class 46 current and electricity - mr. gopie...

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Physics Current and Electricity

Mr Rishi Gopie

Physics by Mr R Gopie

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Current and Electricity Current and Charge

Current is the rate of a directed flow of charge carriers. In a metallic

conductor. Current is due to a directed flow of mobile (i.e. Free) electrons. The

direction of conventional current is that in which positive charge carriers would

move (if they could) and this is opposite to the direction in which the negative

charge carriers (such as electrons) would move.

The S.I. unit of electric current is the ampere and this is defined in terms

of forces exerted between two straight, parallel, current-‐carrying conductors-‐in

fact , this is the current flowing in each such conductor if they are 1 meter apart

and exert equal and opposite forces of magnitude 1N on one another. The unit of

electric charge is the coloumb (C) and this si defined as one ampere second

(since quantity of charge Q = current, I x t) or as the quantity of charge flowing

past a given point in one second when a steady current of one ampere is flowing.

D.C. and A.C. Current exists as direct current, d.c. and as alternating current a.c. D.C represents

a flow of current in one direction or sense only over time while a.c. represents a

flow of current in two opposite directions or senses over time, i.e. flow and

reversal of flow continuously.


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Consider examples of current (I) / Voltage (V) –time (t) graphs representing d.c. and a.c. d.c. On one side of the time axis

a.c. on both side of the time axis

Square wave or pulse d.c


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AC current


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Rectified waveforms

Once the variation is either completely above or completely below the time axis , it is d.c. Once the variation is both above and below the time axis , it is a.c. Consider sinusoidal a.c.

The period T is the time taken to complete one cycle.

The frequency, f, is the number of cycles per second.

Note: T = 1/f and f = 1/T The peak value or amplitude is the maximum value (of V or I) in either direction.


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Typical effects of an electric current include:

1) Heating effects

2) Magnetic effects

3) Chemical effects

Quantities, units, symbols and instruments’ of measurements. Quantity Typical

Symbol Unit & Typical Symbol

Instrument of Measurement

Comments

1) Unit Charge

q or c Coulomb -‐ C

2) Number of Charge carriers

N

3) Total Quantity of charge

Q Coulomb – C

4) Time t Second -‐ s Watch or clock

5) Current I Ampere-‐A Ammeter (or galvanometer)

6) Voltage or Potential Difference

V Volt -‐ V Voltmeter

7) Electromotive force (e.m.f)

E or ε Volt-‐V Voltmeter

8) Resistance R Ohm-‐ Ω Ohmmeter 9) Energy E Joule-‐ J (kWh) Joule meter 10) Work Joule-‐J Joule meter 11) Power P Watt-‐ W


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Equations 1) Q = nq (where q = e and e = 1.6 x 10-‐19 C) 2) Q = IT 3) R = V/I 4) V = IR 5) I = V/R 6) W = QV (where W is work is also electrical energy) 7) P = IV 8) P = I2R 9) P = V2/R 10) E = Pt 11) E = V2t/R

1 kWh = 1000W x 3600s = 3,600,000 J Circuits and Circuit components and their symbols


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Another common component is a potentiometer or potential divider – it is an

arrangement for tapping off a variable or fixed fraction of a fixed applied voltage.

Certain rheostats can be arranged to operate as potential dividers;

Potential Difference (p.d.) /Voltage

In order for current to flow through a component there must exist a

potential difference (i.e. p.d) or voltage across the component. The p.d. / voltage

between or across the ends of a conductor or component is the electrical energy

per unit charge converted to other forms of energy, i.e.

V = E/Q => E = VQ The unit of p.d. is the volt and it is defined as one joule per coulomb. The

maximum voltage that can be obtained between the terminals of an electrical

power supply, such as a cell, is called the electromotive force (i.e. supply) of the

power supply.


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Resistance All components in a circuit offer electrical resistance to the flow of

current-‐ some more than others. Certain components offer very low resistances

and examples of these are connecting wires, switches, power supplies and

ammeters. Other components offer much higher resistances and examples of

these are voltmeters and resistors (fixed and variable).

The resistance of a component such as a resistor in the form of a wire depends

directly on its length and inversely on its area of cross-‐section. Also , the

resistance depends on the nature of the material of which it is made-‐ for

instance, materials such as silver, gold , copper and aluminum have low

resistances. So the longer the specimen of a given material the greater its

resistance and the thinner the specimen, the greater its resistance. The reverse

of both of these ideas is also true. The resistance R , of a component can be

determined from the equation R = V/I, where v is the p.d./voltage applied across

the component and I is the current flowing through the component. The unit of

resistance is the ohm (Ω).

Resistors in Series and Parallel


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Note the following

1) I = I1 = I2 = I3, i.e. the same current flows through components in series.

2) V = V1 + V2 + V3 i.e. the total individual p.d. across components in series is the sum of the individual p.d.s.

3) R = R1 + R2 + R3, i.e. the total resistance of components in series is the sum of the individual resistances.

4) I = I1 + I2 + I3, i.e. the total current through components in parallel is the sum of the individual currents.

5) V = V1 = V2 = V3 , i.e. the total p.d. across components in parallel is the same as that across individual components.


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Ammeters and Voltmeters

An ammeter is an instrument for measuring the current through a

component and so it must be connected in series with the component. In fact, an

ammeter measures and indicates the current flowing through itself and it is

assumed that the same current flows through the component since it is in series

with the ammeter.


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It is essential that the resistance of the ammeter itself be very small

compared with their resistance in the circuit-‐otherwise inserting it into the

circuit will change the very current it is to measure. An ideal ammeter has an

extremely low (close to zero) resistance and hence an extremely low (close to

zero) p.d. across itself.

