class 46 current and electricity - mr. gopie class ·...
TRANSCRIPT
θωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµρτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψυιοπασδφγηϕκλζξ
Physics Current and Electricity
Mr Rishi Gopie
Physics by Mr R Gopie
2
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.
Physics by Mr R Gopie
3
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
Physics by Mr R Gopie
4
Physics by Mr R Gopie
5
AC current
Physics by Mr R Gopie
6
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.
Physics by Mr R Gopie
7
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
Physics by Mr R Gopie
8
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
Physics by Mr R Gopie
9
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.
Physics by Mr R Gopie
10
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
Physics by Mr R Gopie
11
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.
Physics by Mr R Gopie
12
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.
Physics by Mr R Gopie
13
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.
Physics by Mr R Gopie
14
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
Physics by Mr R Gopie
15
Non-‐Ohmic conductors 1) Filament lamp/bulb
2) Carbon resistors
3) Semiconductor Diode
Physics by Mr R Gopie
16
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
Physics by Mr R Gopie
17
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
Physics by Mr R Gopie
18
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.
Physics by Mr R Gopie
19
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).
Physics by Mr R Gopie
20
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.
Physics by Mr R Gopie
21