Electric current
Moving charges need a closed path
1. Electric current: from zero
2. Why does the bulb go out?
A battery, two wires and a bulb: the bulb glows. Now cut one of the wires.
Pick the right answer
Predict: what happens to the bulb?
- A. It stays lit: the battery still has energy
- B. It goes out
- C. It glows more weakly
الشرح
Charges flow around a complete path. Cutting a wire leaves an air gap that the charges cannot cross, so the current stops and the bulb goes out.
3. A flow of charges
Electric current
A flow of electric charges through a conductor or a solution.
Switch (0 = open, 1 = closed) 0
The battery pushes the charges. They flow only round a closed path.
In a metal wire, the moving charges are the free electrons.
4. In a solution, ions move
A solution can carry a current too, if it contains ions.
Each ion is pulled to the plate with the opposite charge. Opposite charges attract.
The ions must be free to move. In a solid salt, they are fixed in place.
5. Check yourself
Pick the right answer
Two plates joined to a battery touch each substance. Which one carries a current?
- A. Solid table salt (sodium chloride)
- B. Sodium chloride solution
- C. Sugar dissolved in pure water
الشرح
Sodium chloride solution carries a current because its ions are free to move to the plates. In solid salt the ions cannot move, and sugar does not form ions in water.
True or false?
In a solution, positive ions move towards the plate joined to the positive terminal.
الشرح
Unlike charges attract and like charges repel, so positive ions move towards the plate joined to the negative terminal, away from the positive plate.
Fill in the blanks
In a copper wire, the charges that move are free _____.
الشرح
In a metal wire, the metal ions stay in their places and the free electrons move between them, so the moving charges are free electrons.
6. Summary
- Electric current is a flow of electric charges.
- It needs a source, such as a battery, and a closed path.
- Wire: free electrons move. Solution: ions move to the oppositely charged plate.
Next: a current through a solution of metal ions can coat an object with that metal.
Can you explain why cutting one wire turns the bulb off?
Reveal
The charges flow only round a closed path. Cutting a wire leaves an air gap that the charges cannot cross, so the current stops and the bulb goes out.