SPI Physics · Free notes

Electricity and Magnetism: SPI Physics

Electricity and Magnetism has 22 questions in the SPI bank: fifteen EASY, six MODERATE and one HARD, all from sample papers and mock tests. Ten are short calculations with V = IR, the series and parallel rules, P = VI or E = Pt, and most of the rest ask which rule or device does a job: where the fuse goes, the field inside a solenoid, what each finger shows. Learn each formula with its units and each rule with the device it belongs to, and these are quick marks.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

SPI home All formulas on one page

Subtopic notes

Formula & revision sheet

8 formulas · 3 reference tables · 22 gotchas across all subtopics — the exam-eve cheat-sheet

Ohm's Law and Resistance

Formulas (3)

Current, charge and potential difference · Current and potential difference $I = \frac{Q}{t} \qquad V = \frac{W}{Q}$
Ohm's law: V = IR · Ohm's law $V = IR \qquad I = \frac{V}{R} \qquad R = \frac{V}{I}$
What a wire's resistance depends on · Resistance of a wire $R = \rho \frac{L}{A}$

Watch out for (6)

Time in minutes
→ Current, charge and potential difference
Dividing the wrong way round
→ Ohm's law: V = IR
Thinking V/I changes along a straight-line graph
→ Ohm's law: V = IR
Leaving out the temperature condition
→ Ohm's law: V = IR
Mixing resistance with resistivity
→ What a wire's resistance depends on
Thicker means more resistance
→ What a wire's resistance depends on

Series and Parallel Circuits

Formulas (2)

Adding resistors in series and in parallel · Series and parallel $R_s = R_1 + R_2 + R_3 \qquad \frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} \qquad R_p = \frac{R_1 R_2}{R_1 + R_2}$
Equal resistors and cutting a wire · n equal resistors $R_s = nR \qquad R_p = \frac{R}{n} \qquad \text{cut into } n \text{ and joined in parallel: } \frac{R}{n^2}$

Watch out for (5)

Adding resistances that are in parallel
→ Adding resistors in series and in parallel
Swapping what is shared
→ Adding resistors in series and in parallel
Splitting the battery voltage between parallel branches
→ Adding resistors in series and in parallel
Forgetting that cutting changes each piece
→ Equal resistors and cutting a wire
Picking series for the minimum
→ Equal resistors and cutting a wire

Electric Power and House Wiring

Formulas (2)

Electric power: P = VI · Electric power $P = VI = I^2R = \frac{V^2}{R}$
Electrical energy and the kilowatt hour · Electrical energy $E = Pt \qquad E\,(\text{kWh}) = \frac{P\,(\text{W}) \times t\,(\text{h})}{1000} \qquad 1\text{ kWh} = 3.6 \times 10^6\text{ J}$

Reference tables (1)

House wiring and safety devices8 rows

Part	What it is or does
Live wire	Red insulation, at 220 V
Neutral wire	Black insulation, at zero potential
Earth wire	Green insulation, joined to the metal body of an appliance and to a plate buried in the ground
Fuse	A wire that melts and breaks the circuit when too much current flows
MCB (miniature circuit breaker)	A switch that trips by itself on too much current, and can be reset
Main switch	A double pole switch that cuts both the live and the neutral wire The earth wire is never switched off.
Short circuit	Live and neutral touch, so the resistance drops and a huge current flows
Overloading	Too many appliances on one socket draw more current than the wiring is meant for

Indian mains: 220 V AC at 50 Hz. Appliances are joined in parallel.

Watch out for (6)

Halving the voltage halves the power
→ Electric power: P = VI
Using milliamperes as amperes
→ Electric power: P = VI
Leaving time in minutes
→ Electrical energy and the kilowatt hour
Calling the kWh a unit of power
→ Electrical energy and the kilowatt hour
A thick fuse wire
→ House wiring and safety devices
Earthing as protection against overload
→ House wiring and safety devices

Magnetic Effects of Current

Formulas (1)

Force on a current in a magnetic field: the motor · Force on a wire and torque on a coil $F = BIL \qquad \tau = NIAB\sin\theta$

Reference tables (2)

The magnetic field of a wire, a loop and a solenoid5 rows

Source	Shape of the field lines
Straight wire	Circles around the wire, centred on it; weaker farther away
Circular loop	Circles near the wire; nearly straight lines at the centre
Solenoid, inside	Parallel straight lines: the field is uniform Uniform inside a long solenoid, not circular.
Solenoid, outside	Like a bar magnet: from the N end round to the S end
Bar magnet	N to S outside, S to N inside, forming closed loops

Circular lines belong to a straight wire; straight parallel lines to the inside of a solenoid.

Electromagnetic induction and the generator6 rows

Fact	Value
Energy change in a generator	Mechanical energy to electrical energy
Rule for the induced current	Fleming's right hand rule
Current reverses	Once every half revolution of the coil Twice in each full turn, not once.
Rings in an AC generator	Slip rings (current keeps alternating)
Rings in a DC generator	Split ring commutator (current flows one way)
AC frequency in India	50 Hz, so the current changes direction 100 times a second

A motor and a generator are built alike: a coil, a magnet and rings. Only the energy change is reversed.

Watch out for (5)

Calling the field inside a solenoid circular
→ The magnetic field of a wire, a loop and a solenoid
Mixing up the clock rule
→ The magnetic field of a wire, a loop and a solenoid
Using the right hand for a motor
→ Force on a current in a magnetic field: the motor
Measuring the angle from the coil's plane
→ Force on a current in a magnetic field: the motor
One reversal per turn
→ Electromagnetic induction and the generator

Question weightage by concept

11 concepts · 22 questions — where the marks actually sit, so you know what to drill first

Ohm's Law and Resistance5 questions · 23%

Concept	Qs	Share
Ohm's law: V = IR	4	18%
What a wire's resistance depends on	1	5%
Current, charge and potential differencefoundation	—	—

Series and Parallel Circuits4 questions · 18%

Concept	Qs	Share
Adding resistors in series and in parallel	2	9%
Equal resistors and cutting a wire	2	9%

Electric Power and House Wiring8 questions · 36%

Concept	Qs	Share
Electric power: P = VI	3	14%
House wiring and safety devices	3	14%
Electrical energy and the kilowatt hour	2	9%

Magnetic Effects of Current5 questions · 23%

Concept	Qs	Share
The magnetic field of a wire, a loop and a solenoid	2	9%
Force on a current in a magnetic field: the motor	2	9%
Electromagnetic induction and the generator	1	5%

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