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We are learning about:
Competencies:
Deduce the relation among potential difference, current and resistance to verify Ohm’s law.
Analyse the I-V graph for different types of conductors and relate its uses in different electronic gadgets.
Scope:
Flow of electric current, resistance and resistors, potential drop, Ohm's Law, heating effect of current).
Objectives:
Construct the relation between electric current and potential difference using appropriate analogy/computer assisted tools/simulation/mobile apps.
Devise an experiment to construct a relationship amongst current, potential difference, and resistance.
Interpret the graph of ohmic and non-ohmic conductors using simulations
Describe the heating effect of electric current and its application in a range of devices to calculate its electric power.
VIDEO LESSONS
NOTES:
1. Electric circuit:
Conductors: Conductors are materials that allows electrons to flow freely from particle to particle. Conductors have lots of free electrons that can flow. More the number of free electrons, better conductor it is.
Example: metals, aqueous solutions of salts.
Aluminum whose atomic number is 13, has 3 free electrons in the valence shell of every atom of Aluminum. Therefore, aluminum is a good conductor.
Insulators: Insulators are materials that obstruct the free flow of electrons from atom to atom and molecule to molecule. Insulators do not conduct electricity since they don’t possess free electrons.
Examples: plastics, paper, rubber, glass and dry air.
Potential difference and electric current:
Electric current is conducted due to the flow of charges in a conductor. Electric current is denoted by letter ( I )
Electric current (I) is the rate of flow of charge (Q) across any cross-section of a conductor.
I=Q/t - where I is current, Q is charge and t is the time.
Current (I) is measured in ampere (A)
The difference in potential can be maintained in electric circuit by connecting a cell or connecting to the mains supply
The potential difference between the two terminals of a cell in an open circuit is called electromotive force or emf (E)
The potential difference between the terminals of the cell in closed circuit is called terminal voltage (V)
The terminal voltage is usually less than the emf of the cell.
Internal resistance (r) of a cell is obstruction offered to the flow of electric current by the electrolyte of the cell.
E=V + Ir
Potential difference is measured in volt (V). Potential difference is referred as voltage.
If one joule of work is done in moving a charge of one coulomb between two points, the potential difference between two points is said to be one volt.
Voltage is defined as work done per unit charge or energy transferred per unit charge.
Voltage(V)=(Work done (W))/(Charge (Q))
Power:
Power is the rate of doing work. In case of electrical circuits, the work is done in moving the charge through the circuit from one terminal to another terminal of the battery or source of emf. Therefore,
Power (P)=Work done on charge(W)/time(t)
Work done (W) = Charge (Q) ×Voltage (V)
P=QV/t
P = VI
Electrical power is the product of voltage and electric current.
Resistance and Resistors:
The opposition offered to the flow of electric current in a conductor is called resistance (R).
Resistance of a conductor depends upon the following:
nature of material of the conductor: Copper and aluminium have lots of free electrons that can move and conduct electric current and offer less resistance as compared to iron and nickel.
length of the conductor: resistance of a wire increases with the increase in length.
temperature of the conductor: The resistance of a conducting wire increases with increase in temperature of the conducting wire.
thickness of the conductor: The resistance is found to decrease with increase in thickness of a conducting wire.
Resistance is measured in ohm and it is denoted by the symbol Ω (omega).
One of the best ways to offer external or additional resistance in a circuit is by using a device called resistor. Resistors are one of the main components of all the electronic appliances and electric circuits. They are made up of various materials like carbon or graphite, nickel-chromium alloy, nickel, tin oxide, etc.
Resistors: Resistors offers additional resistance which controls the flow of current in a circuit.
Potential drop: Potential drop, also known as Voltage drop is defined as the amount of voltage loss in a circuit due to resistance.
Voltage drop results in: poor efficiency, wastage of energy, damage of equipment, fire, accidents,
To reduce voltage drop, the simplest technique used is by increasing the diameter of the conductor.
Ohm's Law relation
Ohm's Law
Ohm's Law
Ohm’s Law: The current flowing through a conductor is directly proportional to the potential difference applied across its ends, temperature and pressure remaining constant.
V=IR
I=V/R
I=V/R
Resistance offered by a conductor is a ratio of potential difference applied across its terminals to the electric current flowing through the conductor.
Current-voltage graphs
Current-voltage graphs
Types of conductors
The conductors which obey Ohm’s law are called ohmic conductors.
The conductors which do not obey Ohm’s law are called non-ohmic conductors.
Different devices have different resistance characteristics. These can be investigated using graphs that show:
current on the vertical axis
potential difference on the horizontal axis
These are known as current-voltage graphs or I-V graphs.
We can determine the behaviour of a conductor by the nature of the graph. The linear graph shows the behaviour of Ohmic conductors and Non-Ohmic conductor is represented by a non-linear graph.
a) I-V graph for a resistor at constant temperature:
The current increases linearly with the increase in voltage. Therefore, it obeys Ohm’s law.
