Notes on electric current and magnets (For Science part of paper II, CTET 2016)
As, only a few days are left for the CTET examination to be held, following notes may be helpful for the student for final revisions of science part of paper II of CTET 2016 examination. The notes are followed by sample questions on the same which may be beneficial to have a look at the pattern and level of questions asked of this section.
•Electric Current: The flow of electric charge is known as electric current. Conventionally, the electric current flows in opposite direction to the movement of electrons. Electric current is expressed by the rate of flow of electric charges. The Rate of flow means the amount of charge flowing through a particular area in unit time.
•Ammeter is an apparatus used to measure electric current in a circuit
•Electric Potential is the amount of electric potential energy at a point is called electric potential.
•The difference in the amount of electric potential energy between two points in an electric circuit is known as voltage, which is equal to the work done per unit charge to move the charge between two points against the static electric field.
•Voltmeter is an apparatus to measure the potential difference or electric potential difference between two points in an electric circuit.
•Ohm’s Law states that the potential difference between two points is directly proportional to the electric current.
•Resistance is a property of conductor due to which it resists the flow of electric current through it, which are used to increase or decrease the electric current. Resistance in a conductor depends on nature, length and area of cross section of the conductor.
•Resistance (R) ∝ length of conductor (l), resistance ∝ 1/Area of cross section of conductor (A), thus, where is the resistivity of the material, Thus, SI unit of resistivity (ρ) is Ω m.
•Very good conducting materials have resistivity in the range of 10−8 Ω m to 10−6 Ω m. Silver has resistivity equal to 1.60 X 10−8 Ω m and copper has resistivity equal to 1.62 X 10−8 Ω m.
•Rubber and glass are very good insulators. They have resistivity in the order of 1012 Ω m to 1017 Ω m.
•Resistivity of materials varies with temperature.
•When resistors are joined from end to end, it is called in series. In this case, the total resistance of the system (Resistors in Series) is equal to the sum of the resistance of all the resistors in the system.
•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.
•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 (Joule’s Law of Heating).
•SI unit of electric power is watt (W).
•1 kWh = 1000 watt x 1 hour = 1000 W x 3600 s ⇒ 1kWh = 3.6 x 106 watt second = 3.6 x 106 J.
•A magnet has two ends called poles, one of which is called a north pole or north-seeking pole, while the other is called a south pole or south-seeking pole.
•The north pole of one magnet attracts the south pole of a second magnet, while the north pole of one magnet repels the other magnet's north pole ( like poles repel, unlike poles attract).
•A magnet creates an invisible area of magnetism all around it called a magnetic field.
•The north pole of a magnet points roughly toward Earth's North Pole and vice-versa. That is because; Earth itself contains magnetic materials and behaves like a gigantic magnet.
•If you cut a bar magnet in half, you get two brand new, smaller magnets, each with its own north and South Pole.
•All magnetic materials contain magnetic moments, which behave in a way similar to microscopic bar magnets.
•If you run a magnet a few times over an unmagnetized piece of a magnetic material (such as an iron nail), you can convert it into a magnet as well. This is called magnetization.
•Diamagnetism is a phenomenon in some materials in which the susceptibility is negative, i.e. the magnetization opposed the magnetizing force.
•In paramagnetism, the atoms or molecules of the substance have net orbital or spin magnetic moments that are capable of being aligned in the direction of the applied field. They therefore have a positive (but small) susceptibility and a relative permeability slightly in excess of one. Paramagnetism occurs in all atoms and molecules with unpaired electrons; e.g. free atoms, free radicals, and compounds of transition metals containing ions with unfilled electron shells.
•Ferromagnetism is a phenomenon in some magnetically ordered materials in which there is a bulk magnetic moment and the magnetization is large.
Sample questions on electric current and magnets:
1. The Resistivity of a wire depends on:
(C) Cross section area
(D) None of the above.
2. Which of the following is not the same as watt?
(A) Joule/ sec
(B) Amperes/ volt
(C) Amperes x volts
(D) (Amperes) 2 x ohm.
3. A circuit contains two un- equal resistances in parallel
(A) The current is same in both
(B) Large current flows in larger resistor
(C) The potential difference across each will be same
(D) Smaller resistance has smaller conductance
4. Four identical resistors are first connected in parallel and then in series. The ratio of the resultant resistance of the first combination to the second will be:
(A) 1 / 16
(B) 1 / 4
5. Tesla is a unit of:
(A) Field strength
(C) Flux density
6. Which of the following materials which is slightly repelled by a magnetic field?
(A) Ferromagnetic material
(B) Diamagnetic material
(C) Paramagnetic material
(D) Conducting material
7. The value of Susceptibility is positive for:
(A) Non¬magnetic materials
(B) Diamagnetic substances
(C) Ferromagnetic materials
(D) None of the above
8. The best evidence which proves that Earth has a magnetic field is:
A. All things fall toward Earth’s center.
B. A compass needle lines up with it.
C. Winds blow from east to west.
D. Earth’s oceans all have currents.
Answers: 1. B 2. B 3. C 4. A 5. C 6. B 7.C 8. B