- Heating Effect: Current through a resistor produces heat, calculated by H = I²Rt or H = VIt (Joule’s Law).
- Heat Applications: Used in devices like irons, heaters, bulbs, and fuses (to stop excess current).
- Power Unit: 1 kWh = 3.6 × 10⁶ J, called 1 unit of electrical energy in electricity bills.
- Short Circuit & Fuse: A short circuit causes a large current; the fuse wire melts to break the circuit and prevent fire.
- MCBs: Miniature Circuit Breakers automatically cut off power during overload or fault conditions.
Definitions [17]
Definition: Current
Current is defined as the rate of flow of charge.
Define the following:
Super conductors
Substances whose resistance decreases tremendously with decreasing temperature and reaches nearly zero near absolute zero are called superconductors; e.g., lead, tin, etc.
Define the following:
Semiconductors
Semiconductors: Substances whose resistance decreases with the increase in temperature are named as semiconductors. E.g. manganin, constantan etc.
Define an electric current.
An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit.
The magnitude of an electric current is the number of electric charges flowing through a conductor in one second.
Define the following:
Electromotive force
Electromotive force: When no current is drawn from a cell, when the cell is in open circuit, the potential difference between the terminals of the cell is called its electromotive force (or e.m.f.).
Define the following:
Conventional current
The movement of the positive charge is called conventional current.
Define the unit of current.
The unit of electric current is ampere (A). When 1 C of charge flows through a conductor in 1 s, it called 1 ampere (A) current.
I = `Q/t`
Define the term resistivity.
The resistivity of a material is the resistance of a wire of that material of unit length and unit area of cross-section.
Definition: Electric Circuit
A continuous and closed path of an electric current is called an electric circuit.
Define the following:
Coulomb
One coulomb is the amount of electric charge transferred by a current of one ampere in one second.
Definition: Conductance
The reciprocal of resistance is called conductllnce. It is denoted by the letter G.
Define one ohm.
One ohm is the resistance of a component when the potential difference of one volt applied across the component drives a current of one ampere through it.
Define temperature coefficient of resistance.
The temperature coefficient is defined as the ratio of the increase in resistivity per degree rise in temperature to its resistivity at T0.
Define Current density.
Current density is a vector quantity, often known as an area vector or cross-sectional area vector, whose value is equal to the electric current flowing per unit area.
J = `"I"/"A"`
S.I unit is A/m2.
Definition: Heating effect of electric current
When a resistor is connected in an electrical circuit, heat is produced in it due to the current. This is known as the heating effect of current.
Define fuse.
Electric fuse is a safety device which is used in household wiring and in many appliances.
Define the following:
Electrolyte
The solution through which the electricity passes is called an electrolyte.
Formulae [1]
Write the mathematical expression for Joule’s law of heating.
The mathematical expression of Joule’s Law of heating is: H = I2 Rt
Where,
H = Produced Heat
I = Current flowing through the device
t = Time taken
r = Resistance of the appliance
Theorems and Laws [2]
State Ohm’s law. Is it always true?
According to Ohm’s law, the current flowing in a conductor is directly proportional to the potential difference across its ends, provided the physical conditions and temperature of the conductor remain constant.
No, it is not always true. E.g., Diode valve, junction diode, etc., do not obey Ohm’s law.
Law: Ohm's Law
Statement: Ohm’s Law
"The electric current flowing through a conductor is directly proportional to the potential difference across its ends, provided the temperature and other physical conditions of the conductor remain constant."
Mathematically,
I ∝ V or V = I R
where:
- V = Potential difference (in volts)
- I = Current (in amperes)
- R = Resistance of the conductor (in ohms, Ω)
Explanation:
When two conductors at different electric potentials are joined by a metallic wire, electrons flow from the conductor at a lower potential (excess electrons) to the one at a higher potential (deficit of electrons). This movement of electrons results in an electric current.
- The current continues to flow until both conductors reach the same potential.
- For continuous current flow, a constant potential difference must be maintained across the ends of the conductor (e.g., using a battery or power supply).
Derivation / Mathematical Proof:
From Ohm’s Law:
I ∝ V ⇒ \[\frac {V}{I}\] = constant
This constant is defined as the resistance (R) of the conductor. Therefore,
V = I R ---(1)
This is the mathematical form of Ohm’s Law.
Special Case:
If the current I = 1 A, then:
V = R
This implies that the resistance of a conductor is numerically equal to the potential difference across it when 1 ampere of current flows through it.
Conclusion:
Ohm's Law provides a fundamental relationship between voltage, current, and resistance in an electric circuit. It is widely used in the design and analysis of electrical and electronic systems.
Key Points
Key Points: Electric Current
- Electricity is a convenient and controllable form of energy widely used in homes, industries, schools, and hospitals.
- Electric current is produced when electric charges flow through a conductor, and it flows only through a closed, continuous electric circuit.
- A switch completes or breaks the circuit; when the circuit is broken, current stops flowing, and devices like bulbs do not glow.
- Electric current is the rate of flow of charge, given by the relation I = Q / t, where Q is charge and t is time.
- In metallic wires, electrons are the charge carriers, but by convention, current flows from the positive to the negative terminal, in the opposite direction to electron flow.
Key Points: Heating Effect of Electric Current
