Electricity

Electricity At a Glance: Points to Remember

Electric Current: The flow of electric charge is known as electric current.
Electric current is carried by moving electrons through a conductor.
Electric current flows in the opposite direction of the movement of electrons.
Electric circuit: Electric circuit is a continuous and closed path of electric current.
Electric current is denoted by letter ‘I’.
Electric current is expressed by the rate of flow of electric charges, i.e. amount of charge flowing through a particular area in unit time.

SI unit of electric charge is coulomb (C).
One coulomb is nearly equal to 6 x 1018 electrons.
SI unit of electric current is ampere (A).
Ampere is the flow of electric charges through a surface at the rate of one coulomb per second.
1mA (milliampere)= 10^-3 A
1μA(microampere)=10^-6 A
Ammeter: An apparatus to measure electric current in a circuit.
Electric Potential: The amount of electric potential energy at a point is called electric potential.
Electric Potential difference: The difference in the amount of electric potential energy between two points in an electric circuit is called electric potential difference.
Electric potential difference is known as voltage, which is equal to the work done per unit charge to move the charge between two points against static electric field. It is denoted by ‘V’.

Where, W = work done and Q = Charge
SI unit of electric potential difference is volt and denoted by ‘V’.

Voltmeter: An apparatus to measure the potential difference or electric potential difference between two points in an electric circuit.
Ohm’s Law: The potential difference between two points is directly proportional to the electric current.

Where R is constant for the given conductor at a given temperature and is called resistance. Resistance is the property of conductor because of which it resists the flow of electric current through it.

SI Unit of resistance is ohm. Ohm is denoted by Greek letter ‘Ω’.
1 ohm (Ω) of Resistance (R) is equal to the flow of 1 A of current through a conductor between two points having potential difference equal to 1 V.

Component that is used to resist the flow of electric current in a circuit is called resistor.
Variable Resistance: The component of an electric circuit which is used to regulate the current; without changing the voltage from the source; is called variable resistance.
Rheostat: This is a device which is used in a circuit to
provide variable resistance.
Resistance in a conductor depends on nature, length and area of cross section of the conductor.
Resistance increases with increase in length of the conductor.
Resistance decreases with increase in thickness of conductor.

Where ρ (rho) is the proportionality constant. It is called the electrical resistivity of the material of conductors.
SI unit of resistivity (ρ) is Ωm
Resistors in Series: When resistors are joined from end to end, it is called in series.
The total resistance of the system is equal to the sum of the resistance of all the resistors connected in series in the system.

Resistors in parallel: When resistors are joined in parallel, the reciprocal of total resistance of the system is equal to the sum of reciprocal of the resistance of resistors.

When electric current is supplied to a purely resistive conductor, the energy of electric current is dissipated entirely in the form of heat and as a result, resistor gets heated. The heating of resistor because of dissipation of electrical energy is commonly known as Heating Effect of Electric Current.
Joule’s Law of Heating: Heat produced in a resistor is directly proportional to the square of current given to the resistor, directly proportional to the resistance for a given current and directly proportional to the time for which the current is flowing through the resistor.

Where, H is heat produced, I is electric current to the circuit, R is the resistance, t is the time.
Heating effect of electric current is used in electric bulb, electric heater, electric iron, etc.
SI unit of electric power is watt (W).
1W = 1 volt x 1 ampere = 1V x 1A
1 kilo watt or 1kW = 1000 W
Consumption of electricity (electric energy) is measures generally in kilo watt.
Unit of electric energy is kilo watt hour (kWh)
1 kWh = 1000 watt X 1 hour = 1000 W x 3600 s ⇒ 1kWh = 3.6 x 106 watt second = 3.6 x 106 J
An electric fuse is a device which is used to limit the current in an electric circuit. The fuse safeguards the circuit and the electrical appliances from being damaged.
The fuse wire is generally an alloy of lead and tin. It has a low melting point and breaks the circuit if the current exceeds a safe value. The thickness and length of the fuse wire depends on the maximum current allowed through the circuit.
It is connected in series in the beginning of the electric circuits.
Electric fuses are always connected in series in an electric circuit. Why?
When the circuit current exceeds a specified value due to voltage fluctuations or short-circuiting, the fuse wire gets heated and melts. Thus it breaks the connection as shown in the figure and no current flows. This prevents damage to the appliance.
P=VI
P/V=I