Physics

medium importance~1 Q in Tier 127 formulas⚡ 12 shortcuts6 subtopics

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Electricity, magnetism and the EM spectrum

⏱ 6 min read🧩 6 question types🎯 21 practice Q

The idea in one minute

Electric current is the flow of charge, pushed by voltage and opposed by resistance. Ohm's law, series and parallel circuits and the heating effect are the core numericals. Magnetism gives us the motor, generator and transformer. The electromagnetic spectrum is a fixed order of waves with fixed uses.

Current, voltage and resistance

Electric current is the flow of charge. $I = \dfrac{Q}{t}$ and its unit is the ampere (A).

A potential difference (voltage) pushes the charges. Its unit is the volt (V). A voltmeter measures it.

Resistance is the wire's opposition to the flow. Its unit is the ohm (Ω).

Ohm's law says $V = IR$ at constant temperature. Double the voltage and the current doubles.

Resistance is $R = \rho\dfrac{L}{A}$ . More length means more resistance. A thicker wire means less. It also depends on the material (resistivity ρ) and on temperature. Metals conduct best. Rubber, plastic and mica are insulators.

Series and parallel circuits

	Series	Parallel
Layout	One loop	Branches across two points
Common to all	Current	Voltage
Total resistance	$R = R_1 + R_2 + \cdots$	$\dfrac{1}{R} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \cdots$
If one bulb fuses	All go dark	Others keep working

In parallel, the total resistance is less than the smallest branch. Homes are wired in parallel. Each appliance gets the full 220 V and works independently.

Example: 2 Ω, 3 Ω and 6 Ω in parallel: $\dfrac{1}{R} = \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{6} = 1$ . So $R = 1$ Ω.

Power, energy and heating

Power is $P = VI = I^2R = \dfrac{V^2}{R}$ . Its unit is the watt. A 100 W bulb at 220 V draws about 0.45 A.

Electricity is sold in kilowatt-hours. 1 unit = 1 kWh = 3.6 × 10⁶ J. Units = watts × hours ÷ 1000.

Joule's law says $H = I^2Rt$ . Heat grows with current squared, resistance and time. Irons, geysers and heaters work this way.

The bulb filament is tungsten. It has a very high melting point.
Heating coils are nichrome. It has high resistivity.
A fuse wire is a tin–lead alloy with a low melting point. It melts on excess current. It sits in series with the live wire.

Household supply

Indian domestic supply is AC at about 220 V (220–230 V) and 50 Hz.

Wire	Colour
Live	Red or brown
Neutral	Black or blue
Earth	Green

The earth wire gives a safe path to the ground for metal bodies.

Magnetism and its devices

A magnet has north and south poles. Like poles repel and unlike poles attract.

Oersted (1820): a current in a wire deflects a compass. Electricity creates magnetism.
Electromagnet: a coil with a soft-iron core. It is temporary. Cranes and bells use it. Steel makes permanent magnets.
Faraday (1831): a changing magnetic field induces a current. This is electromagnetic induction. It runs the generator or dynamo.
Electric motor: current plus a magnetic field gives rotation.
Transformer: changes AC voltage by mutual induction. It works only on AC. Step-up transformers raise voltage for long-distance lines. The current falls, so the I²R loss falls. Step-down transformers lower it near homes.

Rule: Fleming's left-hand rule is for the motor. The right-hand rule is for the generator. Remember "L for Motor, R for Generator".

A galvanometer becomes an ammeter with a low-resistance shunt in parallel. An ammeter is connected in series. A voltmeter has a high resistance in series inside it. It is connected in parallel.

AC, DC and semiconductors

AC reverses direction again and again (mains supply). DC flows one way (cells, batteries).

A rectifier (diode) turns AC into DC. An inverter turns DC into AC.
The transistor (1947, Bell Labs) amplifies and switches signals.
Semiconductors are silicon and germanium. They conduct between metals and insulators.
A pentavalent impurity (phosphorus, arsenic) adds free electrons. That gives n-type.
A trivalent impurity (boron, gallium) leaves holes. That gives p-type.

