ElectricityElectric charge

Conductors and insulators Electric fields

Current, voltage, and resistanceSeries and parallel circuits

Electrical powerMains electricity

Calculating electrical energy

Bogota at night, it is so bright that it can even be seen from space

Electric charge• Electric charge or “electricity” can come from battery

and generators. But some materials become charged when they are rubbed- electrostatic charge.

• Negative and positive charges- Experiments suggest that there are two different and opposite types of el. Charge: positive +, and negative -.

• Where charges come from?• Atoms have electric charges inside them.• A simple model of the atom: there is a central nucleus

made up of protons and neutrons, and electrons orbiting around.

• Nb.el.=nb. Pr…… net charge=0

Conductors and insulators

• Conductors are materials that let electrons pass through them. These free electrons also make metals good thermal conductors. (metals)

• Insulators are materials that hardly conduct at all. Their electrons are tightly held to atoms and are not free to move.

• Semiconductors- These are ”in-between” materials. They are poor conductor when cold, but much better conductor when warm.

Attraction of uncharged objects

• A charged object will attract any uncharged object close to it.

• Earthing- if enough charge builds up on something, el may be pulled through the air and cause sparks- it is dangerous. When earthing that object( to the ground), charge flows away.

• Induced charges-Charges that appear on an uncharged object because of a charged object nearby.

Unit of charge

• The SI unit of charge is the coulomb (C).• 1C=to charge on about 6 million million

million electrons.• Often it is more convenient to measure charge

in microcoulombs• 1microcoulomb (μC)=10^-6C• 1electron charge=-1.6 x 10^-19C• 1 proton charge=+1.6 x 10^-19C

Using electrostatic charge

• Electrostatic precipitators- are fitted to the chimneys of some power stations and factories.

• Industrial inkjet printers• Photocopiers (p:173)

Questions

• 1.What charge does an electron have, + or -?• 2.Would two electrons attract or repel one another?• 3.Two identical metal spheres are placed close to one

another. One is given a large negative charge. The two are then connected by a wire. Use the idea of electric force to explain what happens next.

• 4.How many electrons make 1C of charge?• Example:• The charge on a rubbed polythene rod is, typically,

only about 0.005 μC

Electric field

• We have electric field around the charges.• We use lines with arrows on them, to

represent electric fields.• the field lines always point away from +,and

towards – (p: 174 pictures)

Curves, points, and ions

• When a conductor is charged up, like charges repel, so they collect on the outside.

• The charges are most concentrated near the sharpest curve, electric field is strongest and the field lines are closest together.slike

• At this point electric field is strong enough to ionize the air. Q p175

• If this point touches Van de Graaff generator, charge immediately leaks away from it.

Van de Graaff generator

Ions

• Ions are electrically charged atoms, or group of atoms.• Atoms become ions if they lose or gain electrons.• Most of the molecules in air are uncharged, but not all.• Flames, air movement, natural radiation can all

remove electrons from molecules and ions are formed. Ions can recombine with any free electrons around and become neutral again.

• Normally air is neutral, but with ions air becomes a conductor. In a thunderstorm, the concentrations of different ions may be so great that a very high current may flow through the air, causing a flash of lightning

Current in a simple circuit

• The conducting path through the bulb, wires, switch, and battery is called a circuit.

• There must be a complete circuit for the electrons to flow.

• Turning the switch OFF breaks the circuit and stops the flow.

Measuring current

• A flow of charge is called an electric current.• The higher the current, the greater the flow of

charge.• SI unit of current is the ampere (A).• Current can be measured by connecting an

ammeter into the circuit.• For smaller current, a miliammeter is used.• 1A=1000mA

Charge and current• There is a link between charge and current• Charge=current x time• C=A x s• 1C/s=1A• 1C is the charge that passes when a current of 1A flows for

1s.• The ampere is one of the SI base units.• Current direction- conventional current direction from + to –

round the circuit. • Being negatively charged, electrons are repelled by negative

charge, so are pushed out of the negative terminal of the battery

Potential difference

• PD (voltage) across a cell-the cell has a voltage marked on it. The higher its voltage, the more energy it gives to the electrons pushed out.

