1.
Just as an object placed at different depths of
water experiences different water pressures, or, an object at different
distances from the centre of the earth carries different gravitational potential
energies,

**a charged particle**placed at**different points**of an**electric field**also**experiences different electric forces**acting on it and carries**different electric potential energies**.
2.
The

**potential difference**or**voltage**between 2 points in an electric field is defined as the**work done**in**moving 1 coulomb**of charge from 1 point to the other. Suppose, work, W is done to move a charge, Q from 1 point to another, the**potential difference, V**is given by:**Potential difference = Work done / Charge**;

**V = W / Q**

3.
The

**potential difference**(a.k.a. voltage) between 2 points is 1 volt if 1 joule of work is required to move a charge of 1 coulomb from 1 point to the other:
1 volt = 1 joule / 1 coulomb = 1 J C

^{-1}= 1 V (unit symbol for volt)
4.
Just as it is the pressure difference that
causes water to move from a region of higher pressure to a region of lower
pressure, or an object to move from high ground to low ground due to different
gravitational potential energies, a

**charged particle**too would**move**from a point of higher electric potential (+ve) to another point of lower electric potential (-ve) in an electric field or circuit –**due to the potential difference**between the 2 points.
5.
The

**movement of charged particles**from 1 point to another in an electric field or a circuit due to the potential difference between the 2 points produces an**electric current**. The**direction**of flow of current is by convention also from**positive**to**negative**. The rate of flow of the charge determines the size of the current. But what determines the rate of flow of the charge? Read on…
6.

**Ohm’s law**(by a German physics teacher, Georg Simon Ohm in 1826) states that the**current**flowing through an ohmic conductor**is directly proportional**to the**potential difference**across its ends provided that its temperature and other physical conditions (length, cross-sectional area and material type) remain constant:
I = Constant x V; or,

V = Another constant x I (This
other constant is known as the resistance, R)

**V = R x I = IR**(R is the gradient of a linear V-I graph for ohmic conductor); or

R = V/I (R is the ratio of instantaneous
voltage over current for all conductors); or

I = V/R (I is also the ratio of
voltage over resistance)

**(Yr 2010 SPM P1 Q38 at pg. 240)**
7.
Thus, the

**V-I graphs**:
a.
of all ohmic conductors are linear graphs which
pass through the origin and their gradients are the resistances R of the respective
ohmic conductors – the steeper the gradient, the higher the resistance;

b.
of non-ohmic conductors are non-linear which
pass through the origin and the ratio of V over I at any point of the V-I graph
gives the value of resistance at that point – the higher the ratio, the higher
the resistance.

**(Yr 2006 SPM P1 Q36 at pg. 57)**
8.

**Resistance, R**of a conductor is therefore defined as the**ratio**of the**potential difference**V across the conductor to the**current**I flowing through it. This applies to both ohmic and non-ohmic conductors. The unit of measurement of resistance R is therefore volt per ampere (V A^{-1}) or ohm (Ω).
9.
Factors that affect the resistance R of a
conductor are (experiments at pgs 357 ~ 362):

c.
Its

**length**, l – (directly proportional) the longer the length, the higher the resistance;
d.

**Cross-sectional area**,**A –**(inversely proportional)**the bigger the area, the lower the resistance;**
e.
The

**type of materia**l – resistivity ρ depends on material; and,
f.
Its

**temperature :**
· Most

**Pure metal -**(proportional): the higher the temperature, the higher the resistance.
·

**Alloys**(constantan, nichrome) – resistance increases slightly with temperature increases.
·

**Thermistor –**resistance decreases greatly with slight increase in temperature.
·

**Superconductor**– resistance becomes zero at critical low temperature.

**(Yr 2007 SPM P1 Q35 at pg. 99)**

**(Yr 2009 SPM P1 Q37 at pg. 194)**------------------------------------------------------------------------------------------

**More on Thermistor (***Negative Temperature Coefficient, Thermistor*)**Thermistors**work by translating temperature into resistance, with

**resistance decreasing**as

**temperature increases**(referred to as a '

**negative temperature coefficient**’, or

**NTC, thermistors**).

The

**graph**below illustrates the

**resistance of the thermistor**as

**a function of the temperature**:

**Thermistor Resistance**vs.**Temperature Graph**As can be seen from the graph, the

**resistance**of the thermistor

**drops very quickly**in the temperature range

**0°C to 40°C**- it offers good sensitivity to changes in temperature in this range; however, at much higher temperatures, it will be less sensitive to temperature changes.

10. The resistance

**R**of a resistor of a given material and at a given temperature can be calculated using this relationship:**R =****ρ****l/A,**where**R**is directly proportional to length l and**resistivity****ρ****of the resistor and is inversely proportional to cross-sectional area A of the same.****Resistivity****ρ****of a resistor at a given temperature is a constant dependent on the material of the resistor. Thus,****R =**

**ρ**

**l/A;**or,

**ρ**

**= RA/l**

11.

**Voltmeter**:
g.
It measures

**potential difference**or voltage in**volts**(V);
h.
It is connected

**in parallel**across the resistor or device;
i.
It has

**high resistance**so that the current flowing through it is negligible.
12.

**Ammeter**(or milliammeter):
j.
It measures

**current**in**amperes**(A) (or milliamperes, mA);
k.
It is connected

**in series**with the resistor or component;
l.
It has

**low resistance**so that its existence has insignificant effect on the magnitude of current flowing and to be measured.**13.**

**Measurement of Resistance:**To measure resistance, we usually take the reading of the voltmeter in volts across the resistor over the reading of the ammeter for the current flowing through the resistor. Thus,

**R = V/I**(Instead of using the formula:

**R =**

**ρ**

**l/A**, which we can use too if resistivity

**ρ**

**,**length

**l**and cross sectional area

**A**are all readily and accurately measurable or available)

**14.**

**Superconductors:**

m.

**What are superconductors?****Superconductors**are materials which**offer****no resistance**(i.e. zero resistance) to the flow of current**when**they are**cooled to****below**certain temperatures known as the**critical temperatures for superconductivity**.
n.

**Only some metals show superconductivity**, for examples:__Name of Elements__

__Critical Temperature (K)__

**Zinc**, Zn 0.88

**Aluminium**, Al 1.14

**Tin**, Sn 3.69

**Mercury**, Hg 4.15

**Lead**, Pb 7.26

**Niobium**, Nb 9.2

(

**Some of the best conductors**of electricity at normal temperature like copper, silver, gold**are not superconductors even at absolute zero temperature, 0 K**although their resistance R decreases with temperature)
o.
Once current flows

**in superconductors**, it needs**no further applied voltage**(electric energy per coulomb) to persist flowing – there is**no loss of current**.
p.

**Superconductors can produce**magnets with magnetic field strengths > 10 times that of the best normal electromagnets. These**superconducting magnets**are useful:
iii.
to produce

**computer chips which are faster and smaller**.
q.

**Superconducting wires**or cables**increase the efficiency of electrical power**transmission as loss of energy as heat is greatly reduced.
r.
Students must be able to recognize the R
(Resistance) – T (Temperature) graphs of normal conductors (pl see below for copper), NTC thermistor (pl see below), RTD (below) and those of
superconductors (pg. 365).

------------------------------------------------------------------------------

-------------------------------------------------------------------------------

------------------------------------------------------------------------------

-------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------

By: tutortan1@gmail.com (edited on 24/05/16)

## No comments:

Post a Comment