Electrical Circuits and Electrical Safety
Electrical Circuits and Electrical Safety
A Complete Guide for Grade 7 Students
17.1 Circuit Diagrams and Components
Circuit diagrams use symbols to represent electrical components. This makes it easier to design and understand circuits.
| Symbol | Name | Function |
|---|---|---|
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Cell | Single battery (1.5V). Long line = positive (+), short line = negative (-) |
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Battery | Multiple cells connected together (e.g., 3V battery = two 1.5V cells) |
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Resistor | Slows down electricity (like a narrow pipe for water) |
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Switch | Opens or closes the circuit (like a drawbridge) |
Open, Closed, and Short Circuits
Open Circuit: The path is broken (e.g., switch is off). No electricity flows. Example: A flashlight with a broken wire won't turn on.
Closed Circuit: The path is complete (e.g., switch is on). Electricity flows normally. Example: Your bedroom light turns on when you flip the switch.
Short Circuit: Electricity takes a "shortcut" (e.g., a wire bypasses a lightbulb). Too much electricity flows, which can overheat wires or cause fires! Example: If a live wire touches a metal casing (like in a broken toaster), it creates a short circuit.
17.2 Series Circuits
In a series circuit, all components are connected in a single loop. There's only one path for electricity to flow.
Key Rules for Series Circuits
- Current is the same everywhere: I₁ = I₂ = I₃ = ...
- Voltage adds up: V_total = V₁ + V₂ + ... + Vₙ
- Resistance adds up: R_total = R₁ + R₂ + ... + Rₙ
Real-life Example: Old Christmas lights were often in series. If one bulb blew out, the whole string went dark because the circuit was broken!
17.3 Parallel Circuits
In a parallel circuit, components are connected in separate branches. Electricity has multiple paths to flow.
Key Rules for Parallel Circuits
- Voltage is the same across all branches: V₁ = V₂ = V₃ = ...
- Current splits between branches: I_total = I₁ + I₂ + ... + Iₙ
- Combined resistance is less than smallest resistor: R_total < R₁, R₂, ...
Real-life Example: Your home lights are in parallel! If one bulb blows out, the others stay on because each has its own path.
17.4 Action and Use of Circuit Components
Potential Divider
A potential divider uses two resistors in series to get a smaller voltage from a bigger one.
Thermistors
NTC thermistors decrease resistance as temperature increases. Used in thermostats and temperature control circuits.
Light-Dependent Resistors (LDRs)
LDRs decrease resistance as light increases. Used in automatic streetlights that turn on at night.
Real-life Example: Streetlights use LDRs. During the day: bright light → low resistance → circuit off. At night: dark → high resistance → circuit on.
17.5 Electrical Safety
Common Electrical Hazards
- Damaged insulation: Exposed wires can cause electric shocks
- Overheating cables: Caused by overloaded sockets or thin wires
- Damp conditions: Water conducts electricity (never use hairdryers near water)
- Excess current: From overloading plugs and sockets
Safety Features
Fuses
A thin wire that melts when too much current flows, breaking the circuit. Must be replaced after blowing.
Earthing (Grounding)
Earth wire (green/yellow) connects metal casing to ground. If live wire touches casing, current flows through earth wire instead of through you.
Double Insulation
Appliances with two layers of insulation use two-pin plugs (no earth wire). Common in phone chargers and plastic-cased devices.
Important Safety Rules:
- Never use electrical devices with wet hands
