The most important relationship in electrical science is that between the applied e.m.f or p.d and the current established by it. This relationship is known as Ohm’s law and it was discovered by Georg ohm in 1826. George ohm who was the first scientist to develop the idea of resistance, carried out experiments with different metal wires to discover how the current through each depended on the p.d. applied across its ends. If a test is conducted, in which a circuit is setup with an ammeter and a voltmeter and if a metal is add to the circuit then a graph is drawn of the voltage against current values, and we notice and realize the three points mentioned below:-
- The graph is a straight line passing thorough the origins.
- When we double the voltage, the current is automatically doubled.
- Dividing the p.d. by the current always gives the same value which is 5V/A .
It is important to realize that such graphs apply only to certain metals. Metals, which produce such graphs, are called ohmic conductors. This is because temperature does not affect the rate of resistance in the metal specimen. Experiments with other wires produce similar results. These can be summed up in a law now known as Ohm’s law, which states:
“The current flowing through a metal conductor is directly proportional to the p.d across its ends provided the temperature and other physical conditions remain constant.”
The current passed by a wire in a circuit was proportional to the p.d across the wire., so for a wire, p.d./current is a constant provided the temperature stays the same.
There are generally two types of metals, ohmic and non-ohmic. A conductor with a resistance, which changes with current, is a non-ohmic conductor. For example, a diode is a component which allows current to pass easily in one direction but not in the opposite direction and there
The conditions for ohmic conductors is that the temperature should always be kept constant and if it is not constant it will lose its ohmic characteristics. It is also important to note that for ohmic conductors, that current which is input to the wire must be kept at a minimal value and any increase in the value of current will cause heat to be produced and lead to an increase in resistance. If this happens then the metals will lose its ohmic characteristics. The heat which is produced due to an increase in current is known as the heating effect.. When more current is added, the will be more chance for these charge carriers to bump against the metals atoms which causes vibrations of the atoms. As the atoms vibrate they release the potential energy that they have in the form of kinetic energy. The atoms, in turn take in the heat, as energy, and they vibrate even more causing more heat to produce. The amount of heat produced will then reach a point before it stops. The vibration of the atoms impedes the movement of the charge carriers, and thus resistance increases which means that a metal is no longer ohmic, and it does not act like an ohmic conductor.
Ohms Law is a mathematical equation that shows the relationship between Voltage, Current and Resistance in an electrical circuit. It is stated as:
V = I x R
R = V / I
I = V / R
Where
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V = Voltage
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I = Current (I stands for INTENSITY)
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R = Resistance
Current is the movement of electrons in a circuit .. Current is the movement of electrons in a conductor. Current is measure in amps and voltage in volts. While current is measured using an ammeter, voltage is measured using a voltmeter. A current in a metal was actually a flow of negatively charged particles, called electrons moving in the opposite direction, but it is still normal practice to use the conventional direction from +ve to –ve terminal of the cell. The p.d. or voltage across the terminals of the battery indicates the potential energy given to each coloumb of charge pushed out.
This is a piece of metal which is made up of ‘atoms’.
Each atom is made up of a nucleus that has a certain number of protons and neutrons with an equal numbers of electrons in orbit.
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Notice that in the copper atom, the outside shell has only one electron. The outer shell of any atom is called the valence shell. When the valence electron in any atom gains sufficient energy from some outside force, it can break away from the atom and become what is called a free electron.
The following picture shows a complete circuit that the electrons can follow to produce current.
The circuit symbol for a resistor is:
Some devices have a constant resistance and are known as Ohmic resistors. In others, the flow of current may affect the internal structure of the device and so alter its resistance. When this occurs, the ratio of V/I is not constant and so the graph of Voltage vs Current for the device is not a straight line passing through the origin.
A length of wire for Ohmic conductors:- the graph is a straight line through the origin, so the current is proportional to the voltage. Hence the resistance (=V/I) is constant showing that the wire is an Ohmic conductor. Instead of taking one pair of values for Voltage and current, it is better to take a series of value before plotting voltage against current graph. The resistance of the wire can be obtained from the graph.
