Changing circuits 2
1. Changing Circuits (Time-Varying Circuits)
What is a Changing Circuit?
A changing circuit is an electrical circuit in which current, voltage, or resistance changes with time.This happens when:
A switch is opened or closed
Components like capacitors or inductors are present
The power supply varies (AC supply)
Temperature or physical conditions change
2. Types of Changing Circuits
(A) Switching Circuits
When a switch is turned ON or OFF, the circuit does not instantly reach its final state.
Example:
Turning on a fan
Switching on a bulb
Charging a capacitor
(B) RC Circuits (Resistor + Capacitor)
Charging a Capacitor
Initially, current is maximum
As the capacitor charges, current decreases
Finally, current becomes zero
Voltage across capacitor increases gradually
Time constant (τ):
τ=RC\tau = RCτ=RC
It decides how fast the circuit changes
Discharging a Capacitor
Stored charge flows through resistor
Current and voltage decrease exponentially
Energy is dissipated as heat in the resistor
(C) RL Circuits (Resistor + Inductor)
When Switched ON
Current increases slowly
Inductor opposes change in current (self-induction)
When Switched OFF
Inductor tries to keep current flowing
Can cause sparks (back EMF)
Time constant:
τ=LR\tau = \frac{L}{R}τ=RL
(D) RLC Circuits (Resistor + Inductor + Capacitor)
Used in radios, tuning circuits
Can show oscillations
Energy transfers between capacitor and inductor
3. Why Circuits Do Not Change Instantly
Because of:
Capacitance (stores electric charge)
Inductance (resists change in current)These properties create transient states before steady state.
4. Applications of Changing Circuits
Power supplies
Chargers
Radio and TV tuning
Signal processing
Electronic timing circuits
ELECTRICAL RESISTANCE
1. What is Electrical Resistance?
Electrical resistance is the opposition offered by a conductor to the flow of electric current.
Symbol: R Unit: Ohm (Ω)
2. Ohm’s Law
V=IR
Where:
V = Voltage
I = Current
R = Resistance
This law holds at constant temperature.
Types of Resistance
(A) Fixed Resistance
Value does not change
Example: Carbon resistors
(B) Variable Resistance
Value can be adjusted
Example: Rheostat, Potentiometer
(C) Non-Ohmic Resistance
Does not follow Ohm’s Law
Example: Diodes, filament lamps
Applications of Resistance
Heaters
Bulbs
Current control
Voltage division
Protection of circuits