Electricity formulas are equations used to calculate current, voltage, resistance, charge, power, and electrical energy in a circuit. Students use these formulas to solve numericals from electricity, circuits, and household energy consumption.
Every electricity numerical gives two values and asks for the third. The student who knows which formula connects those two values solves the question in under a minute. This article covers all electricity formulas with symbols, SI units, and solved examples so students can move from formula to answer without confusion.
Key Takeaways
| Concept |
What to Remember |
| Electric current |
Rate of flow of electric charge |
| Voltage |
Work done per unit charge |
| Resistance |
Opposition to current flow |
| Ohm’s law |
Relation between voltage, current, and resistance |
| Power |
Rate of electrical energy use |
| Electrical energy |
Total energy consumed over time |
| SI units |
Current in ampere, voltage in volt, resistance in ohm |
Electricity Formulas With Units for Quick Revision
Electricity formulas with units help students avoid basic calculation errors. Most mistakes happen when students mix symbols, SI units, or time conversions.
| Quantity |
Formula |
SI Unit |
| Charge |
Q = It |
Coulomb (C) |
| Electric current |
I = Q/t |
Ampere (A) |
| Potential difference |
V = W/Q |
Volt (V) |
| Work done |
W = VQ |
Joule (J) |
| Resistance |
R = V/I |
Ohm (Ω) |
| Ohm’s law |
V = IR |
Volt (V) |
| Electric power |
P = VI |
Watt (W) |
| Power using current |
P = I²R |
Watt (W) |
| Power using voltage |
P = V²/R |
Watt (W) |
| Electrical energy |
E = Pt |
Joule (J) |
| Commercial energy |
1 kWh = 3.6 × 10⁶ J |
Kilowatt-hour |
| Series resistance |
R = R₁ + R₂ + R₃ |
Ohm (Ω) |
| Parallel resistance |
1/R = 1/R₁ + 1/R₂ + 1/R₃ |
Ohm (Ω) |
| Resistivity |
ρ = RA/l |
Ohm metre (Ω m) |
Basic Electricity Formulas Students Should Know
Basic electricity formulas form the base for circuit-based questions. Learn the meaning first, then memorise the formula and unit.
Electric Current Formula
Electric current is the rate at which electric charge flows through a conductor.
Formula: I = Q/t
Where:
I = electric current
Q = electric charge
t = time
The SI unit of current is ampere (A). One ampere means one coulomb of charge flows in one second.
Example:
If 10 C charge flows in 5 seconds, current = 10/5 = 2 A.
Charge Formula
Charge can be calculated when current and time are known.
Formula: Q = It
This formula helps when a question gives current for a fixed duration. Always convert minutes into seconds before substitution.
Example:
If current is 3 A for 10 seconds, charge = 3 × 10 = 30 C.
Voltage Formula
Voltage, also called potential difference, is work done per unit charge.
Formula: V = W/Q
Where:
V = potential difference
W = work done
Q = charge
The SI unit of voltage is volt (V). One volt means one joule of work moves one coulomb of charge.
Ohm’s Law Formula and Resistance Formula
Ohm’s law is one of the most important electrical formulas. It connects voltage, current, and resistance in a simple circuit.
Ohm’s Law Formula
Formula: V = IR
Where:
V = voltage
I = current
R = resistance
Ohm’s law means current is directly proportional to voltage when resistance stays constant. If voltage increases, current also increases.
The same formula can be rearranged as:
I = V/R
R = V/I
Use I = V/R when voltage and resistance are given. Use R = V/I when voltage and current are given.
Resistance Formula From Ohm’s Law
Resistance is the opposition offered by a conductor to electric current.
Formula: R = V/I
The SI unit of resistance is ohm (Ω). A conductor has 1 ohm resistance when 1 volt produces 1 ampere current.
Example:
If voltage is 12 V and current is 3 A, resistance = 12/3 = 4 Ω.
Resistance Formula Using Length and Area
Resistance also depends on the material, length, and area of a conductor.
Formula: R = ρl/A
Where:
R = resistance
ρ = resistivity
l = length of conductor
A = area of cross-section
A longer wire has more resistance. A thicker wire has less resistance. This is why electric wires use good conducting materials.