Filament lamp :-
The graph of I against V becomes less and less steep as the current increases from zero. So the value of V/I increases as the current increases. Hence the resistance of the filament increases with increasing temperature. The filament becomes hotter as its current increases. Like any other metal, the filament lamps resistance increases with increased temperature. The graph below proves this:-
The third graph shows about diode and that the diode conducts in one direction only. This direction, called the forward direction, allows conduction much more easily than in the reverse direction. In the reverse direction, a diode has a very high resistance. Below shows the nature of the graph in which current is plotted against current.
Semiconductors, of which silicon and germanium are the two best known, are insulators if they are very pure, especially at low temperatures. Their conductivity increases at high temperatures. This is the opposite to the case of metals. In metals the number of electrons available for conduction, i.e. to carry current, is fixed. As the temperature increases the resistance increases due to increased vibration of the atoms making electron flow more difficult. In semiconductors (and carbon) this effect is more than the ‘freeing’ of more charges for conduction as the temperature increases. As a result the resistance decreases at higher temperatures.
The resistance of some semi conducting materials decreases when the intensity of light falling on them increases. This property is made use of in light dependent resistors, LDRs. The I—V graph for an LDR is similar to that of a thermistor.
The action of an LDR depends on the fact that the resistance of the semiconductor cadmium sulphide decreases as the intensity of the light falling on it increases. When the light from a lamp falls on the ‘window’ of the LDR, its resistance decreases and the increased current lights the lamp. LDRs are used in photographic exposure meters. The Light Dependent Resistor (or LDR) is the input or sensing part of the circuit. The resistance between the LDR legs changes according to how much light falls on the surface of the the LDR. When it is in the dark the resistance is high so not much current can flow . When it is in the light the resistance is low and the current can flow more.
The resistance of an LDR decreases as the intensity( brightness of the light falling upon increases). In Bright light an LDR has a low resistance. In the dark an LDR has a high resistance.
The LDR drops its resistance when light shines onto it. Commonly, the LDR is made of a voltage divider circuit so that its change in resistance translates to a change in the output voltage. The thermistor acts in much the same way, except that it responds to a temperature increase by dropping its resistance.
In the circuits above, an increase in light intensity or an increase in temperature result in VOUT also increasing. If it is desired that VOUT decreases with light or heat, the transducer can be made the other half of the voltage divider.
In general, an increase of temperature increases the resistance of metals, as for the filament lamp but decreases the resistance of semiconductors. The resistance of most thermistors decreases if their temperature rises,i.e. their I---V graph bends up. If a resistor and a thermistor are connected , then the voltage across the resistor increases as the temperature of the thermistor increases; the circuit can be used to monitor temperature, for example in a car radiator. Thermistor contains semi conducting metallic oxides whose resistance decreases when the temperature rises due to heating the thermistor directly or to passing a current through it. The graph shown below proves this.
The resistance of a thermistor decreases as the temperature rises. At low temperatures a thermistor has high resistance and at high temperatures the thermistor would have low resistance.
Because of the very large number of atoms that interact in a solid material, the energy levels are so closely spaced that they form bands. The highest energy filled band, which is similar to the highest occupied molecular orbital in a molecule (HOMO), is called the valence band. The next higher band, which is similar to the lowest unoccupied molecular orbital (LUMO) in a molecule, is called the conduction band. The energy separation between these bands is called the energy gap, Eg.
energy bands and gaps:-
The filling of these bands and the size of the energy gap determine if a material is a conductor (a metal), a semiconductor, or an insulator. In metals there is no energy gap between filled and unfilled energy levels. A significant number of electrons are thermally kept into empty levels, creating holes in the filled band. The electrons in a conduction band and the holes in a valence band can move throughout the material, allowing it to easily conduct electricity. In semiconductors Eg is small, but large enough so that a fairly small number of electrons are in the conduction band due to thermal energy, and these materials conduct poorly. In insulators Eg is large so that electrons are not promoted to the conduction band due to thermal energy, and these materials do not conduct electricity.