Current flows only in a closed loop with a source of potential difference
Definition
Electric current
Electric current is the rate of flow of electric charge.
- For charge to flow, the circuit must be closed and must contain a source of potential difference, such as a cell or battery.
- A closed circuit has a complete, unbroken path, so if an open switch or a gap breaks it, no charge flows and there is no current.
- The source of potential difference is what pushes the charge around, so a circuit without one gives no continuous current.
Key Idea
A current exists only when there is a complete closed circuit that contains a source of potential difference.
Current is the rate at which charge flows past a point
- The size of the current tells you how quickly charge flows past a point in the circuit.
- A larger current means more charge passes each second; a smaller current means less charge passes each second.
- Current is measured in amperes, A\text{A}A, and charge flow is measured in coulombs, C\text{C}C.
- A current of 1 A1\ \text{A}1 A means 1 C1\ \text{C}1 C of charge passes a point every second.
Charge flow, current and time are linked by Q=ItQ = ItQ=It
- Charge flow, current and time are linked by the equation Q=ItQ = ItQ=It, where:
- QQQ is the charge flow in coulombs, C\text{C}C
- III is the current in amperes, A\text{A}A
- ttt is the time in seconds, s\text{s}s
- This is one of the equations you need to recall and use, so learn it.
- It rearranges to find the current, I=QtI = \dfrac{Q}{t}I=tQ, or the time, t=QIt = \dfrac{Q}{I}t=IQ.
Example
A current of 0.50 A0.50\ \text{A}0.50 A flows through a lamp for 444 minutes. Calculate the charge that flows.
State the equation:
Q=It Q = It Q=ItConvert the time to seconds:
t=4×60=240 s t = 4 \times 60 = 240\ \text{s} t=4×60=240 sSubstitute and calculate:
Q=0.50×240=120 C Q = 0.50 \times 240 = 120\ \text{C} Q=0.50×240=120 CThe charge that flows through the lamp is 120 C120\ \text{C}120 C.
Current is the same at every point in a single loop
- In a single closed loop the current has the same value at every point.
- This is because charge is not used up as it passes through a component, so the rate of flow of charge stays the same all the way round.
- For example, if the current leaving a cell is 0.40 A0.40\ \text{A}0.40 A, it is still 0.40 A0.40\ \text{A}0.40 A before and after every component in that loop.
Common Mistake
- Do not say the current is "used up" by a lamp or resistor: current is the rate of flow of charge, and charge is not used up in a single loop.
- Always convert the time to seconds before using Q=ItQ = ItQ=It, multiplying minutes by 606060.
Exam technique
- In a calculation, write the equation first, substitute values with consistent units, and give the unit in your answer.
- To explain why the current is the same at two points in a loop, say that charge is not used up, so the rate of flow of charge stays the same.
Self review
- What is electric current?
- What two things must a circuit have for charge to flow continuously?
- State the equation linking charge flow, current and time, with units.
- What does a current of 1 A1\ \text{A}1 A mean in coulombs per second?
- Why is the current the same at every point in a single closed loop?