The potential drop across a resistor is uniform at a constant temperature as the potential drop depends upon the magnitude of resistance in a circuit which is relatively fixed.
Uses: In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines
b) I-V graph for a filament lamp:
With the increase in the current, the filament becomes hotter and the collision between the electrons increases. Thus, there is corresponding increase in the resistance of the filament and the graph is flattened at higher current.
The current- voltage graph for a filament lamp is not linear and hence does not obey Ohm’s law.
c) I-V graph for a diode:
A diode is an electronic device which allows electric current to flow through it in only one direction. The I-V graph for a diode is not linear and hence a diode does not obey Ohm’s law.
Uses: Diodes can be used as rectifiers, signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulators, and oscillators.
d) I-V graph for a thermistor:
Thermistors are temperature sensitive resistors. Thermistors are made up of semiconductor material with a resistance that is especially sensitive to temperature. Since the current- voltage graph for a thermistor is not a straight line, it also does not obey Ohm’s law.
Uses: Thermistors are used as temperature sensors. They can be found in every day appliances such as fire alarms, ovens and refrigerators. They are also used in digital thermometers and in many automotive applications to measure temperature.
e) I-V graph for LDR:
A photoresistor or light-dependent resistor (LDR) or photocell is a light-controlled variable resistor. LDR has high resistance in the absence of light and low resistance in the presence of light.
The current- voltage graph for an LDR does not obey Ohm’s law as the resistance does not remain constant.
An LDR or light dependent resistor is also known as photo resistor, photocell, photoconductor. It is a one type of resistor whose resistance varies depending on the amount of light falling on its surface. When the light falls on the resistor, then the resistance changes. These resistors are often used in many circuits where it is required to sense the presence of light. These resistors have a variety of functions and resistance. For instance, when the LDR is in darkness, then it can be used to turn ON a light or to turn OFF a light when it is in the light.
Uses: alarm clocks, street lights, light intensity meters, burglar alarm circuits.
Heating effect of current: When an electric current is passed through a conductor, the conductor becomes hot after some time and produce heat. This happens due to the conversion of some electric energy passing through the conductor into heat energy. The electrons collide with other electrons in the course of their motion. During the collisions, some of the kinetic energy transforms into heat energy This effect of electric current is called heating effect of current.
Applications of Heating Effect of Current
The heating effect of current is used in various electrical heating appliances such as electric bulb, electric iron, room heaters, geysers, electric fuse etc.
The heating effect of current was studied experimentally by Joule in 1941. After doing his experiments Joule came to the conclusion that the heat produced in a conductor is directly proportional to the product of square of current (I^2), resistance of the conductor (R) and the time (t) for which current is passed. Thus,
H =I^2Rt
Exercise Questions
Fill in the Blanks
The current through an electric circuit will decrease if the _______________across the circuit decreases, keeping the resistance of the circuit constant
Potential difference is measured in ________________
Electric current is conducted due to the flow of ___________ in a conductor.
Electric current flows from region of __________ potential to ______________ potential.
When one joule of work is done in moving a charge of one coulomb between two points, the potential difference between two points is _________________
The opposition offered to the flow of electric current in a conductor is called___________
The device used to obstruct or regulate the passage of electric current is called a _______________
The amount of voltage loss in an electric circuit due to resistance is called _____________
The conductors which obeys ohm's law are called _________________
The device which allows electric current to flow through it in only one direction is a ___________________
In rice cooker the resistance wire have ______________ resistance and ____________ melting point.
The rate at which energy is being consumed by the appliance is also called _____________
Multiple Choice Questions
Take this quiz question: Try to score full mark by attempting multiple times. Then copy the question and the correct answer in your exercise book.
Long answer questions
Q1. Observe the circuit diagram given below and find out what is wrong with the circuit diagram. Draw the circuit diagram correctly.
Q2. The relationship between current (I), voltage (V), and resistance R was established by German physicist Georg Simon Ohm in 1827. However, Ohm’s law is not a universal law. Differentiate between ohmic conductor and non-ohmic conductor.
Q3. You have a 24 Volt battery and placed across a 8 ohm resistor, what will the current be?
Q4. Find the resistance of a filament of a lamp rated at 240 V and 60 W.
Q5. An air-conditioner in Gelephu operates on a 240 V circuit and draws 15 A current. Determine the power rating of such a conditioner.
Q6. What would happen to its resistance when the length of a conductor is doubled by keeping its thickness same. Explain with supporting relationship between length and resistance of a conductor.
Q7. State Ohm’s law. Is this law obeyed by all types of conductors? Give examples to support your answer.
Q8. State the factors affecting resistance of a conductor.
Q9. All electrical appliances when used for a longer time get heated. What could be the reason for this?
Q10. Copper is better conductor than aluminium. Why is aluminium wires preferred over copper wires in long distance transmission of electricity?
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