3.1.1Electricity
Electric Current
5 min read 5 parts Lesson 1 of 16
Section 1 of 5Charge and Current
Electric current is the movement of charged particles around a complete circuit.
Charge carriers
- In metal wires the carriers are free electrons, which drift through the wire when a voltage is applied.
Charge
- Electric charge is measured in coulombs (C); one electron carries a very small charge.
Current
- Current is the charge passing a point in a circuit each second, so it measures the rate of flow.
The ampere
- A current of 1 ampere means that 1 coulomb of charge passes a point every second, so 1 A = 1 C/s.
Section 2 of 5The Equation Q = It
Charge, current and time are linked by one simple relationship.
The Equation Q = It
Equation
- Q = It, where Q is charge in coulombs, I is current in amperes and t is time in seconds.
Finding current
- Rearranging gives I = Q ÷ t, useful when the charge and time are known from a question.
Finding time
- Rearranging again gives t = Q ÷ I, which tells you how long a given charge takes to flow.
Time units
- Minutes and hours must be converted to seconds before substituting into the equation.
| Quantity | Symbol | Unit |
|---|---|---|
| Charge | Q | coulomb (C) |
| Current | I | ampere (A) |
| Time | t | second (s) |
Section 3 of 5Measuring Current
An ammeter is connected so that all the current being measured passes through it.
In series
- An ammeter is always placed in series, in the same loop as the component being investigated.
Same everywhere
- In a simple series loop the current has the same value at every point around the circuit.
Small currents
- Many electronic circuits use milliamperes, where 1 mA = 0.001 A, so convert before calculating.
Direction
- Conventional current is drawn from positive to negative, opposite to the actual direction of electron flow.
Section 4 of 5Worked Example
Question
- A current of 0.50 A flows through a lamp for 2.0 minutes.
- Calculate the charge that passes through the lamp.
- Convert the time to secondst = 2.0 × 60 = 120 s.
- Write the relationship Q = It.
- Q = 0.50 × 120 = 60 C.
Answer The charge that flows is 60 C.
Section 5 of 5Exam Checklist & Tips
3.1.1 – Key Points for the Exam
- Charge
- A property of matter measured in coulombs;
- moving charge makes a current.
- A property of matter measured in coulombs;
- Current The rate of flow of charge, measured in amperes.
- Coulomb
- The unit of electric charge;
- 1 C passes a point each second when the current is 1 A.
- The unit of electric charge;
- Ammeter A meter connected in series that measures current.
Common Mistakes
- Connecting an ammeter in parallel; it must be placed in series with the component.
- Using minutes directly in Q = It instead of converting to seconds.
- Forgetting to change milliamperes to amperes, for example using 25 instead of 0.025.
- Saying current is used up by a lamp; the same current returns to the cell.