Electrical Circuit – GCSE Physics
Introduction
- An Electrical circuit is a closed path that allows electric current to flow through it.
- It connects electrical components using conductors to perform a specific function using electricity.
- Electrical circuits make it possible to control and distribute electrical energy safely and efficiently.
Where it is used:
What is an Electrical Circuit?
- An Electrical circuit is a closed loop path that allows electric current to flow, allowing energy to power devices and systems.
- It connects some components so that electricity can do useful work.
- Without electrical circuits, we could not use electricity to light our homes, charge devices, run machines, or operate communication systems.
Basic Electric Circuit:
Components used in an electrical circuit with their symbols:
What are Series and Parallel Circuits?
Series Circuit
- In Series circuit, the components are connected end-to-end in a single path, so the same current flows through all of them.
Example:
- A simple flashlight with two batteries.
Parallel Circuit:
- In Parallel circuit, the components are connected across multiple paths, so voltage is the same across each branch, but current can vary.
Example:
- Home appliances (lights, fans, etc.) wired separately to the mains.
What is Potential Difference, Current & Resistance?
Potential Difference:
- Definition: The force required for the flow of electrons in a circuit is called potential difference.
- Unit: Volts (V)
- Provided by: A cell or battery.
Current:
- Definition: The flow of electric charge (usually electrons) through a conductor is called current.
- Unit: Amperes (A)
- Provided by: A cell or battery pushing charges through the circuit.
Resistance:
- Definition: The property of a material that opposes the flow of electric current.
- Unit: Ohms(Ω)
- Provided by: Resistors and the materials of wires/components in the circuit.
What is Ohm’s Law?
- Ohm’s Law states that the current through a conductor is directly proportional to the potential difference across it, if the temperature remains constant.
- It is written as:
Where:
- V = Potential difference
- I = Current
- R = Resistance
Solved Example:
Solved Example: Problem: If a circuit has a current of 3 A and a resistance of 3 Ω, find the voltage across it.
Solution:
Step #1: Given
- A = 3A
- R = 3Ω
Step #2: Using formula
Step #3: Plug the values
Therefore, the voltage across the circuit is 9V.
Final Answer: 9V
How are Charge, Current & Time Related?
Charge
- Definition: A measure of the total current that flowed within a certain period of time. It is carried by particles like electrons and protons.
- Symbol: Q or q
- Unit: Coulomb (C)
How to Calculate Charge:
- Formula:
Where,
- Q = Charge
- I = Current
- t = Time
Solved Example:
Problem: A current of 5 A flows through a circuit for 6 seconds. Calculate the charge that flows through the circuit.
Solution:
Step #1: Given
- A = 5A
- t = 6s
Step #2: Using formula
Step #3: Plug the values
Final Answer: 30 C
How are Energy, Voltage & Charge Related?
Energy
- Definition: An electricity, energy is the work done when charge moves through a voltage.
- Symbol: E
- Unit: Joule (J)
How to Calculate Charge:
- Formula:
Where:
- Q = Charge
- E = Energy
- V = Voltage
Solved Example:
Problem: A charge of 15 C moves through a voltage of 9 V. Calculate the energy transferred.
Solution:
Step #1: Given
- Q = 15Q
- V = 9V
Step #2: Using formula
Step #3: Plug the values
Therefore, 135 joules of energy are transferred.
Final Answer: 135 Joules
Solving Questions with Circuit Diagrams
Steps to Solve Question with Circuit Diagram
- Step #1: Identify given values in the circuit.
- Step #2: Choose the correct formula.
- Step #3: Substitute values and calculate the unknown.
Solved Example:
Problem: A circuit has a 12 V battery and a 4 Ω resistor in series. Find the current.
Solution:
Step #1: Identify given values in the circuit.
- V = 12 V
- R = 4Ω
Step #2: Choose the correct formula.
- Using Ohm’s Law:
Step #3: Substitute values and calculate the unknown.
- Substitute the values:
Therefore, the current flowing in the circuit is 3 A.
Final Answer: 3 A
Solved Example:
Problem: A parallel circuit has a 10 Ω resistor and a 20 Ω resistor. The current through the 20 Ω resistor is 0.6 A. Calculate:
1. The current through the 10 Ω resistor, and
2. The total voltage of the cell.
Solution:
Step #1: Identify given values in the circuit.
- R1 = 10Ω
- R2 = 20Ω
- I2 = 0.6 A
Step #2: Choose the correct formula.
- In parallel circuits,
- Voltage across each branch is the same:
Step #3: Substitute and calculate.
- The voltage across the resistors:
Using,
The voltage across the 10 Ω resistor is also 12 V.
- The current through the 10 Ω resistor:
Using,
The current through the 10 Ω resistor is 1.2 A.
- The total current from the cell:
In parallel:
The total current from the cell is 1.8 A.
Solved Example:
Problem: A battery of 9 V is connected in series with a 3 Ω resistor. Calculate the charge flowing through the circuit in 4 seconds.
Solution:
Step #1: Identify given values in the circuit.
- V = 12V
- R = 4Ω
- t = 4s
Step #2: Choose the correct formula.
- To find charge (Q), use:
First, find current (I) using Ohm’s Law:
Step #3: Substitute and calculate.
- Find the Current:
- Find the Charge:
The charge flowing through the circuit in 4 seconds is 12 C.
Final Answer: 12 C
Frequently Asked Questions
It is a closed path that allows electric current to flow.
Ohm’s Law states that:
V = I × R
Electric charge is a property of particles like electrons and protons that causes them to experience a force in an electric field.
It is the work done to move unit charge between two points. Measured in volts (V) using a voltmeter.
It is the opposition to the flow of electric current. Measured in ohms (Ω) using an ohmmeter.
Current stays the same in all parts of the circuit. It has only one path to flow.
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