surfaces of filament bulbs and fluorescent lamps from time to time