Electricity

the main junior cert lesson page on electricity is here

Static Electricity Electric Charge

Protons and electrons of atoms have a property called electric charge that produces a force field. The type of charge on the proton is given the title of positive The electron has an opposite type of charge, negative, The strength of the protons positive charge is equal in strength of the electron’s charge.

A proton has an electric charge of +1; an electron’s charge is –1. Atoms are neutral because they have the same number of protons and electrons. The electron (-) is the only particle in an atom that moves.

Law of Attraction and Repulsion of Electric Charge

Like charges repel and unlike charges attract.

(+) « » (+) (+)» «(-) (-) « » (-)

Neutral objects become charged by gaining or losing electrons.

Charging By Friction

Investigation: using simple materials to generate static electricity.

Gain Electrons: become negatively charged. Electrons (-) > Protons (+)

Lose Electrons: become positively charged. Electrons (-) < Protons (+)

(a) Rub a perspex rod with dry fur such as wool.

The perspex becomes positively charged.

The fur negatively charged.

Electrons transferred from the perspex to the fur.

The perspex goes positive as it lost electrons.

The fur goes negative because it gained elecrons. Remember: only the electrons move and electrons are negative

(b) Rub a polythere rod with dry cotton.

The polythene becomes negatively charged.

The cotton positively charged.

Electrons transferred from the cotton to the polythene.

The polythene goes negative as it gained electrons.

The cotton goes positive because it lost elecrons. Again: only the electrons move and electrons are negative (-).

Charging involves the movement of electrons from one object to another.

The object that gains the electrons becomes negatively charged.

The object that loses the electrons becomes positively charged.

Static Electricity is a form of energy due to an electric field produced by a stationary electric charge.

Perspex, fur, polythene and cotton are electrical insulators. Electrical insulators do not allow electric charges to flow through them. Therefore even when charged insulators are held by hand they stay charged. They cannot lose their charge because electrons cannot flow into them from the Earth or away from them to the Earth.

Demonstration of the Forces Between Charged Objects

Positively charge two perspex rods by rubbing each with fur. Negatively charge two polythene rods by rubbing each with dry cotton. Suspend a positive rod by a dry thread so that it hangs horizontal. In the hand bring the second positive rod close to one end of the hanging positive rod and record what happens. Bring a negative rod close to one end of the hanging positive rod and record what happens. Repeat for a hanging negative rod.

Conclusion: like charges repel, unlike charges attract

Effects of Static Electricity

1. Similarly charged objects repel.

2. Objects of opposite charge attract.

3. Charged object attract neutral ones.

a) Charged rod attracting tiny pieces of paper, dry hair, sawdust, deflecting a water from the tap

b) Gluing a charged balloon to the wall.

4. Can cause a spark – a sudden flow of electric current due to a discharge of static electricity.

Lightning is a giant spark heating the air to 30,000°C and traveling up at 140,000 kms-1 & down at 1,600 kms-1.

Earthing is the connection of a conductor to the Earth by way of another conductor. A charged conductor that is electrically insulated from the Earth will stay charged. It will lose its charge if it is then connected to the Earth by another conductor. A positively charged conductor will gain electrons from the Earth becoming neutral. A negatively charged conductor will lose electrons to the Earth becoming neutral. Earthing prevents the charging of the conductor object. The outer metal casing of some electrical appliances is earthed. This is to prevent the outer casing staying charged if an electrical fault occurs. The outer casing would become charged if the live wire made contact with it. Because of its low resistance a large current will flow along the earth wire to Earth. This large current breaks the circuit by tripping the circuit breaker open or melting the fuse - a current cannot flow in a broken circuit. The metal casing is neutral as current is no longer flowing into it.

Electrical Conductors and Insulators

Electrical conductors allow electric charges to flow through them e.g. metals and graphite are good conductors as they allow electrons (-) to flow easily through them. Ionic solutions and melts are good conductors because the positive and negative ions can flow in the liquid. An electrical insulator does not allow the flow of electric charges through them e.g. wood, rubber, plastic (perspex, polythene), fur, glass, paper,.

Uses of Static Electricity

Fingerprinting, spray painting, photocopying, crop spraying and in the removal of smoke from chimneys.

