Work And Energy – Energy
* Energy is defined as the capacity to do work.
* The various forms of energy include kinetic energy, potential energy, thermal or heat energy, electrical energy and chemical energy.
* Energy stored in an object due to its position or place is known as potential energy.
* The gravitational potential energy of an object is the work done in raising it from the ground to a certain point against gravity. It is calculated using the expression PE= mgh.
* The energy possessed in a moving object is called kinetic energy. It is calculated by the formula:
EK = 1/2 mv 2
Where,
m is the mass of the object
v is the velocity of the object
* Energy can be converted from one form to another.
* Law of conservation of energy: Energy can neither be created nor destroyed, but can change its form.
* Power (P) is the rate of doing work. It is calculated using the expression:
P =w/t
Where,
W is the work done
t is the time taken
* We express larger rates of energy transfer in kilowatts (kW).
Electricity – Current Electricity – Basics
* Electricity current is uniform flow of charges in a particular direction.
* The rate of flow of charge o uniform motion of electrons:
(I = W/t)
* Potential difference is the work done in moving a unit charge between two points in an electric field against the direction of force:
(v = W/Q )
* Electric resistance is the opposition to the flow of electrons by the conductor.
Factors affecting electric resistance are:
* The length of the conductor
* The cross sectional area of the conductor
* The resistivity of conductor material
* The temperature of the conductor
Thus, ( R = p (1/a))
* Resistivity of a material is the resistance offered by a conductor having unit length and unit area of cross section.
Ohm’s law states that the current passing through a conductor is :
* Direct proportional to the voltage in the conductor and
* Inversely proportional to the resistance of the conductor.
Thus, I = v/r
Electricity – Current Electricity – Circuits
* In parallel circuits:
V = V1 = V2 = V3
Itotal = I1 + I2 + I3
1/Req = 1/R1 + 1/R2 + 1/R3
Electricity – The Heating Effect of Electricity
* Heating effect of electricity: When current passes through a conductor, the conductor gets heated as electrical energy is converted into heat energy.
* A good heating element should have high resistivity, a high melting point and should show negligible variations in resistance due to temperature changes.
* An electric fuse is used to break the flow of electric current when the device is overloaded. Its mechanism is based on the heating effect of electricity.
* Joule’s laws: The amount of heat produces in a conductor is equal to the product of the amount of current squared, the resistance of the conductor, and the time of which current passes through the conductor.
Thus:
H = i2Rt
Electricity – The Magnetic Effects of Electricity
* When an electric current passes through a conductor, magnetic field is created around the conductor.
* A magnetic field is the extend of space surrounding a magnet where the magnet’s effect can be felt.
* Magnetic field lines represent the lines of action of the force acting on a unit North Pole placed in a magnetic field.
*
Maxwell’s right hand grip rule: Holding a conductor in the right hand with the fingers curled around it and the thumb stretched out:
* The thumb denotes the direction of the current flow.
* The direction of the curling of the fingers indicates the direction of the magnetic field lines.
* Maxwell’s corkscrew rule: On turning, if a right- handed corkscrew advances in the direction of current, then the direction of rotation of its head gives the direction of the magnetic field lines.
* A magnetic field caused by the current- carrying conductor consists of sets of concentric lines of force.
The direction of the magnetic field lines depends on the direction of the current passes through the conductor.
* A solenoid consists of an insulted conducting wire wound on a cylindrical tube made of plastic or cardboard.
* An electromagnet is a magnet made up of a coil of insulated wire wrapped around a soft iron core that is magnetised only when current flows through the wire.
Fleming’s left hand rule: When the fore finger, middle finger and thumb of the left hand are stretched such that they are at right angles to each other then:
* The fore finger gives the direction of the magnetic field.
* The middle finger points in the direction of the current.
* The thumb gives direction of the force acting on the current- carrying conductor placed in the external magnetic field.
An electric motor converts electrical energy into mechanical energy using the effect of electricity.
The Magnetic Effects of Electric Current – Electro Magnetic Induction
* Electromagnetic induction (EMI) is the process of generating an electromotive force by moving the conductor through a magnetic field.
* The electromotive force generated due to electromagnetic induction is called induces emf.
* The current due to induced emf is called induced current.
* Michael Faraday hypothesised that current induced in a coil is due to changing magnetic field.
Factors effecting EMI
* Direction of movement of the magnet.