• The scientific name for voltage is potential difference. PD can be measured by connecting a voltmeter.

• 1V=1J/1C…1V represents 1J of potential energy given to each coulomb of charge.

Cells in series

• To produce a higher PD, several cells can be connected in series.

• PDs around a circuit.• The electrons flow through two bulbs, they lose

some of potential energy in the first bulb. The rest in the second.

• Moving round a circuit, from one battery terminal to the other, the sum of the PDs across the components is equal to the PD across the battery.

Components in parallel• There is another way to connect two lamps to a

cell. Place them side by side, you can see that both lamps shine brightly.- they are in parallel

• When two lamps are connected in series, they are dim.

• “in series”-”end-to- end”• “in parallel”-”side- by- side”• Current in parallel circuit, each lamp gets its own

share of the current. This shows that it is easier for the current to flow.

Resistance• Equation:• Resistance(Ω)=PD across conductor (V)/current through conductor

(A)• SI unit of resistance is the ohm (Ω)• Some factors effecting resistance:• 1)Length- doubling the length of a wire doubles its resistance• 2)Cross-sectional area: Thin wire has more resistance then a thick

one.• 3)Material: a nichrome wire has more resistance than a copper wire

of the same size.• 4)Temperature: For metal conductors, resistance increases with

temperature.• For semiconductors, it decreases with temperature.

Resistance components

• Resistors: In simple circuits, they reduce the current. In more complicated circuits, such as those in radio, TV, computers, they keep current and PDs at the same level.

• Variable resistors (rheostats): used for varying current.• Thermistors: have a high resistance when cold but

much lower resistance when hot.• Light-dependent resistors (LDRs): have a high

resistance in the dark but a low resistance in the light• Diodes: have an extremely high resistance in one

direction but low resistance in the other.

Resistance• V, I, R equations:• R=V/I R- resistance; V- PD; I-current• V=IR• I=V/R• Ohm’s law• The current is proportional to the PD.• Experiment results shows:• Graph of current against PD is a straight line through

the zero point• If the PD doubles, the current doubles• PD/current, always has the same value

More about resistance factors• 1) A and B copper wires have the same cross-sectional

area and temperature, but• B has2 x length of A• B has 2 x resistance of A• 2) C has 2 x cross-sectional area of B• C has ½ x resistance of B (p:184)• Proportionality problems:• R=ρ x l/A ρ- constant for the material resistivity • If wires A and B are made from the same material, ρ is

the same, and we can do:• resistanceA x areaA/lengthA=res.B x

ar.B/le.B

Series and parallel circuits

• (p:186)• Bulbs in series:• Bulbs share the PD-dimly• If the bulb is removed the other goes out- circuit

is broken• Bulbs in parallel:• Each gets the full PD• If one bulb is removed, the other keeps working-

it is part of unbroken circuit.

Basic circuit rules

• In series:• Current through each of the component is the

same• The total PD is the sum of the PDs across each of

them• In parallel:• PD across each of the components is the same.• The total current in the circuit is the sum of the

currents in the branches.

Cell arrangements

• If cells are connected in series, the total PD is the sum of the individual PDs

• PD across parallel cells is only the same as from one cell, but together the cells can deliver a higher current.

Combined resistance of resistors in series and in parallel

• In series:• R=R1+R2

• In parallel:• 1/R=1/R1+1/R2• R=R1 x R2/R1+R2• (work p:189)

Electrical power

• Power=energy transformed/ time taken • (Example: If the battery is supplying 5 joules of

energy every second, power is 5 watts.)• In symbols: P=VI (equation)• Units: W=VA ( 1kW=1000watts)• Power is the rate at which energy is transformed

( changed from one form to another)• Appliances have a power rating marked on them.• (Work p: 190, 191)

Power dissipated in a resistor• When a current flows through a resistor, it has a heating

effect. Electrons lose potential energy, which is changed into thermal energy.