Current Electricity and Voltage

Electricity is a form of energy & can be converted to other forms of energy.

a) Electrical to Kinetic: electric motor turning a fan.

b) Electrical to Heat: electric fire and heating element in a kettle.

c) Electrical to Light: light bulb.

d) Electrical to Magnetic: electromagnet.

e) Electrical to Chemical: recharging a car battery.

Other forms of energy can be converted to electricity.

a) Kinetic to Electrical: wind turbine, dynamo and hydroelectric stations.

b) Light to Electrical: solar cell.

c) Chemical to Electrical: simple cell, dry cell and battery.

Electricity is a Very Versatile Form of Energy

Electricity is the most useful form of energy in today’s technological world.

1. Electricity is easily converted to many other needed forms of energy.

2. Electricity is easily transferred from its source to where it is needed.

3. Electricity is easily formed from other sources of energy.

Electric Current

An electric current is a flow of electric charge or electrons. In order for an electric current to flow it needs

(a) complete pathway (or circuit) of conductors,

(b) potential difference (voltage) – difference in potential from one end of the circuit to the other end.

Test Electrical Conduction in a Variety of Materials and Classify Each Material as a Conductor or Insulator.

On one conducting wire from a battery terminal have a switch and a light bulb. Connect the positive terminal of a battery to one end of the material by conducting wire. Connect the negative terminal of the battery to the other end of the material. Close the switch. Record whether the bulb lights or not.

Bulb lights. → Conductor Bulb does not light. → Insulator:

Direct and Alternating Current

Direct Current (d.c) The electric charges flow continuously in one direction only. D.C. is produced by batteries.

Alternating Current (a.c.)The electric charges flow in one direction for a short time, stops and then flows in the opposite direction for a short time and then the cycle repeats. The number of cycles per second is the frequency and in measure in Hertz. (Hz) In Ireland the frequency is 50 Hertz – the current flows in one direction for 0.01 seconds, stops and flows in the opposite direction for 0.01 seconds – each cycle is one fiftieth of a second.

Alternating current is produced by a.c. generators, also known as alternators, where kinetic energy is converted to electrical energy. A bicycle dynamo is an example of an alternator. At power stations electricity for industry, business and homes is produced by very large alternators.

Advantages of Alternating Current

1. Alternating current is much easier and cheaper to produce.

2. It is very easy to convert to d.c. if a direct current is needed to run an appliance.

3. The voltage of an a.c. supply is easily changed; stepped up or stepped down.

4. It is much easier and cheaper to transfer. (Less wasted as heat)

Measuring Electric Current

Electric Current Symbol: I Unit: Ampere (Amp) Measuring Instrument: Ammeter

Ammeter Symbol: a Ammeter Position: ‘in series’ i.e on the same path as the electrical appliance.

Potential Difference or Voltage Unit: Volt Measuring Instrument: Voltmeter

Voltmeter Symbol: Circle with a V in it. Voltmeter Position: ‘in parallel’

Potential difference is also known as voltage. The voltage in the mains supply in Ireland is 230 volts a.c.

Measuring Resistance of Metallic Conductors

Resistance measures the opposition the object offers to the flow of electricity.

Resistance Symbol: R Unit: ohm Ω Measuring Instrument: ohmmeter

ohmmeter Position: ‘in series’ Conductors have low resistanace. Insulators have high resistance.

Set Up a Simple Electric Circuit Using Appropriate Instruments to Measure

a) Currrent

b) Potential Difference (Voltage)

c) Resistance

Establish the Relationship Between Current, Potential Difference and Resistance.

Set up an electric circuit – battery, conducting wire, switch, rheostat (variable resistor), ammeter and resistor in series; a voltmeter is placed in parallel to the resistor.

  1. Set the rheostat at maximum resistance.
  2. Close the switch and record the current and voltage.
  3. Use the rheostat to obtain a different current – record the new current and voltage.
  4. Continue to use the rheostat to obtain new currents and new voltages.
  5. Record all the data in a chart and graph the results putting voltage on the x-axis.

There is a direct relationship between voltage and current: V/I = constant. The value of the constant is a direct indication of the resistance of the ‘conductor’. Therefore the relationship between V, I and R is: V/I = R

Calculate the resistance of a conductor if the current flowing in it is 3 A and the potential difference across the ends of the conductor is 12 V.

