This section deals with electricity and magnetism that is very important for competitive exams. Read this portion and then at the end, you will get to solve some MCQs.
Electricity and Magnetism For Competitive Exams
In the forthcoming section, we have discussed the concepts like Charge, conductors and insulators, potential difference, electric current, magnetism, types of magnetic materials etc.
Charge
Charge is the basic property associated with matter due to which it produces and experiences electrical and magnetic effects. Similar charges repel each other and opposite charges attract each other. The SI unit of charge is coulomb (C).
Conductors
Conductors are those materials, which allow electricity to pass through them. Metals like silver, iron, copper and earth acts like a conductor. Silver is the best conductor.
Insulators
Insulators are those materials which do not allow electricity to flow through them. Metals like wood, paper, mica, glass, ebonite are insulators.
Electric Current
Electric current refers to the directional flow of charged particles, such as electrons or ions, moving through an electrical conductor or space.
Electric current is denoted by I, and it is defined as the rate of flow of electrons. I = Charge moved/ time taken
Its unit is Ampere (A), and one Ampere = One coloumb/ one Ampere.
Even though electric current has a direction as well as the magnitude, It is still a scalar quantity.
Electrons or ions are in motion at room temperature, but they do not constitute an electric current, as their motion is haphazard and not in a particular direction. In order to make electrons move in a particular direction, we need to use an energy source/voltage source called battery.
The magnitude of electric current is measured by an Ammeter.
Potential Difference
Between two points A and B, the electric current can flow only if there is a potential difference between them. This potential difference is denoted by V.
The units of potential difference are Volts (V) and it is measured by a device called Voltmeter.
Resistance
The opposition offered to the flow of electric current is called Resistance. It is denoted by R.
Resistance is denoted by R. Its units are Ohm (Ω). It can be measured by using a device called Ohmmeter.
When resistances R1, R2, R3 are connected in series, the net resistance can be calculated as Rnet = R1 + R2 +R3
When resistances R1, R2, R3 are connected in parallel, the net resistance can be calculated as Rnet = 1/R1 + 1/R2 + 1/R3
Heat produced due to electric current
When an electric current passes through a wire of some resistance, it leads to the generation of heat. This heat is used for a variety of purposes like lighting, heating etc.
An electric bulb works on the principle of generation of heat by an electric current. In an electric bulb, electric current is passed through a high resistance Tungsten filament. This produces a lot of heat and light.
An electric bulb makes a bang when it is broken because there is a vacuum inside the electric bulb, when the bulb is broken air rushes at great speed from all sides to fill the vacuum. The rushing of air produces a noise generally referred to as the bang.
ent.
Electric Fuse
An Electric Fuse is an Electric device which interrupts the flow of excess current in an Electric circuit. It is used to prevent damages of electrical appliances. It is usually in the form of a thin wire.
Electric fuse should have high resistivity and low melting point, so that it would melt whenever an excessive current passes through a circuit, and electrical appliances get disconnected.
Magnetism
All magnets have two poles, north seeking end or North Pole and south seeking end or South Pole. Like poles of a magnet repel each other, while the unlike poles attract each other.
A magnet with a single pole (magnetic monopole) can not exist. If we try to break a magnet with two poles, the resultant magnets will also have two poles.
The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south to the geographic north. When a bar magnet is suspended freely, it points in north-south direction. The tip which points towards the geographic North is called the North pole and the tip which points towards the geographic South is called magnet’s south pole.
Magnetic Materials: The materials that are attracted towards a magnet are called magnetic materials e.g,. iron, nickel, cobalt.
Non magnetic Materials: The materials that are not attracted towards a magnet. e.g., Mercury, water
A freely suspended bar magnet always rests in the North South direction.
Diamagnetic Substances
Diamagnetic substance are the substances which when placed in magnetic field, acquire feeble magnetism opposite to the direction of the magnetic field. A diamagnetic substance does not have unpaired electrons and is not attracted to a magnetic field. Examples- Gold, Diamond, Copper, Water, Mercury etc.
Paramagnetic Substances
Paramagnetic substance are the substances which when placed in magnetic field, acquire feeble magnetism in the direction of the magnetic field. Example- Al, Na, Mn etc.
Ferromagnetic Substances
Ferromagnetic substance are the substances which when placed in magnetic field, are strongly magnetized in the direction of the magnetic field. Examples- Iron, Cobalt, Nickle
Curie Temperature
The Curie temperature (TC), or Curie point, is the temperature at which certain materials lose their permanent magnetic properties, to be replaced by induced magnetism.
Magnetic Lines
Isogonic lines : Isogonic lines are lines on the Earth’s surface along which the declination has the same constant value, and lines along which the declination is zero are called agonic lines.
Isoclinic lines: Isoclinic lines are imaginary lines on the earth’s surface connecting points where the earth’s magnetic field has the same angle.
The aclinic line: The aclinic line is the magnetic equator, where the magnetic field is inclined neither north or south, so it’s a special case of an isoclinic line.
Isodynamic line: A line on a map connecting points of equal strength of the earth’s magnetic field.
Electricity and magnetism are not two separate quantities. They are inter-related and we can produce magnetic field from electric field and electric field from magnetic field. for example, in case of light (electro-magnetic waves), varying electric field produces varying magnetic field and vice versa.
Electric force and the magnetic force together constitute the Electro-magnetic force. This force is one among the 4 fundamental forces of nature.
Elecro-magnetic force is the second strongest fundamental force in nature.
Best MCQs On Electricity and Magnetism
In the following section, you will get to solve some top MCQs on electricity and magnetism.
What is the unit of Electric current?
- Volt
- Ampere
- Ohm
- Joule
Ampere
Which of the following is actually a work done per unit charge?
- Electric current
- Potential difference
- Resistance
- Work done
Potential difference
At room temperature and without the application of any voltage source, the electrons in a metal are:
- In random motion
- In an ordered motion
- At rest
- none
in random motion
The electric filament of an electric bulb is made up of :
- Lead
- Tungsten
- Nichrome
- Copper
Tungsten. Its symbol is W
The heating coil of an electric heater is made up of:
- Lead
- Tungsten
- Nichrome
- Copper
Nichrome. It is an alloy of Nickel (80%) and Chromium (20%)
Which of the following is a statement of Ohm’s law?
- V= IR
- V= I2R
- I= VR
- R= VI
V =IR
Magnetic permeability is maximum for:
- Diamagnetic substances
- Ferromagnetic substances
- Paramagnetic substances
- All of them
Ferromagnetic substances
Which of the following is suitable for making permanent electromagnets?
- Soft iron
- Copper
- Stainless steel
- Hardened steel
Soft Iron. It is due to the reason that it retains magnetism permanently
A bar magnet has:
- One pole
- Two poles
- Three poles
- None of the above
Two poles
Which of the following quantities is neither a scalar nor a vector quantity?
- Charge
- Electric Current
- Potential difference
- Resistance
Electric current.
If a bar magnet is suspended freely, its North pole will approximately:
- align itself towards the geographic South
- align itself towards the geographic North
- align itself towards along the equator
- will stay upright
align itself towards the geographic North
Three resistances each of 2 ohm, are connected in series. What is the net resistance of the combination?
- 2 ohm
- 4 ohm
- 6 ohm
- 2/3 ohm
6 ohm