The electromagnetic spectrum

EM waves need no medium. They travel at $c = 3 \times 10^8$ m/s in vacuum. In order of increasing wavelength:

Gamma rays, X-rays, Ultraviolet, Visible, Infrared, Microwaves, Radio waves.

Wave	Use
Gamma	Cancer treatment, sterilising tools
X-rays	Bone imaging, airport scanners
Ultraviolet	Sterilisation, vitamin D in skin
Visible	Seeing, lasers
Infrared	TV remotes, night vision, heaters
Microwaves	Microwave ovens, radar, satellite links
Radio	Broadcasting, mobile phones

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common4 practice Q

Ohm's law and series or parallel circuits

How to spot it:

The question gives two or three resistors and asks total resistance, gives voltage and resistance and asks current, or asks what stays the same in series or parallel.

V=IR,\quad R_s=R_1+R_2,\quad \frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}

Method

In series, add the resistances. The current is common.
In parallel, add the reciprocals, then invert. For two resistors use R₁R₂/(R₁+R₂).
Three equal resistors R in parallel give R/3.
For current, use I = V/R.

Why it works:

One path shares current (series). Branches share voltage (parallel).

Try this

Resistors of 2 Ω, 3 Ω and 6 Ω are joined in parallel. Find the total resistance.

Show solution

$\dfrac{1}{R} = \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{6}$ .
$= \dfrac{3+2+1}{6} = 1$ .
So R = 1 Ω, less than the smallest resistor.

Answer

1 Ω

Type 2very common4 practice Q

Power, kWh and the heating effect

How to spot it:

The question gives appliance watts and hours, H = I²Rt, 'the filament is made of ...', 'why is the fuse wire low-melting', or transmission steps.

P=VI=I^2R=\frac{V^2}{R},\quad H=I^2Rt,\quad 1\ \text{unit}=1\ \text{kWh}

Method

For energy, use kW × hours = units. Multiply by the tariff for rupees.
For heat, use H = I²Rt with t in seconds.
Filament is tungsten. Coil is nichrome. Fuse is a low-melting alloy in series with the live wire.
Long lines run at high voltage so the current and I²R loss stay small.

Why it works:

Watts times hours gives energy. Current squared gives heat.

Try this

A 2 kW heater runs for 3 hours. How many units does it use?

Show solution

Units = kW × hours.
2 × 3 = 6.

Answer

6 kWh, that is 6 units

Type 3very common4 practice Q

Electricity and magnetism devices

How to spot it:

The question says 'The electric motor works on ...', 'a generator is based on ...', 'a transformer works on ...', asks which Fleming rule applies, or who found electromagnetic induction.

Method

Oersted: a current makes a magnetic field.
Motor: a magnetic force on a current-carrying wire. Use the left-hand rule.
Generator: electromagnetic induction (Faraday). Use the right-hand rule.
Transformer: mutual induction. AC only. Step-up raises V and lowers I.

Why it works:

Each device turns one effect of electricity or magnetism into work.

Try this

The working of an electric motor is based on:

Show solution

A wire carries current inside a magnetic field.
The field pushes the wire.
The left-hand rule gives the direction.

Answer

The magnetic force on a current-carrying conductor

Type 4common3 practice Q

Household wiring and safety

How to spot it:

The question says 'Appliances in a house are connected in ...', asks the colour of the earth wire, the job of the fuse, or mains voltage and frequency in India.

Method

Homes use parallel wiring. Each appliance gets full voltage and has its own switch.
Live is red or brown. Neutral is black. Earth is green.
The fuse is low-melting and sits in series with the live wire. It melts first on overload.
India uses AC at about 220 V and 50 Hz.

Why it works:

The wiring plan protects both the appliance and the person.

Try this

Which colour of insulation is used for the earth wire in India?

Show solution

Live is red or brown.
Neutral is black.
Earth is the remaining one.