R = V/I

= 12 V

3 A = 4 Ω

Calculate the current flowing in a conductor if its resistance is 5 Ω and the potential difference across the ends of the conductor is 30 V.

I = V/R

= 30 V

5 Ω = 6 A

Calculate the voltage across a conductor if its resistance is 4 Ω and the current flowing through it is 2 A.

V = R x I

= 4 Ω x 2 A

= 8 V

Effects of an Electric Current

An electric current cannot be seen. The moving charges, electrons or ions, are too small to be seen.

An electric current can be suspected by noticing its classical effects – heating, magnetic, chemical.

Heating Effect of an Electric Current

a) A electric ‘heating element’ is connected to a battery but the switch is open. Measure the temperature of the water. Close the switch. The circuit is complete so an electric current will flow. Note the change in the thermometer: the mercury thread expands along the thermometer. The temperature of the water is increasing.

Therefore the electric current is heating the water.

A fuse is a deliberate ‘weak link’ for safety in an electric circuit and it is designed to break the circuit stopping the flow of current if the current has gone too high. The higher the current the greater is the heating effect.

Excessive current overheats and melts the wire in the fuse.The wire is no longer intact, the circuit is broken and

current stops flowing. Fuse wire is made of low melting point metal.

Other Everyday Examples of The Heating Effect

1. Heating Water In The Home: electric kettle, immersion heater, electric shower, washing machine,.

2. Electric fire, electric cooker, electric toaster electric iron, electric light bulbs etc

The Chemical Effect of an Electric Current

Connect two graphite carbon electrodes to a battery. The connection of the insulated wires to the electrodes must be waterproof. Place the electrodes into acidulated water and cover each with a test tube of water. Close the switch. The circuit is complete so an electric current will flow. Note the colourless gas bubbles given off at the graphite electrodes. Note that twice as much gas is collected in one tube – the tube over the anode, the positive electrode.

Test the gases. The gas in the anode tube relights a glowing splint – it is oxygen.

The gas over the cathode tube burns with a pop – it is hydrogen.

The electric current has broken water (H2O) into hydrogen and oxygen. The cathode is the negative electrode.

Hydro-gen O+ygen

Everyday Applications of the Chemical Effect

1. Electroplating: putting a fine layer of metal on materials that are electrical conductors.

Plating with silver, chromium, tin and copper plating to protect or enhance the beauty of objects.

2. Extraction of metals from their ores. 3. Electro-painting e.g. of cars.

The Magnetic Effect of an Electric Current

Place a plotting compass beside a long straight conducting wire that is connected to a battery and the switch is open so an electric current is not flowing. Note the position of the magnetic needle in the magnetic compass.

Close the switch. The circuit is complete so an electric current will flow.

Note: the compass needle is pointing in a different direction.

Open the switch – the current stops. The compass needle swings back to pointing north again.

Electric Circuits

Electric Current is a flow of electric charge. In order for an electric current to flow it needs

(a) complete pathway (or circuit) of electrical conductors,

(b) potential difference (voltage) – difference in potential from one end of the circuit to the other end;

Series Circuit offers only one path for the electric current. All the conductors are connected one after the other.

Cheap christmas tree lights are example. A circuit in a torch is another. The current is the same at all points in a series circuit. If a series circuit has two bulbs the current flowing through each is the same and only half the current if only one bulb was in the circuit. If one bulb should ‘blow’ then the circuit is broken and the current stops so the other bulb stops lighting.

Parallel Circuit There are two or more paths for the electric current. The current in the different paths may be the same or different. If a parallel circuit has two bulbs in parallel then each bulb receives as much current as on bulb in a series circuit – this is twice the current that a bulb gets if there a two bulbs in series. If a bulb ‘blows’ in one path the other bulb keeps lighting.

Function of a Switch `Complete circuit when closes and so an electric current will flow in the circuit. When open it breaks the circuit and the electric current stops flowing.

Electricity in the Home

The Mains Supply is a large underground and insulated electric cable containing two thick insulated wires, the live wire (L) and the neutral wire (N). The neutral wire is earthed, usually at the local substation, where the supply voltage neutral wire. Parallel circuits run from the consumer unit and supply electricity to all parts of the home. Each parallel circuit is protected by a fuse or a circuit breaker.

