Electricity and Magnetism: Current, Circuits and Magnetic Effects
Current and the meters that measure it, Ohm's law and resistance, series and parallel, heating and chemical effects, magnets, motors, generators and safe home wiring.
Distribution transformers on a pole. They step down the high AC voltage of the supply line to the lower voltage used in homes. A transformer works on electromagnetic induction, so it works only with AC. Image: Rsparks3, Wikimedia Commons · CC0
Every switch, fan and phone charger runs on electric current, the flow of charge in a wire. A current heats a wire, can split a liquid, and turns a coil into a magnet. A moving magnet can, in turn, make a current. These links run motors and power stations. Exams ask about meters and units, Ohm's law, series and parallel, the fuse, the kilowatt hour, magnets and home wiring.
On this page you will learn:
Current, potential difference and the meters: ammeter, voltmeter, galvanometer
Ohm's law, resistance, series and parallel, and the capacitor
Heating effect, the bulb, the kilowatt hour, cells and electroplating
Magnets, the right-hand thumb rule, the motor and the generator
AC and DC, home wiring, fuse, short circuit, earthing and lightning
220 V
Home supply in India
AC at 50 Hz
1 kWh
1 unit on the bill
= 3.6 × 10⁶ J
V = IR
Ohm's law
At a steady temperature
Current and the meters that measure it
Rule in one line: the ammeter goes in series, the voltmeter in parallel.
KEY TERM
Electric current
Current is the rate of flow of charge: I = Q/t. In a metal wire, electrons carry the charge. A cell keeps a potential difference (voltage) across the wire. This push drives the charge round the circuit. By custom, current flows from the + end to the − end, opposite to the electrons.
Cell→Voltage→Current
Quantity
SI unit
Measured with
Current (I)
Ampere (A)
Ammeter, joined in series
Potential difference (V)
Volt (V)
Voltmeter, joined in parallel
Very small current
Ampere (A)
Galvanometer: detects it and its direction
Resistance (R)
Ohm (Ω)
Ohmmeter
Charge (Q)
Coulomb (C)
—
An ammeter has a very low resistance, so it does not slow the current. A voltmeter has a very high resistance, so it takes almost no current.
An ammeter. Its dial is marked A for ampere, the unit of current. An ammeter is always joined in series, so the whole current passes through it. Image: Anders Jonsson, Wikimedia Commons · CC BY-SA 3.0
QUICK CHECK
PYQ · PPSC SEVE 2016 — Electric current is measured by:
Tap an option to answer
Answer: D — Ammeter
An ammeter measures current. An anemometer measures wind speed; a commutator is a part of a motor.
QUICK CHECK
PYQ · PPSC ICS 2016 — The voltmeter is a measuring instrument used for measuring electric potential difference between two points in an electric circuit. It is always connected in ________ across the points between which the potential difference is to be measured.
Tap an option to answer
Answer: B — Parallel
A voltmeter compares two points, so it is joined across them, in parallel.
QUICK CHECK
PYQ · PPSC OHE 2016 — A measuring instrument used for detecting and measuring small currents in a conductor is:
Tap an option to answer
Answer: B — Galvanometer
A galvanometer detects very small currents and shows their direction. An ammeter is a galvanometer adapted for larger currents.
Ohm's law, resistance, series and parallel
Rule in one line: in series resistances add; in parallel the total is less than the smallest.
KEY TERM
Ohm's law
The voltage across a metal wire is directly proportional to the current in it, if its temperature stays the same: V = IR. R is the resistance, the wire's opposition to current. Double the voltage and the current doubles. Double the resistance and the current halves.
V→I→R→V = IR
Quantity
Formula
Unit in other units
Current
I = Q/t
ampere = coulomb/second
Charge
Q = It
coulomb = ampere·second
Resistance
R = V/I
ohm = volt/ampere
Power
P = VI
watt = volt·ampere
Learn each unit as a mix of others. Exams ask you to match them.
KEY TERM
What resistance depends on
Resistance grows with the length of a wire and falls with its thickness: R = ρl/A. ρ is the resistivity of the material, in Ω m. A long, thin wire resists more than a short, thick one. Resistance also changes with temperature.
Resistivity of some metals
Silver
1.60
Copper
1.62
Aluminium
2.63
Tungsten
5.20
Iron
10.0
Values at 20 °C from the class 10 NCERT table. Silver conducts best, but copper and aluminium are cheaper, so wires use them.
SERIES VS PARALLEL
Series
Parallel
Path
One path, end to end
Side by side branches
Current
Same in each
Splits up
Voltage
Splits up
Same on each
Total R
R1 + R2 + …
1/R = 1/R1 + 1/R2 + …
One fails
All stop
Others work
To get the lowest resistance from a set of resistors, join them all in parallel. To get the highest, join them in series.
