O Level Revision : Combined Science - Science in Energy Uses - Electrical Energy

Electrostatics

• It is the study of static electricity (stationary electric charges).
• Electrostatics is associated with electrical charges that are static (i.e. at rest).
• Effects of electrostatics are observable in dry insulators. This is because charges inside insulators are not allowed to move significant distances. If there is any movement the displacement is negligible.
• Electrostatics is the study of electrical charges at rest.

Concept of charge

• Electrical charge is a property associated with sub-atomic particles (refer to information on atoms).
• There are two types of charges namely positive (caused by protons located in the nucleus) and negative (caused by electrons in orbits/shells around the nucleus).
• In an atom the number of protons is equal to number of electrons-thus making an atom neutral
• All matter is composed of neutral atoms.
• A material is said to be charged if there has been transfer of electric charge. The transfer causes an imbalance of the charges.
• The electrons are the atomic particles that move to create charge.
• Negative charge is the result of acquiring electrons and positive charge the result of losing electrons.

Conductors and insulators

• In electrical terms materials are classified either as conductors or insulators.
• Conductors are materials that allow electrical charges to pass through them. Metals are good examples of conductors.
• Metals have free electrons that can easily be transferred through the entire metal.
• Insulators on the contrary have firmly bound electrons. Insulators therefore do not permit passage of electrical charges through them. Examples of insulators include plastics, rubber, wood, paper, glass and many other non-metals.
• Such materials are good insulators when they are dry.

Attraction and repulsion

Experiments and activities can be performed to illustrate behavior of charged bodies. a)    Plastic rule(r) —pieces of paper activity

The rule is charged by rubbing against a cloth but the papers are neutral. Pieces of paper are attracted to the rule(r) because there is charge redistribution in the paper as shown in Fig. 13.1 below

Plastic ruler attracting pieces of paper

b) Detecting charges

Electrical charge can be detected using a device called a gold leaf electroscope.

Gold leaf electroscope

Earthed metal cage

When the device is neutral, the flexible gold leaf hangs in a vertical position. If charged body is brought closer to the rod, the gold leaf is elevated to an angle Ø. The change in angle indicates attraction or repulsion of charged bodies.

1. Attraction →   In the presence of a neutral or oppositely charged body.
2. Repulsion →   In the presence of a body with a charge similar to the electroscope.

Note: Repulsionistheonlywaytobesureofthechargeofabody!Repulsiononlyoccurs between like charges.

• Silk or woolen clothes produce sparks or crackling sounds when rubbed by hands or against dry hair.
• Dust or dry grass particles are attracted to dry iron-pressed polyester trousers in dry season.

Methods of charging

a) Rubbing

• When two insulators are rubbed against each other, the heat generated is changed to kinetic energy that excites surface electrons.
• This energy causes transfer of the electrons between the two materials.
• Donor material becomes positive, an example is PERSPEX.
• Acceptor material becomes negative, an example is POLYTHENE.

b) Induction

• When a charged insulator is brought near another neutral insulator, charge redistribution occurs due to either attraction or repulsion of charges.
• If a connection to earth is permitted bodies become charged.

Lightning

• Lightning is a natural example of effects of electrostatically charged clouds that will be discharging.
• Clouds (made of water vapour) rub against each other just before a storm.
• The rubbing results in transfer of electrons and creation of positive or negative clouds.
• Heavy and charged clouds descend towards the Earth and charges virtually “jump” across the air gap (from ground to a positive cloud or from a negative cloud towards the Earth).
• A spark and sound are simultaneously produced in the process.
• The light is observed first because light waves travel faster than sound waves.
• The lightning is basically associated with accumulation of large amount of charges and therefore high voltage.

Dangers of lightning

The discharge process occurs via the tallest object in the vicinity and its dangers include;

• electrocution,
• fires,
• destruction of property, and
• death.

Precautions

During thunder storms:

(i)   Do not be the tallest target – do not go on high land/hill. (ii)  Do not seek shelter under a tree.

(iii) Disconnect roof-top TV aerials.

(iv) Do not go near water sources or metal fences. (v)  Construct lightning conductors.