A voltmeter is an instrument used to measure p.d. (i.e. voltage) across a

component and so it is connected in parallel with the component. in fact , a

voltmeter measures and indicated the p.d. across itself and it is assumed that this

is the same p.d. across the component since it is in parallel with the voltmeter.

It is essential that the resistance of the voltmeter be very large compared with

any other resistance in the circuit ( especially the resistance of the component

across which it is connected) otherwise it will itself alter the very p.d. it is to

measure by drawing a significant current away from the component. So an ideal

voltmeter has an infinite (i.e. extremely high) resistance and hence draws a

negligible (almost zero) current.


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Ohm`s Law This law states that the p.d. applied across a metallic conductor is directly

proportional to the current through the conductor, provided that physical

conditions such as strain, temperature and illumination, remain constant, so V∝I

and V/I = a constant, i.e. the resistance, R, of the metallic conductor.

For a metallic conductor at constant temperature there is a linear relationship

between V and I and a graph of V against I, or I against V, (known as the V-‐I or I-‐V

characteristic), is a straight line through the origin (0,0). Conductors with such

V-‐I or I-‐V, graphs are known as ohmic conductors.

The slope of a V-‐I graph gives the resistance, R, of the conductor and that of a I-‐V

graph give the reciprocal of the resistance, 1/R, of the conductor.

Conductors, which do not have V-‐I or I-‐V graphs that are straight lines through

the origin are called non-‐ohmic conductors. Consider typical I-‐V characteristics

for both ohmic and non-‐ohmic conductors;

Ohmic Conductors

1) Metallic conductors (such as pure metals and alloys at constant temperature)

2) An aqueous solution of copper sulphate with copper electrodes


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Non-‐Ohmic conductors 1) Filament lamp/bulb

2) Carbon resistors

3) Semiconductor Diode


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Consider a typical circuit for investigating Ohm`s law and deriving a conductor V-‐I or I-‐V characteristic:

House Circuits

Within the house, the connecting cables (themselves insulated-‐

usually with white plastic) contain three insulated wires-‐ one of these is

the live wire, L, (covered with brown plastic insulation), another is the

neutral wire, N, (covered with blue plastic insulation), and the third is the

earth or ground wire, (covered with green/yellow plastic insulation),

which is earthed (i.e. grounded) at the house


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There are three single cables from the pole to the house – two live and

one neutral. Each live cables carries 110 V (r.m.s) a.c. and by using both

220V (r.m.s) a.c. can also be obtained. Most appliances require 110 V

(r.m.s) a.c. and the two live 110 V (r.m.s) a.c. wires share the distribution

of energy to these appliances. Certain appliances however, such as some

electric stoves, some dryers and air conditioners require 220V (r.ms.) a.c.

and for these, thicker connecting wires and special sockets (with

matching plugs) must be used.

With the house, a ring main circuit exists – with the live and

neutral wires running in two complete rings around the house. Circuits

are tapped off from these rings such that all circuits tapped off are in

parallel of each other and with the main supply. So each circuit/appliance

operates at the mains voltage.

The earth wire is a safety device to prevent an electrical shock to a

person in the event of this person touching the metal case or housing of

an appliance, which has made contact with a live wire. The earth wire

provides a safe alternative path (rather than through the person`s body)

for the current to the earth. In addition, the large current (due to the small

resistance), which flows in the earth wire, may cause the fuse (in the L

wire-‐E wire circuit) to blow and so break the circuit and turn of the

current-‐thus rendering that circuit safe.

A fuse or a circuit breaker is a safety device to minimize a

possibility of an overload (i.e. excess) of current flowing through a given


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appliance or circuit-‐such an occurrence can lead to i) appliance damage ii)

electrical fires.

A fuse can exist as a metal strip, which melts (i.e. “blows”). When a

current above a particular value (i.e. the fuse rating) flows through it acts

as a circuit breaker, which switches off when a current above a particular

value (i.e. the breaker rating) flows through it. The former type generates

on the heating effect of a current while the latter type operates on the

magnetic effect of a current.

The fuse or circuit breaker rating must be greater than the

operating current required, but a close as possible to this operating

current so that the fuse/circuit breaker will “blow” (i.e. melt) switch off

before overloading can occur.

Switches and fuses must always be placed in the live wire

Advantages of the parallel connection of domestic appliance include:

1) A malfunction of one appliance does not affect the operation of other

appliances.

2) Each appliance can be independently controlled

3) Each appliance can be operated at its rated power

4) All appliances operate at the same voltage-‐thus appliances can be

standardized with respect to operating voltage.


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Adverse effects of an incorrect or fluctuating supply to an appliance include:

1) Current surges which can cause overload and thus lead to i) damage to an

appliance ii) electrical fires

2) Current/voltage underload, which can result in an appliance operating

below its rated power, or even not operating at all.

Ways of reducing waste electrical energy include;

1) Switching off all appliances (e.g. lights) when not in use.

2) Ensuring that all heating /cooling appliances (e.g. electrical stoves

/refrigerators) are adequately insulated.

3) Operating appliances (such as air-‐conditioners) at their lowest possible

power rating that will still achieve the objective of using the appliance.

4) Adequately insulating all buildings/rooms that use air conditioners.

Electronics – the diode

A diode is a device with little or no resistance in one direction (i.e. forward bias)

and very high resistance in the opposite direction (i.e. reverse bias).


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A diode conducts in only one direction, i.e. when forward biased it will therefore

rectify a.c. i.e. convert a.c. to d.c. – the d.c. that results is referred to as half-‐wave

rectified a.c. A typical circuit giving this output is

Four diodes connected in a special circuit called a bridge rectifier can produce

full wave rectified a.c.


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class 46 current and electricity - mr. gopie...

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