A CAPACITOR STORES CHARGE
A capacitor is two metal plates with an insulator between them. It stores charge and gives it back when needed, as in a camera flash. A resistor controls current. A transistor switches or boosts signals. An ammeter only measures current.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Match List-I (Physical Quantity) with List-II (Unit of Measurement): a-Current, b-Resistance, c-Power, d-Charge with I-Ampere·second, II-Volt·ampere, III-Coulomb/second, IV-Volt/ampere.
Tap an option to answer
Answer: A — a-III, b-IV, c-II, d-I
Current = coulomb/second (III). Resistance = volt/ampere (IV). Power = volt × ampere (II). Charge = ampere × second (I).
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Four resistance 1Ω, 2Ω, 4Ω and 4Ω have been joined in a circuit to obtain lowest resistance of the combination of the four. Its value will be:
Tap an option to answer
Answer: A — 0.5 Ω
Lowest means all in parallel: 1/R = 1/1 + 1/2 + 1/4 + 1/4 = 2. So R = 0.5 Ω, less than the smallest (1 Ω).
QUICK CHECK
PYQ · Punjab Police Constable 2021 — If you want to store charge, which of the following device you will use?
Tap an option to answer
Answer: C — Capacitor
A capacitor stores electric charge on its two plates.
Heating and chemical effects of current
Rule in one line: heat = I²Rt, and the bill is counted in kilowatt hours.
KEY TERM
Heating effect (Joule's law)
A current I in a resistance R for time t makes heat H = I²Rt. Double the current and the heat becomes four times. Irons, toasters, kettles, heaters, bulbs and the fuse all use this effect.
I²→R→t→H
Part
Material
Why
Bulb filament
Tungsten
Melts only at about 3380 °C, so it can glow white-hot
Gas inside a bulb
Nitrogen and argon
Do not react, so the filament lasts longer
Heater and iron coils
Nichrome (nickel + chromium)
High resistivity; does not burn away when red-hot
Fuse wire
Tin-lead alloy
Low melting point; melts when current is too high
House wires
Copper or aluminium
Low resistivity; little heat is wasted
Which material goes where. Alloys resist more than pure metals, so heaters use them.
A filament bulb. The thin coil is tungsten, which melts only at about 3380 °C, so it can glow white-hot. The bulb is filled with an inert gas such as nitrogen or argon, so the filament does not burn up. Image: KMJ, Wikimedia Commons · CC BY-SA 3.0
KEY TERM
Power and the unit on your bill
Electric power is P = VI, in watts. The commercial unit of energy is the kilowatt-hour (kWh). It is the energy a 1000 W appliance uses in 1 hour. 1 kWh = 1 unit = 3.6 × 10⁶ J. A watt is a unit of power; a kWh is a unit of energy.
WORKED EXAMPLE
A 100 W bulb used for 10 hours uses 100 × 10 = 1000 Wh = 1 kWh, that is 1 unit. A 1000 W heater used 2 hours a day for 30 days uses 1 × 2 × 30 = 60 units.
KEY TERM
Cells and the chemical effect
A cell turns chemical energy into electrical energy. In a common dry cell, a zinc can is the − end and a carbon rod is the + end. Between them is a moist paste of ammonium chloride and zinc chloride, the electrolyte. Current also does the reverse: it can break a liquid apart. This is electrolysis.
ELECTROPLATING WITH COPPER
Solution
Copper sulphate in a beaker
↓
+ end
A copper plate; it slowly dissolves
↓
− end
The object to be coated
↓
Result
A thin copper layer on it
Taps and cycle rims get chromium; food cans get tin. Zinc on iron is galvanising, but it is usually done by dipping, not by current.
QUICK CHECK
PYQ · PSSSB Election Kanungo 2018 — Which of the following gases is inside an electric bulb?
Tap an option to answer
Answer: B — Nitrogen
Filament bulbs are filled with nitrogen and argon, which do not react. Oxygen would burn the hot filament.
QUICK CHECK
PYQ · PPSC ARO 2022 — The commercial unit of energy is:
Tap an option to answer
Answer: C — kilowatt-hour
The kilowatt-hour is the 'unit' on the electricity bill. Watt and kilowatt are units of power, not energy.
QUICK CHECK
PYQ · PPSC SEVE 2016 — In a dry cell (battery), which of the following are used as electrolytes?
Tap an option to answer
Answer: A — Ammonium chloride and Zinc chloride
The paste in a common zinc-carbon dry cell is ammonium chloride with zinc chloride.
QUICK CHECK
PYQ · PPSC EO 2018 — A battery or cell converts _________ energy into electrical energy
Tap an option to answer
Answer: B — Chemical
Inside a cell, chemical reactions between the electrodes and the paste push charges round the circuit. So a cell changes chemical energy into electrical energy.
