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7.2 Current And Voltage

Current,I

 

From ,Current ,I is the rate of flow of charge, Q 

 

Current = Charge

Time 

 

Or

 

clip_image001

 

The unit of current, I , is   Ampere (A)

1A= 1 C s-1

 

Current is said  “ A current flows through…….”

 

The current is measured by an ammeter and it must be connected  in series with the component  which we want to find the current through the component.

 

For example, if we want to measure the  current through a bulb : 

 

clip_image003

Potential difference (Voltage) , V

 

An electric circuit must have a cell or some source of electrical energy ,if there is to be a current.  The energy to move the electrons  or to produce current we call “ voltage  or potential difference” 

 

The potential difference across a conductor is work (energy)  to carry a unit  charge  

 

Potential difference = work = energy

charge          charge 

 

Or

 

clip_image004 

 

The unit  of potential difference,  V = Volt(V) 

 

1 V  =  1 J C-1

Potential difference is said  “ A  potential difference across……”

 

The potential difference is measured by a voltmeter  and it must be connected  in parallel with the component  which we want to find the potential difference between the component.

 

For example, if we want to measure the  difference across a bulb :

 

 

 

clip_image006 

 

 

 

Example 1 

 

A battery is switched on for a few moments. In that time 40 C pass through the battery and they carry away  240 J  of electrical energy . What is the potential difference across the battery? 

 

Solution 

 

 

 

Example 2 

 

A current of 8 A flowing through an electric heater for 50 minutes convert  5.76 X 10 6 J of electrical energy into heat energy.

Calculate

 

(a)           the total charge circulated through the heater

 

(b)           the potential difference across the heater

 

Solution

 

 

The relationship between potential difference,V and current,I in a metal conductor – Ohm’s Law

 

The  relationship between voltage, V and current ,I in a metal conductor is discovered by George  Ohm is called as Ohm’s law. 

 

Ohm’s law state” The potential across a metal conductor is directly proportional to the current flowing through the conductor , provided that its temperature remains constant.

 

clip_image008

Or   V a I  , if  T remains constant 

 

The experiment to investigate the relationship between the potential difference ,V and the current,I in a metal conductor 

 

Hypothesis:

 

The potential difference across a metal conductor

 

increases  as the current in the metal conductor increases.

 

Aim of the experiment :

 

To investigate the relationship between the potential difference ,V and current, I in a metal conductor

 

Variables in the experiment:

Manipulated variable: the current

 

Responding variable: the potential difference

Fixed variable: temperature

 

List of apparatus and materials:

 

Ammeter, voltmeter, rheostat, conductor wire, connection wires ,dry cells and switch.

Arrangement of the apparatus:

clip_image010 

 

The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

The switch is closed.

 

The reading of the voltmeter ,V and  the ammeter, I is recorded.

 

The experiment is repeated 5 times for with different  value of, I, by adjusting the rheostat

 

Tabulate the data:

 

I

 

 

 

 

 

 

 

 

 

 

 

 

 

V

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis the data:

Plot the graph  V against  I

 

clip_image012

 

 

The Ohmic conductors and non-Ohmic conductor

 

Conductors which obey the Ohm’s law is called Ohmic conductors.

 

clip_image014

 

clip_image016 

 

Resistance, R

Based on the Ohm’s law,

 

V a I

 

V = k I

 

k = V

 

I

 

k  is replaced by R

 

Hence definition of resistance is ; 

 

Resistance is the ratio of the potential difference to the current.

 

Or

Resistance = Potential difference

 

Current

 

Or

 

clip_image017

 

The S.I. unit is  Ohm (W) 

 

1 W =  1 V A-1

 

 

 

clip_image019

Note : For a metal conductor ,if the temperature and the other physical properties are remain unchanged , the resistance of the conductor not depend  on the current ,I and the potential difference, V.

 

The factors affect the resistance of a conductor 

 

(a)  The length of the conductor( l)

 

The resistance is directly proportional to the Length of the conductor

 

( R a I )

 

clip_image021

 

(b)    The cross-sectional area of the conductor (A)

The resistance is inversely proportional to  the cross-sectional area of the conductor

 

( R a 1 )

 

A

 

clip_image023

 

(c)     The type of  the  material  of the conductor

 

Different material of the conductors have the resistance .

 

clip_image025

 

For good conducting material such as silver and cooper have low resistance.

 

For good insulating material such as rubber has high resistance.

 

 

 

(d)    Temperature

 

For metal conductors such as cooper ,iron , the resistance is increases linearly to the temperature.

 

For alloys such as constantan and  nichrome  the temperature not affected the resistance.

 

For semiconductor materials such silicon and germanium  an also termistor the resistance decreases when its temperature is decreased.

 

 

clip_image027

Resistivity , ρ

 

R a I   and    R a 1

 

A

 

R a l Þ R = ρ l

 

A                          A

 

ρ   = RA

 

l

 

As ρ increases  ,  R increases.

 

 

S.W.G ( Standard Wire Gauge ) of a conductor  wire

 

When the number of S.W.G. higher the thickness of the wire decreases .

The experiment to investigate the relationship between the resistance and the length of  a metal conductor 

 

Hypothesis:

 

The resistance of metal conductor increases as the length increases

 

Aim of the experiment :

 

To investigate the relationship between the resistance and the length of  a metal conductor.

Variables in the experiment:

Manipulated variable: the length of the metal conductor

 

Responding variable: the resistance

Fixed variable: temperature, the cross-sectional area and type of material

 

List of apparatus and materials:

 

Ammeter, voltmeter, conductor wire, connection wires ,dry cells, metre rule and switch.

Arrangement of the apparatus:

clip_image029

The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

The length of the conductor wire is measured by a ruler = l

 

The switch is closed.

 

The reading of the voltmeter ,V and  the ammeter, I is recorded.

 

The resistance is calculated , R = V

 

I

 

The experiment is repeated 5 times for with different  length of the wire

 

Tabulate the data:

 

I

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis the data:

Plot the graph  R against  l

 

clip_image031

Types of resistor

 

Fixed resistor :

 

Common materials is used in fixed resistor are alloys such as nichrome and constantan.

 

clip_image033

 

Rheostat :

 

This has a coil  resistance wire wound a ceramic tube. A sliding contact ca be moved to any position along the coil. This changes the length of the wire and as a result the resistance changes.

 

clip_image035

Potentiometer

As voltage divider. When the length increases , the resistance increases and as the result the potential differences (voltage) increases .

 

clip_image037

Thermistor :

 

A  thermistor is a resistor whose resistance changes with temperature. Its decrease  in resistance as the temperature rises.

 

clip_image039

Light dependent resistor (LDR)

The resistance of LDR changes depends on the brightness of light falling on it. As the brightness of light falling on it increases the resistance decreases.

 

clip_image041

 

Superconductor 

 

Superconductor is a metal conductor decreases when its temperature  decreases.

 

The resistance decreases with temperature but the resistance suddenly becomes zero when it is cooled below a certain temperature called the critical temperature, tc

 

clip_image043

 

These materials offer no resistance to flow of current and act as perfect conductors with zero resistivity. Once the current is set up in these materials, its need no applied voltage to persist flowing without any loss..

 

The example of superconductor materials are aluminium (Tc= 1.2 K), Plumbum (Tc= 7.2 K), Niobium (Tc= 9.3 K), Nb3Sn (Tc= 18 K) , Nb3Ge (Tc= 23 K) and

 

YBa2Cu3O2 (Tc= 100 K).

 

Its used in transportation, medicine ,industry and army .

 

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