Answer

Green

Type 5very common3 practice Q

EM spectrum order and uses

How to spot it:

The question says 'Arrange gamma to radio by wavelength', 'TV remote uses ...', 'which wave is used in radar / microwave oven' or 'UV makes which vitamin'.

Method

Learn rising wavelength: gamma, X, UV, visible, IR, microwave, radio. Frequency runs the other way.
Match uses: gamma for cancer therapy, X-ray for bones, UV for sterilising and vitamin D, IR for remotes, microwave for ovens and radar, radio for broadcast.
All EM waves travel at 3 × 10⁸ m/s in vacuum and need no medium.

Why it works:

The order and the uses are fixed facts.

Try this

Which electromagnetic wave is used in TV remotes?

Show solution

Remotes send invisible beams.
They lie just beyond the red end of visible light.

Answer

Infrared

Type 6occasional2 practice Q

Semiconductors, rectifiers and electronics

How to spot it:

The question says 'n-type is doped with ...', 'which device converts AC to DC', or asks about transistors and semiconductor materials.

Method

Semiconductors are silicon and germanium.
Pentavalent doping (P, As) gives n-type, with extra electrons. Trivalent doping (B, Ga) gives p-type, with holes.
A diode is a rectifier (AC to DC). An inverter changes DC to AC. The transistor (1947) amplifies and switches.

Why it works:

The number of outer electrons in the impurity decides n-type or p-type.

Try this

A pure silicon crystal doped with a pentavalent impurity such as phosphorus becomes:

Show solution

Phosphorus has one extra outer electron.
It gives a free electron to the crystal.
Free electrons are negative carriers.

Answer

n-type

Formula sheet

Ohm's law

V = IR

V is voltage, I is current, R is resistance

Series resistance

R_s = R_1 + R_2 + \cdots

the same current flows through each

Parallel resistance

\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + \cdots

two resistors: R₁R₂/(R₁+R₂)

Electric power

P = VI = I^{2}R = \frac{V^{2}}{R}

P is power in watt

Joule heating

H = I^{2}Rt

t is time in seconds

Resistance of a wire

R = \rho\frac{L}{A}

ρ = resistivity

Shortcuts that save time

⚡ Fleming's hands

L comes before M, so Left goes with Motor. G is for generator, so Right goes with Generator.

Example

Which Fleming rule gives the direction of the force in an electric motor?

Show solution

The motor uses a force on a current-carrying wire.
Left for Motor.

Answer

Fleming's left-hand rule

⚡ Spectrum order

Good X-rays Use Visible Infra-Micro Radios. The wavelength rises along the list. Frequency and energy fall.

Example

Which has the longest wavelength: X-rays, UV, microwaves or gamma rays?

Show solution

Order by rising wavelength: gamma, X-ray, UV, microwave.
The last one is the longest.

Answer

Microwaves

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Thinking a fuse wire should have a high melting point.

It must melt easily, so use a low-melting alloy.

Mistake 02

Saying a transformer works on DC.

It needs changing current, so it works only on AC.

Mistake 03

Connecting an ammeter in parallel.

An ammeter goes in series. A voltmeter goes in parallel.

Mistake 04

Adding reciprocals but forgetting to invert the answer in parallel.

Take 1/R first, then flip it to get R.

Mistake 05

Using the right-hand rule for the motor.

Motor uses the left hand. Generator uses the right hand.

Quick revision

Read this the night before the exam.

I = Q/t and V = IR. R = ρL/A: longer means more, thicker means less.
Series: same current, resistances add. Parallel: same voltage, total R is smaller. Homes are parallel.
P = VI. 1 unit = 1 kWh. H = I²Rt. Filament is tungsten, coil is nichrome, fuse is low-melting in the live wire.
Oersted: current gives magnetism. Faraday, 1831: induction gives the generator. Left hand for motor, right for generator.
A transformer works on AC only. Step-up is used for transmission. A rectifier turns AC to DC. The transistor is from 1947.
EM waves by rising wavelength: gamma, X, UV, visible, IR, microwave, radio.

Practice: 21 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 4 min · wrong answers go to your mistake notebook automatically.

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