Current,I
From ,Current ,I is the rate of flow of charge, Q
Current = Charge
Time
Or
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 :
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
The unit of potential difference, V = Volt(V)
1 V = 1 J C1
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 :
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.
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:
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
The Ohmic conductors and nonOhmic conductor
Conductors which obey the Ohm’s law is called Ohmic conductors.
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
The S.I. unit is Ohm (W)
1 W = 1 V A^{1}
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 )
(b) The crosssectional area of the conductor (A)
The resistance is inversely proportional to the crosssectional area of the conductor
( R a 1 )
A
(c) The type of the material of the conductor
Different material of the conductors have the resistance .
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.
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 crosssectional 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:
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
Types of resistor
Fixed resistor :
Common materials is used in fixed resistor are alloys such as nichrome and constantan.
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.
Potentiometer
As voltage divider. When the length increases , the resistance increases and as the result the potential differences (voltage) increases .
Thermistor :
A thermistor is a resistor whose resistance changes with temperature. Its decrease in resistance as the temperature rises.
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.
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, t_{c}
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 (T_{c}= 1.2 K), Plumbum (T_{c}= 7.2 K), Niobium (T_{c}= 9.3 K), Nb_{3}Sn (T_{c}= 18 K) , Nb_{3}Ge (T_{c}= 23 K) and
YBa_{2}Cu_{3}O_{2 }(T_{c}= 100 K).
Its used in transportation, medicine ,industry and army .
Filed under: Fizik
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