Magnets, motors and generators
Rule in one line: a current makes a magnetic field; a changing magnetic field makes a current.
Every magnet has a north and a south pole. Break it, and each piece still has both.
Like poles repel; unlike poles attract. Repulsion is the sure test of a magnet.
A freely hung magnet points north–south. A compass needle is a small magnet.
Iron, nickel and cobalt are pulled by magnets. Magnetite is a natural magnet.
Field lines run from N to S outside a magnet. They never cross. They crowd where the field is strong.
Iron filings around a bar magnet trace its magnetic field lines. The filings crowd at the poles, where the field is strongest. The lines curve round from one pole to the other. Image: MikeRun, Vera Wurmsdobler, Wikimedia Commons · CC BY-SA 4.0
KEY TERM
Right-hand thumb rule
Oersted saw a compass needle turn near a wire carrying current. So a current makes a magnetic field. Hold the wire in your right hand with the thumb along the current. Your curled fingers then show the direction of the field lines, which circle the wire.
Thumb→Current→Fingers→Field
PERMANENT MAGNET VS ELECTROMAGNET
Permanent magnet
Electromagnet
Made of
Steel or special alloy
Coil round soft iron
Magnetism
Always on
Only while current flows
Strength
Fixed
More turns or current = stronger
Uses
Compass, fridge door
Bell, crane, motor
A long coil of many turns is a solenoid. With current, it acts like a bar magnet.
MOTOR VS GENERATOR
Electric motor
Electric generator
Energy change
Electrical → mechanical
Mechanical → electrical
Works on
Force on a current in a field
Faraday's induction
Fleming's rule
Left hand
Right hand
Found in
Fan, mixer, pump
Power station, dynamo
The force on a wire in a magnetic field is largest when the wire is at right angles to the field, and zero when it is parallel.
KEY TERM
Electromagnetic induction
Michael Faraday found that moving a magnet in or out of a coil makes a current in it. The faster the change, the bigger the current. A generator turns a coil in a magnetic field to make electricity this way. A transformer also works on induction: it steps AC voltage up or down.
Moving magnet→Changing field→Induced current
QUICK CHECK
PYQ · Punjab Police Constable 2021 — In right-hand thumb rule, thumb point towards ______.
Tap an option to answer
Answer: D — The direction of current
The thumb points along the current; the curled fingers show the magnetic field.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — When a current carrying aluminum rod is placed in a magnetic field, the displacement of the rod is largest when the direction of current is:
Tap an option to answer
Answer: B — at right angle to the direction of the magnetic field
The force is largest at 90° to the field and zero when parallel to it. This is the idea behind Fleming's left-hand rule.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Which one of the following converts mechanical energy into electrical energy using Faraday's Law?
Tap an option to answer
Answer: B — Electric generator
A generator uses induction to turn motion into electricity. A motor does the opposite. A rectifier changes AC into DC.
Home electricity and safety
Rule in one line: Indian homes get 220 V AC at 50 Hz (NCERT value; the official standard is 230 V), guarded by a fuse or MCB and an earth wire.
DIRECT CURRENT VS ALTERNATING CURRENT
DC
AC
Direction
One way only
Keeps reversing
Source
Cell, battery
Power station
Used in
Torch, phone
Home mains
AC can be stepped up by a transformer and sent far with little loss. That is why power stations make AC.
Wire
Colour
Job
Live
Red (brown in new wiring)
Brings current at 220 V
Neutral
Black (blue in new wiring)
Completes the circuit
Earth
Green
Joined to a metal plate deep in the ground; takes leaking current away from a metal body
Homes are wired in parallel, so each appliance gets the full 220 V and has its own switch.
WHAT A FUSE DOES IN A SHORT CIRCUIT
1
Insulation wears off
↓
2
Live touches neutral
This is a short circuit
↓
3
Resistance falls to nearly zero
Current shoots up
↓
4
Fuse wire melts
Heating effect breaks the circuit; fire is avoided
Overloading is different: too many appliances on one socket. The fuse guards against both. An MCB does the same job but simply switches off, and can be switched on again.A porcelain rewirable (kit-kat) fuse. A thin fuse wire of low melting point is fixed between the screws. If too much current flows, the wire heats up and melts, and the circuit breaks. This is the heating effect of current. Image: Anihl, Wikimedia Commons · Public domain
QUICK CHECK
PYQ · PPSC ICS 2016 — An electric fuse is based on
Tap an option to answer
Answer: A — Heating effect of current
Too much current heats the thin fuse wire until it melts and breaks the circuit.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Short-Circuiting occurs in the circuit when:
Tap an option to answer
Answer: C — Live wire and neutral wire come into direct contact with each other
Live touching neutral is a short circuit. Many appliances on one socket is overloading — a different fault.
KEY TERM
Earthing
Metal-bodied appliances such as irons, toasters and fridges are earthed. If the live wire touches the metal body, current flows to the ground through the earth wire. It does not flow through the person who touches it.
