Rapid Revision - Class 7 Science- Chapter 14 - Electric Current and Its Effects - CBSE Class Notes Online

Revision Notes -Chapter 14 – Electric Current and Its Effects

What is Electric Current?
- It’s like water flowing through a pipe but with electricity flowing through wires and cables.
- Essential for powering various appliances, from light bulbs to computers.
How is Electricity Produced and Sent to Us?
- Generated in power stations, which are like ‘electricity factories’.
- Travels through a network of wires and poles, or sometimes underground cables, to reach our homes and schools.
Understanding Electric Circuits
- An electric circuit is a path that allows electricity to flow smoothly.
- Key parts include wires (paths for electricity), a bulb (shows if electricity is flowing), and a switch (controls the flow, like a gate).
Closed vs Open Circuit
- Closed Circuit: When the switch is ON, the circuit is complete, and the bulb lights up.
- Open Circuit: When the switch is OFF, the circuit is open, stopping electricity, and the bulb stays off.
When a Bulb Doesn’t Light Up
- Even if the switch is ON, a bulb might not light up if it’s fused (broken filament inside).
Circuit Diagrams
- They are like blueprints for electric circuits, showing how components are connected using symbols.
- Helps us understand and build circuits easily.
Electronic Components and Symbols
- Basic elements of a circuit include cells/batteries, switches, and bulbs.
- Each has a standard symbol, like parallel lines for cells/batteries and a break in a line for switches.
- These symbols are a universal language for understanding circuits globally.
What is an Electric Cell?
- It’s a basic source of electricity, like in torches, clocks, and toys.
- Provides about 1.5 volts, much less than our home electricity (about 220 volts).
Understanding Batteries
- A battery has several electric cells joined together.
- Used when one cell’s power isn’t enough, like in car batteries, which are rechargeable.
How Cells are Combined: Series vs Parallel
- Series Connection: Cells lined up end-to-end (positive to negative). Increases voltage. Example: Two cells give 3 volts together.
- Parallel Connection: Cells side by side, with all positives and negatives connected. Increases capacity (how long they last), but voltage stays the same.
The Heating Effect of Electric Current
- Passing current through a device can make it hot.
- More heat with higher resistance (like nichrome wire) or more current.
- This is why bulbs light up and heaters get warm.
Practical Uses of the Heating Effect
- In Bulbs: The filament gets hot and glows, giving us light.
- In Heaters and Irons: They get hot to warm things up or iron clothes.
- In Electric Fuses: They melt to break a circuit if too much current flows, preventing fires or damage.
Magnetic Effect of Electric Current
- Discovered by Hans Christian Oersted.
- When electric current passes through a wire, it creates a magnetic field, like how a magnet works.
- Straight Wire: Current flowing through it creates a magnetic field around the wire.
- Coiled Wire: More turns in the coil increase the magnetic effect.
- With Iron Core: Wrapping the coil around an iron rod makes the magnetic field even stronger.
Lightning and Its Connection to Magnetism
- Lightning is a powerful electric discharge during a storm.
- It happens when air currents and water droplets move in opposite directions in clouds, creating a charge.
- Lightning can create natural magnets called lodestones from the mineral magnetite.
How Electric Bulbs and Fuses Work
- Electric Bulb: Has a filament inside that gets very hot and glows when current passes through, providing light.
- Electric Fuse: A safety device in circuits to prevent too much current flow. It has a wire that melts and breaks the circuit if current is too high, stopping electricity flow to prevent fires or damage.
Compact Fluorescent Lamps (CFLs)
- More energy-efficient than regular bulbs.
- Produce less heat and more light, saving energy.
- Work by converting electric current into ultraviolet light, which then makes the fluorescent coating inside glow.
- Look for the ISI mark for safety and efficiency.
Electric Bell: How It Works
- Consists of a U-shaped electromagnet and an armature connected to a clapper.
- Pressing the bell’s switch completes the circuit, magnetising the electromagnet.
- This attracts the armature, hitting the gong to make sound.
- When the armature moves, it breaks and then remakes the circuit, causing continuous ringing.
Electromagnets
- Created by passing electric current through a coil wrapped around an iron core.
- Can be turned on or off and their strength adjusted by changing the current or coil turns.
- Used in many devices like electric bells and motors.
Electrical Safety
- Main hazards: Overloading and short circuits.
- Overloading: Too many appliances in one socket, causing wires to overheat and potentially start fires.
- Short Circuits: Happen when wire insulation wears out, causing wires to touch and overheat.
- Safety Devices:
  - Fuses: Break the circuit if too much current flows, using a wire that melts.
  - MCBs: Turn off automatically when current is too high and can be reset.
- The ISI mark on appliances indicates safety and quality.