Current electricity

• Current electricity results from moving charges.
• Current electricity can be made to do work.
• Voltage is the work done per unit charge moved between any two points in a circuit.
• It is the amount of other forms of energy converted to electrical energy per unit charge moved in a circuit.
• Voltage is calculated using the formulae below:

Voltage  =               Work     
                       Charge moved

V           =            W    
                            Q ,  

where V = voltage, W = work done, Q = charge moved

Units are joules per Coulomb J/C, also called a volt, symbol V

• Cells, batteries and generators are sources of voltage (also called electromotive force, emf).
• P.D. is referred to when electrical energy is converted to other forms. For example, across a resistor in a circuit. Voltage and potential difference (p.d) are measured by a very high resistance meter called a voltmeter.
• A voltmeter is always connected in PARALLEL/ACROSS an electrical component, e.g. across a lamp, resistor or a battery.

Electric current

• Electric current is the rate of flow of electrical charges.
• Current is the amount of electrical charges moving per unit time.
• Current is calculated as :

Current  =           Charge 
                             Time

• Units are C/s  =   Cs-1

I =   Q   
        T ,

where Q = Charge in Coulombs, T = time in seconds.

• I is measured by a low resistance meter called anAmmeter, symbol    A    . Sl units of current are Amperes, shortened as amps or just A.
• 1A =  1Cs-1
• An ammeter is always connected in SERIES with the power source.
• What is conventional current?
• NB: In all circuits and discussions, reference will be made to conventional current.

Resistance

• It is the opposition to flow of electric current in a circuit.
• Resistance is defined also as the ratio between Voltage and Current R = V/1.
• This relation between voltage and current was established by a scientist called George Ohm, hence resistance has units of Ohms (Ω).
• Ohm’s law states that ‘Vo1tage is directly proportional to current, if temperature (or resistance) is constant.’
• Conductor materials that are specifically meant to offer resistance to flow of charges are called resistors.
• These can be connected in any manner series, parallel or combination.

A circuit

**  Rheostat = variable resistor- regulates current flowing in the circuit.

**  The ammeter is in series with and  the Voltmeter in parallel to/across the fixed

resistor whose value is to be found

**  The switch is only closed to take readings and immediately opened so that temperature of resistor does not increase.

The triangle below summarizes the relationship among the three quantities.

     V

I           R

Circuits

(i)    V =   IR

(ii)   I   =     V   R

(iii)  R  =     V I

• A circuit is a complete connection of conductors that permit electrical charges to flow.
• There are two types of circuits namely series and parallel.
• The two can-be compared as shown in Table 13.1 below:

Differences between series and parallel series

Cells

• Electrical cells are sources of voltage. In cells chemical energy is converted to electrical energy.
• A simple cell consists of;
  • two electrodes
  • an electrolyte.
• Electrodes are the terminals made from two different metals from the reactivity series.
• Carbon can also be used as an electrode since it’s a non-metal that conducts electricity.
• The two electrodes are the ANODE and CATHODE.
• Metals high up in the series form cathodes while bottom metals form anodes.
• An electrolyte is an ionic compound in solution form or paste form that conducts electricity.
• Salt solutions and diluted acids can be used as electrolytes.

The simple cell

A simple cell.

• In a simple cell, voltage decreases with time.
• This is due to polarization. Polarization is the accumulation of gas bubbles on electrodes.
• Hydrogen gas collects on the cathode when an acid electrolyte is used.
• The gas accumulates and acts as an insulator which reduces conduction and hence voltage of a cell.
• Magnitude of voltage can also be varied by altering the difference between metals on the
• electrochemical (reactivity) series.
• An electrochemical series is simply an arrangement or order of metals according to the electrode potentials.

 Electrochemical series

• Primary cell cannot be recharged.
• Secondary cells are rechargeable.

Dry cell

• A cell phone and car battery consist of secondary cells.

Water that is not distilled may contain salts. These salts may react with sulphuric acid and contaminate the electrolyte.

Secondary cell (lead acid accumulator)

• To recharge a secondary cell its positive is connected to the positive of the charger and negative to negative of the charger - i.e. in parallel with the charger.
• Electrolyte level in a car battery should be checked regularly and topped up using distilled water only.
• Acar battery should not be kept in a discharged state for too long otherwise plates will become ‘sulphated’ and unable to recharge.

Photovoltaic cell

• These are solar cells that change light energy to electrical energy.
• Solar cells and any other type of cells can be connected in series to produce high voltage or in parallel to deliver a high current.