Lightning over Oradea, Romania. Charges build up in clouds. When they grow very large, they jump to the ground as a giant electric spark. A lightning conductor on a tall building leads this charge safely into the earth. Image: Mircea Madau, Wikimedia Commons · Public domain
SAFETY IN A THUNDERSTORM
A house, or a car or bus with windows shut, is safe. Stay away from open fields, tall lone trees, poles and metal objects. If caught outside, squat low; do not lie flat. Do not bathe or touch metal pipes during the storm.
More facts asked once in a whileTap to open
A compass needle's north pole points north. So the Earth's magnetic pole near the geographic north is really a magnetic south pole.
Heating, hammering or dropping a magnet weakens it. Bar magnets are stored in pairs with soft-iron keepers across the ends.
In a DC motor, split rings (the commutator) reverse the current every half turn, so the coil keeps turning one way.
1 horsepower = about 746 W.
Glass rubbed with silk becomes positive; a plastic rod rubbed with wool becomes negative. An electroscope detects charge.
Johann Schweigger of Germany built the first galvanometer, in 1820. It was named after Luigi Galvani, who studied electricity in frog legs.
Traps and practice
Last revision — where the options try to fool you.
COMMON TRAPS
Ammeter in series, voltmeter in parallel. Do not swap them.
In parallel the total resistance is LESS than the smallest one, not the average.
The kWh is a unit of energy; the watt and kilowatt are units of power.
Tungsten for the filament, nichrome for the heater, low-melting alloy for the fuse.
Thumb = current in the right-hand thumb rule; fingers = field.
Short circuit = live touches neutral. Overloading = too many appliances.
QUICK CHECK
PYQ · Punjab Police SI 2021 — The first mechanical/sensitive galvanometer was discovered by which of the following scientists?
Tap an option to answer
Answer: A — Johann Schweigger
Schweigger built it in 1820, soon after Oersted's discovery. Galvani only gave the device its name; Torricelli made the barometer.
CHECK YOURSELF
Resistors of 2 Ω, 3 Ω and 6 Ω are joined (a) in series and (b) in parallel. Find the total in each case.
Show answer
(a) 2 + 3 + 6 = 11 Ω. (b) 1/R = 1/2 + 1/3 + 1/6 = 1, so R = 1 Ω — less than the smallest.
FLASHCARDS
Electricity flashcards
Measures current
Ammeter (in series)
Measures voltage
Voltmeter (in parallel)
Detects tiny currents
Galvanometer
Stores charge
Capacitor
Commercial unit of energy
Kilowatt-hour (kWh)
Fuse works on
Heating effect
Motion into electricity
Generator
Earth wire colour
Green
QUICK BYTES
V = IR; ammeter series, voltmeter parallel; galvanometer for tiny currents
Series: add; parallel: total below the smallest
H = I²Rt; 1 kWh = 1 unit = 3.6 × 10⁶ J
Bulb: tungsten filament, nitrogen and argon gas
Dry cell paste: ammonium chloride + zinc chloride
Thumb = current; force largest at 90° to the field
India: 220 V AC, 50 Hz; live red, neutral black, earth green
❧
Frequently Asked Questions
Which instrument measures electric current?
An ammeter, joined in series. A galvanometer detects very small currents. A voltmeter measures potential difference and is joined in parallel.
Why is a voltmeter joined in parallel?
It measures the difference between two points, so it must sit across them. Its high resistance means it takes almost no current.
What is Ohm's law?
At a steady temperature, the voltage across a conductor is proportional to the current through it: V = IR. R is the resistance in ohms.
How do you get the lowest resistance from a set of resistors?
Join them all in parallel. The total is then less than the smallest one. For example, 1, 2, 4 and 4 ohms in parallel give 0.5 ohm.
What is the commercial unit of electrical energy?
The kilowatt-hour (kWh), called one 'unit' on the bill. 1 kWh equals 3.6 × 10⁶ joules.
Which gas is filled in an electric bulb, and why?
Nitrogen and argon. They do not react with the hot tungsten filament, so it lasts longer.
What does a capacitor do?
It stores electric charge on two metal plates separated by an insulator, and releases it when needed.
What is the right-hand thumb rule?
Hold a straight wire in your right hand with the thumb along the current. The curled fingers show the direction of the magnetic field round the wire.
What is the difference between a motor and a generator?
A motor turns electrical energy into motion. A generator turns motion into electrical energy, using Faraday's electromagnetic induction.
What is a short circuit, and how does a fuse help?
A short circuit happens when the live and neutral wires touch directly. The current shoots up. The fuse wire, working on the heating effect, melts and breaks the circuit.
Now sit the questions they actually set
Previous-year PPSC questions on this topic, free to read with the worked answer.