* Orientation of the magnet
* Speed of movement of the magnet
* Strength of the magnet
* Cross sectional area if the conductor
* Faraday’s law states that the magnitude of the induced emf, E, in any closed circuit is directly proportional to the rate of the change of the magnetic flux, φ, through the circuit.
* Fleming’s right hand rule states that if the index finger points in the direction of the magnetic field and the thumb indicates the direction of the motion of the conductor, then the middle finger indicates the direction of the induced current flow in the conductor.
* An electric generator is used to convert mechanical energy, using electromagnetic induction.
* Alternating current (AC) is the current induced by an AC generator. AC current changes direction periodically.
* Direct current (DC) always flows in one direction, but its voltage may increase of decrease.
Electrical components and wires fitted in a household to supply electricity to various appliances form a domestic electric circuit.
* Domestic electric circuits are connected to this network through main supply cables, commonly known as mains.
* The main supply cable has two wires: Live wire and Neutral wire.
* Separate circuits may be used for power lines supplying 15A and 5 A current.
* Domestic electric circuits also have earth wires to save users from severe electric shocks.
* An electric fuse is a safely device used to protect an electric circuit against excessive current.
Electric Current and Its Effects – The Heating Effect of Electric Current
* When an electric current flows through a wire, it produces heat. This is known as the heating effects of electric current. The electric iron, electric room heater, the toaster, hair dryer, electric stove, geyser, immersion water heater and food warmer are some of the appliances that work on the heating effect of electric current.
* Coils of wire used for producing heat in the appliances are known as ‘heating elements’.
* The ‘ISI’ mark is granted by the Bureau of Indian Standards (BSI) to various products for quality. We should always buy appliances with an ‘ISI’ mark as it ensures that the appliance is safe and will ensure minimum wastage of energy.
The amount of heat produced in a wire by passing an electric current depends upon its:
* Length
* Thickness
* Duration of flow of current
* Nature of material
* An electric fuse is a safety device used to protect an electric appliance against excessive current.
* When the wire in fuse breaks, it is said to have blown.
Excessive current in a circuit may be caused by:
* Overload
* Short Circuit
* If the appliances are connected to the same socket, then the load on that socket increases as many appliances are drawing current from it, leading to an overload in the circuit.
* If, due to wear and tear, the insulation of two wires is broken, and these wires touch each other directly when carrying current, a short circuit may result.
* Overloading and Short circuits can cause a fire.
* Nowadays, Miniature Circuit Breakers (MCBs) are used in place of fuses.
* These are switches that switch off automatically when the current in a circuit exceed a certain maximum limit and turns off the current.
Electrical Energy and Power
* The potential difference (v) between points A and B in a conductor is expressed as: V = V (A) – V (B)
* The amount of charge ΔQ flowing from A to B in a time interval Δt is expressed as: ΔQ = I Δt
* A change in potential energy between points A and B is expressed as: ΔUpot = Final PE – Initial PE
* An increase in kinetic energy k OF the changes in the conductor is expressed as: Δk = ΔUpot
* The amount of energy dissipated is expressed as: ΔW = I ΔtV
* The energy dissipated per unit time is termed as the power dissipated represented by: P = IV
* Power dissipated or lost (P) is also called Ohmic loss in a conductor of resistance R carrying a current I.
* To minimise the power loss in transmission cables, very high voltage is produced at the generation point and transmitted almost up to the point where the power is utilised.
* Very high voltage is converted to low voltage using a step-down transformer placed very near to the point of utilisation of power.
Introduction to Electro Magnetic Induction
* Electricity and Magnetism are inter-related and the energies linked with them are also inter- convertible.
* Whenever there is relative motion between the magnet and a coil, an electric current is induced in the coil.
Transformers
* A transformer is a device used to increase or decrease the voltage of an alternating current.
* Transformers work on the principle of mutual induction. According to the principle of mutual induction, when a coil carrying a time – varying current is placed close to another coil, a current is induced in the other coil.
* For an ideal transformer, the ratio of output voltage to input voltage is equal to the ratio of input current, which is equal to the ratio of the number of turns in the secondary coil to that in the primary coil.
* If the secondary coil has a greater number of turns than the primary, then the transformer is called a step-up transformer. If the secondary coil has a less number of turns the primary, then the transformer is called a step-down transformer.