• Scientifically speaking, energy is dissipated in the resistor.• Another useful version of the electrical power equation:• Power=PD x current• But: PD= current x resistance• power= current x resistance x current P=I^2R(resistors have resistance measured in ohms-Ω)(work p:191)

Mains electricity (1)

DC (one-way; direct current)•AC (alternating current; mains current)•It flows backwards and forwards, backwards and forwards…. 50 times per second, in some countries. The mains frequency is 50Hz- hertz.•In other countries, the mains frequency is 60Hz.AC is easier to generate than DC ( like that from a battery).

Live, neutral wire, switch, earth, double insulation, plugs

• Live-or hot, or active wire, alternately negative and positive, making the current flow backwards and forwards through the circuit.

• Neutral (or cold) wire- it is kept at zero voltage by the electricity supply company.

• Switch- fitted in the live wire. Because, if it would be in neutral wire, live wire would be live with switch off.

• Fuse- this is a thin piece of wire which overheats and melts if the current is too high. Similar to the switch.(circuit breaker instead of a fuse.)

• Earth- wire. This is a safety wire. It connects the metal body of things, to earth.

• Double insulation-some appliances do not have an earth wire. Case is made of plastic.

Plugs• Plugs are safe and simple way of connecting appliances

to the mains.• - plugs with two metal pins- live and neutral, earth

connection made by two metal contacts at the edge.• - some plugs have a third pin for the earth connection• A few countries use a three-pin plug with a fuse inside• Fuse value:• Example:• Kettle: 2300W, 230V• Current=power/voltage=2300W/230V=10A• So a 13A fuse is needed

Mains electricity-2• Circuits around the house• The electricity supply company's cable into each house

contains a live and a neutral wire.• In the consumer unit, these wires branch into several

parallel circuits; it also contains an earth wire.• In the consumer unit (fuse box, breaker box) – circuit

breaker is an automatic switch when the current rises above the specified value.

• For extra safety, some circuit can be fitted with a residual current device (RCD)

• Mains sockets- is protected by its own fuse or circuit breaker in the consumer unit (p:194)

• Two- way switches (p:195)

Safety first

• Old frayed wiring.• Long extension leads• Water in sockets or plugs• Accidentally cutting cables

• If an accident happens, and someone is electrocuted, you must switch off at the socket and pull out the plug before giving any help.

Electrical energy calculations• Power=energy transformed/ time taken [W]=[J]/[s]• Power=PD x current• [W]=[V][A]• Calculating energy• Energy transformed=power x time• [J]=[W][s]• Energy transformed=PD x current x time• [J]=[V][A][s]• Measuring energy in kilowatt-hours• Energy supplied=power x time• [kWh]=[kW][h] • Example: 1kW is 1000W; 1h is 3600s• So if 1kW appliance is used for 1h• Energy supplied= 1000W x 3600s= 3 600 000 J• 1kWh=3 600 000J

Central electricity• The “electricity meter” in a house is an

energy meter. The more energy you take, the more you have to pay.

• Example:

• energy supplied=42935kWh-41710kWh=1225kWh=1225units

• Cost of energy supplied=1225 x 10=12250cu

Present Meter reading

PreviousMeter reading

Units used Cost per unit (cu)INCL tax

Cost(cu)

42935 41710 1225 10 12250

• Example 1:• If energy is 10cu per unit, what is the cost of

running a 2kW heater for 3 hours?• Energy supplied=power x time=2kW x 3h=6 units• Total cost=6 x 10cu= 60cu• Example 2:• If energy costs 10cu per unit, what is the cost of

running a 100W lamp for 30 minutes?• 100W=0,1kW• 30min=0,5h• Energy supplied=power x time=0,1kW x 0,5h=0,05 unit• Total cost= 0,05 x 10cu= 0,5cu• Work (p:197,198,199)