Electrical Safety in the Home

Fuses and Circuit Breakers are safety devices to break the circuit if the current goes too high. Excessive current causes the wire in the fuse melts, the circuit is broken and the current stops flowing. Modern consumer units contain circuit breakers. Circuit breakers work much faster than fuses & when the fault has been corrected are reset by pressing a switch commonly known as ‘trip switches’ and test regularly by pressing the test button.

Correct Wiring of a Plug

A special parallel circuit in the home is a ring main circuit or loop line. An electrical appliance can tap into this circuit when their fused flat 3-pin plug is inserted into a power socket that is normally mounted on a wall. A ring main circuit may have up to ten power sockets. When the plug is inserted a connection is made between the live and neutral wire of the supply and so current will flow through the appliance when switched on.

Insulation Colour of Wires

Earth (E): yellow-green Live (L): brown Neutral (N): blue Note: the fuse is on the live wire.

The colours have been chosen so they can be easily distinguished by normal sighted and by colour-blind people.

The earth wire in the power socket is usually connected to a naked metal water pipe in the house and so it is earthed. Make sure you can label a diagram of a correctly wired plug.

Power Rating of Electrical Appliances

Power is the ‘work rate’ or work done per second. Power is the amount of energy converted to other forms of energy every second. Power is measured in joules per second, Watts. One watt = one joule of energy converted to other forms of energy each second. 100 W stamped on a bulb means that 100 joules of electrical energy are converted to other forms of energy each second (provided it is connected to the correct voltage supply).

Rating: the power of the appliance in watts or kilowatts. The power (number of Watts(W)) of an electrical appliance is indicated on the outer surface of the appliance along with other electrical features such as the required voltage. The rating is only true at the voltage mentioned on the appliance.

The Rating of Some Household Electrical Appliances

Appliances of different rating will use different amount of energy in the same time. The higher the rating the more electrical energy they use each minute. The higher the rating the greater the cost of running the appliance

The Kilowatt-hour is the unit of energy used by electricity companies to calculate ‘your’ bill (kWh).

The energy in kilowatt hour is the same as 1,000 watts for an hour, Which is 1,000 joules every second for one hour =1,000 J/s x 3,600 s = 3,600,000 joules or 3.6 million joules.

Calculating Costs of Using Domestic Appliances

First: Calculating the number of ESB Units i.e. kilowatt-hours

Equation: ESB Units = Rating (kW) x Time (hours)is reduced to 230V. The electricity supply is alternating current (a.c.). All fuses and switches must be on the live wire for safety. A main fuse is on the live wire in the mains supply cable.

A special meter measures the quantity of electrical energy supplied to the home. The energy unit is the kilowatt-hours (kWh). The mains cable then passes on to the consumer unit that is also known as a distribution box or fuse box. The consumer unit contains a special main switch which when open breaks both the live wire and

Question: Calculate the number of Units used if an electric fire rated at 3 kilowatts was on for 5 hours.

Answer: ESB Units = Rating (kW) x Time (hours)

= 3 kW x 5 h

= 15 kWh

Question: Calculate the number of Units used if a television rated at 200 watts was on for 12 hours.

Answer: ESB Units = Rating (kW) x Time (hours)

= (200/1,000) kW x 12 h

= 0.2 kW x 12 h

= 2.4 kWh

Next: Calculating the cost of using an electrical appliance.

Equation: Total Cost = Rating (kW) x Time (h) x Unit Cost (c)

Question: Calculate the cost of using an electric fire rated at 3 kilowatts, on for 5 hours at 13 c per unit.

Answer: Total Cost = Rating (kW) x Time (h) x Unit Cost (c)

= 3 kW x 5 h x 13 c/kWh

= (3 x 5 x 13) c

= 195 c

Electric Bill

The Number of Units (kWh) used is calculated by makings two readings of the meter. The difference in these readings is the number of units used in the last payment period. The cost per unit (1kWh = 12.76c is on the bill and so the bill is easily calculated. Payment Due = Difference in Last Two Readings x Unit Cost