Power Formula in Electricity
Power tells how fast electrical energy changes into heat, light, sound, or mechanical energy. Bulbs, fans, heaters, and chargers all have power ratings.
Electric Power Formula
Formula: P = VI
Where:
P = electric power
V = voltage
I = current
The SI unit of power is watt (W). One watt means one joule of energy is used per second.
Power Formula Using Current and Resistance
Using Ohm’s law, power can also be written as:
Formula: P = I²R
Use this power formula when current and resistance are given.
Power Formula Using Voltage and Resistance
Another useful power formula is:
Formula: P = V²/R
Use this formula when voltage and resistance are given.
Example:
If a device works at 220 V and has resistance 110 Ω, power = 220²/110 = 440 W.
Electrical Energy Formula
Electrical energy is the total energy consumed by a device over time. It depends on power and duration of use.
Electrical Energy Formula in Joules
Formula: E = Pt
Where:
E = electrical energy
P = power
t = time
The SI unit of energy is joule (J). If power is in watts and time is in seconds, energy comes in joules.
Example:
A 100 W bulb used for 10 seconds consumes 100 × 10 = 1000 J.
Electrical Energy Formula in kWh
Household electricity bills use kilowatt-hour as the commercial unit of energy.
Formula: Energy in kWh = Power in kW × Time in hours
1 kWh = 3.6 × 10⁶ J
Example:
A 1 kW heater used for 2 hours consumes 2 kWh of electrical energy.
Series and Parallel Resistance Formulas
Series and parallel circuits use different resistance formulas. Identify the circuit type before solving the question.
Series Resistance Formula
In a series circuit, resistors connect one after another.
Formula: R = R₁ + R₂ + R₃
The total resistance in series is greater than each individual resistance. Current remains the same in all resistors, while voltage gets divided.
Parallel Resistance Formula
In a parallel circuit, resistors connect across the same two points.
Formula: 1/R = 1/R₁ + 1/R₂ + 1/R₃
The total resistance in parallel is less than the smallest individual resistance. Voltage remains the same across each branch, while current gets divided.
Electricity Formulas With Symbols
Symbols make electrical formulas shorter, but they can confuse students during revision. Learn each symbol with its meaning before solving numericals.
I means electric current.
Q means electric charge.
t means time.
V means potential difference or voltage.
W means work done.
R means resistance.
P means electric power.
E means electrical energy.
ρ means resistivity.
l means length of conductor.
A means area of cross-section.
Use the same symbols consistently while solving. Do not use W for both work and watt in the same step without context.
Electricity Formulas With Symbols
- I — electric current
- Q — electric charge
- t — time
- V — potential difference
- W — work done
- R — resistance
- P — electric power
- E — electrical energy
- ρ — resistivity
- l — length of conductor
- A — area of cross-section
Do not use W for both work and watt in the same solution without clarifying context.
Solved Examples on Electricity Formulas
Solved examples help students understand which formula fits each question. Practise the method before memorising shortcuts.
Example 1: Find Electric Current
Question: A charge of 24 C flows through a wire in 6 seconds. Find the current.
Formula: I = Q/t
Solution: I = 24/6 = 4 A
Answer: The current is 4 A.
Example 2: Find Resistance
Question: A circuit has voltage 10 V and current 2 A. Find resistance.
Formula: R = V/I
Solution: R = 10/2 = 5 Ω
Answer: The resistance is 5 Ω.
Example 3: Find Electrical Energy
Question: A 200 W device runs for 5 seconds. Find energy used.
Formula: E = Pt
Solution: E = 200 × 5 = 1000 J
Answer: The device uses 1000 J of electrical energy.
Quick Revision List of Electricity Formulas
Revise these Electricity Formulas before solving numerical questions.
Current: I = Q/t
Charge: Q = It
Voltage: V = W/Q
Work done: W = VQ
Ohm’s law: V = IR
Resistance: R = V/I
Resistivity: ρ = RA/l
Power: P = VI
Power: P = I²R
Power: P = V²/R
Energy: E = Pt
Commercial unit: 1 kWh = 3.6 × 10⁶ J
Series resistance: R = R₁ + R₂ + R₃
Parallel resistance: 1/R = 1/R₁ + 1/R₂ + 1/R₃