3.2.1Movement and Interactions
Electric Current — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 10 parts Lesson 18 of 59
Part 1 of 8Electric current
Electric current
Electric current the rate of flow of electric charge, which is the charge passing a point per unit time.
Unit
- Unit the ampere (A), where 1 A = 1 C/s.
Condition
- Condition a current flows only when there is a complete circuit.
In metals
- In metals the current is a flow of negatively charged electrons through the lattice of metal ions.
Part 2 of 8Current equation
Equation
- I = Q ÷ t, with I in A, Q in C and t in s.What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
Rearranged forms
- Q = I × t and t = Q ÷ I.
Meaning
- Meaning the size of the current is the charge passing through a component each second.
Part 3 of 8Conventional current and electron flow
1Conventional current
- Conventional current it is taken as the flow of positive charge from the positive terminal to the negative terminal of a cell.
2Electron flow
- Electron flow electrons are negative, so they flow from the negative terminal to the positive terminal.
3Directions
- Directions conventional current and electron flow are in opposite directions.
Part 4 of 8Measuring current
Ammeter
- Ammeter it measures current and is connected in series with the component.
Analogue ammeter
- Analogue ammeter it has a needle and scale, with typical ranges of 0 to 1 A and 0 to 5 A.
- View the scale at 90° to avoid parallax error.
- Check the needle is at zero first, or subtract any zero error from every reading.
Digital ammeter
- Digital ammeter it shows digits, reads small currents in mA or μA and is more precise.
- The display may flicker between values, so a judgement is needed about which to record.
- Check for a zero error before use.
Part 5 of 8Direct current
Direct current (d.c.)
Direct current (d.c.) a steady current that always flows in the same direction round a circuit.
Source
- Source cells and batteries produce d.c.
Terminals
- Terminals a d.c. supply has a fixed positive and a fixed negative terminal.
Part 6 of 8Alternating current
Alternating current
Alternating current (a.c.)
Alternating current (a.c.) a current that continuously changes direction, going back and forth round a circuit.
Source
- Source generators produce a.c.
Terminals
- Terminals the two terminals are identical and keep swapping between positive and negative.
Frequency
- Frequency the number of times per second the current changes direction and returns.
Mains
- Mains the UK mains supply is a.c. at 50 Hz and about 230 V.
Part 7 of 8Comparing d.c. and a.c.
| Direct current | Alternating current |
|---|---|
| Flows in one direction | Changes direction continuously |
| Produced by cells and batteries | Produced by generators and the mains supply |
| Fixed positive and negative terminals | Two identical terminals |
Part 8 of 8More from the specification
1
Current flow and electrostatic field lines
- Current flow and electrostatic field lines uniform electric field lines accelerate free electrons, transferring electrical power across conductive circuit loads.
2
Electric charge
- Electric charge it is carried by electrons and protons, and charged objects are produced by friction as a polythene rod gains electrons.
3
Electric current
- Electric current the rate of flow of charge, I = Q / t, measured in amperes.
4
Potential difference and resistance
- Potential difference and resistance energy is transferred per coulomb across a component, and resistance is V / I.
5
Electric field
- Electric field a charged plate or object has a field in which another charge feels a force.
6
Electroscope
- Electroscope a gold leaf rises when charged because the leaf and plate carry the same charge and repel.
7
Field line
- A line showing the direction of the force on a north pole.
8
Electromotive force
- The electrical work done by a source in moving a unit charge around a complete circuit.
9
Gold-leaf electroscope
- A device that detects charge by the rising of a gold leaf.
10
Electric charge
- Electric charge it is carried by electrons and protons, and charged objects are produced by friction as a polythene rod gains electrons.
11
Electric current
- Electric current the rate of flow of charge, I = Q / t, measured in amperes.
12
Potential difference and resistance
- Potential difference and resistance energy is transferred per coulomb across a component, and resistance is V / I.
13
Electric field
- Electric field a charged plate or object has a field in which another charge feels a force.
14
Electroscope
- Electroscope a gold leaf rises when charged because the leaf and plate carry the same charge and repel.
15
Resistance
- How difficult it is for current to flow through a material.
16
Polythene
- Material that becomes negative when rubbed with cloth.
17
Electroscope
- Device for detecting charge using a gold leaf.
18
Conductor
- Material that lets current flow.
19
Acetate
- Material that becomes positive when rubbed with cloth.
20
Voltage
- A measure of the electrical energy supplied or changed in a circuit.
21
Insulator
- A material that does not conduct heat or electricity.
22
Potential difference
- The work done by a unit charge passing through a component.
Key terms
| Term | Meaning |
|---|---|
| Electric current | The rate of flow of electric charge. |
| Ampere | The unit of current, equal to one coulomb per second. |
| Conventional current | Current taken as flowing from the positive to the negative terminal. |
| Direct current | A current that flows in one direction only. |
| Alternating current | A current that continuously changes direction. |
| Ammeter | An instrument connected in series that measures current. |
| Frequency | The number of cycles of an a.c. supply per second, in hertz. |
Avoid these mistakes
1
Connecting an ammeter in parallel — it must be in series.
2
Saying current is used up in a component — the same current leaves as enters in a series circuit.
3
Saying conventional current flows from negative to positive — it flows from positive to negative.
4
Forgetting to check an ammeter for zero error before using it.
5
Using minutes instead of seconds in I = Q ÷ t.
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