Electric Circuits
Introduced| English |
|---|
| circuit/ˈsɜːkɪt/ |
| electromotive force/ɪˌlektrəʊˈməʊtɪv fɔːs/ |
A battery, a wire, a bulb — the smallest complete loop
- Connect a battery to a bulb with wires and the bulb lights.
- Break the wire anywhere and it goes dark at once.
- Current needs an unbroken circuit 电路 — a closed loop.
- This lesson sets up the parts every circuit is built from.
The battery provides EMF
- A battery pushes charge with an electromotive force 电动势 (EMF), symbol $\varepsilon$.
- EMF is the energy given to each coulomb, measured in volts.
- It is not really a "force" — it is energy per charge.
- The battery is the pump that keeps the current flowing.
Build a circuit
Connect a battery to bulbs and see current flow only when the loop is complete.
The EMF of a battery is:
EMF is energy per charge, in volts — not really a force.
The component that gives energy to the charges is the ____.
The battery supplies the EMF that drives the current.
The loop must be complete
- Charge can only flow if there is a path back to the battery.
- Any break — a gap, an open switch — stops the current everywhere.
- A switch simply makes or breaks that loop on demand.
- No closed loop, no current.
Opening a switch anywhere in a simple loop stops the current everywhere.
A break anywhere opens the loop, so no current flows at all.
A ____ makes or breaks the loop to turn a circuit on or off.
A switch opens or closes the loop on demand.
Reading circuit symbols
- A battery is a long and short line; a resistor is a rectangle (or zig-zag).
- A bulb is a circle with a cross; a wire is a plain line.
- These symbols let us draw any circuit clearly.
- A neat diagram is the first step to solving a circuit.
Select all true statements about circuits.
Complete loop, EMF per charge, battery gives / resistors take. A break stops the whole loop.
The energy journey
- The battery gives each charge energy (a voltage rise).
- Components like bulbs and resistors take that energy (a voltage drop).
- Around a full loop, the energy given equals the energy used.
- That balance becomes Kirchhoff's loop rule later.
Around a complete loop, the energy the battery gives each charge:
Energy given equals energy used — the loop-rule balance.
A bulb won't light when connected to a battery. The switch is open.
- An open switch breaks the loop, so no current flows.
- Close the switch to complete the circuit and the bulb lights.
EMF is energy per charge (volts), not a force despite its name. And a battery only drives current when the circuit is a complete loop — a single broken wire stops the current in the whole circuit, not just at the break.
A circuit is a closed loop driven by a battery's electromotive force (EMF, $\varepsilon$) — energy per charge in volts. Current flows only when the loop is complete. The battery gives energy; resistors and bulbs take it, balancing around the loop.