Current & Charge
Current & Charge
- A complete circuit already contains mobile charge carriers. Closing the switch makes them drift around the circuit.
- Current tells us how quickly charge passes a point.It does not measure how much energy each charge carries.
Carriers and direction
- In a metal, the mobile carriers are negative electrons. The positive ions remain fixed in a lattice.
- In an electrolyte, positive and negative ions both move and both contribute to the current.
- Conventional current is drawn in the direction a positive charge would move: from the positive terminal towards the negative terminal. Electron drift is in the opposite direction.
Definition
1 markWhat is meant by electric current?
Model answer: The rate of flow of electric charge.
Charge passing each second
Symbols
- = charge passing a point (C)
- = current (A)
- = time (s)
- One ampere means one coulomb passes the point each second. Therefore.
- A current-time graph gives charge from the area under the graph. For constant current, this area becomes .
Worked example
Keep time in seconds
A current of 0.50 A flows for 2.0 minutes. Calculate the charge.
- Convert time: .
- .
Answer
Charge comes in fixed units
- The magnitude of the charge on one electron is the elementary charge .
- Charge is quantised: the charge on an isolated object is a whole-number multiple of . Test a value by dividing it by ; the result must be an integer.
Your turn9702/22/M/J/25 Q6(b)(i)
A current of 0.030 A flows for 4.0 minutes. Calculate the charge that passes. [2] (9702/22/M/J/25 Q6(b)(i))
Show worked answer
- Convert 4.0 minutes to 240 s.
- .
Answer