4.2.5Electricity and Magnetism
Current — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions
5 min read 10 parts Lesson 9 of 31
Part 1 of 8Definition of current
Electric current
- Electric current the rate of flow of electric charge, or the charge passing a point per unit time.
Carriers
- Carriers in a metal wire the carriers are free electrons; in a salt solution they are ions.
Unit
- Unit the ampere (A), where 1 A = 1 C s−1.
- Smaller currents use milliamps (1 mA = 10−3 A) and microamps (1 µA = 10−6 A).
Part 2 of 8The equation I = Q / t (Extended)
I
Q / t
What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
- I is current (A), Q is charge (C) and t is time (s).
- Rearranged, Q = I t and t = Q / I.
Charge from a graph
- Charge from a graph the area under a current–time graph equals the charge passed.
- For a steady current the area is a rectangle, I × t.
- For a changing current the area can be estimated by counting squares.
Part 3 of 8Conventional current and electron flow
1Conventional current
- Conventional current flows from the positive terminal of the cell, round the circuit, to the negative terminal.
2Electron flow (Extended)
- Electron flow (Extended) electrons are negative, so they drift the opposite way, from negative to positive.
3Circuit symbols
- Circuit symbols arrows on diodes and ammeters follow conventional current.
Part 4 of 8Conduction in a metal wire
1Free electrons
- Free electrons a potential difference across the wire gives the electrons a small net drift towards the positive end.
2Drift speed
- Drift speed the drift is slow, often less than a millimetre per second.
3Instant response
- Instant response a lamp lights at once because the electric field travels through the wire at close to the speed of light and pushes all the electrons together.
4Same current in a thin section
- Same current in a thin section the current is the same along a wire, so the electrons drift faster where it is thinner.
Part 5 of 8Measuring current
Ammeter
- Ammeter measures current and is connected in series so the whole current passes through it.
Low resistance
- Low resistance a good ammeter has very low resistance so it does not change the current.
Analogue ammeter
- Analogue ammeter read from directly in front to avoid parallax error.
Digital ammeter
- Digital ammeter gives a direct reading and may flicker between values.
Zero error
- Zero error check the reading is zero before starting or subtract the zero value.
1
Break the circuit at the point where the current is to be measured.
2
Insert the ammeter in the gap with its + terminal towards the + side of the cell.
3
Close the switch and read the current.
4
Convert any reading in mA to A before using it in equations.
Part 6 of 8Current in a series circuit
Same everywhere
- Same everywhere all the charge entering a component leaves it, so ammeters before and after a lamp read the same.
Energy, not charge
- Energy, not charge a lamp transfers energy from the charge but does not use up the charge.
Parallel circuits
- Parallel circuits the current splits at a junction and the branch currents are re-united further on.
Part 7 of 8Effects of a current
Heating
- Heating a current heats a conductor, as in a kettle element.
Magnetic
- Magnetic a current produces a magnetic field, as in an electromagnet.
Chemical
- Chemical a current causes chemical change, as in electrolysis.
Part 8 of 8More from the specification
More from the specification
Electrostatic Charging by Friction
- When insulating materials like a polythene rod or an acetate rod are rubbed with a dry cloth in an electrostatic experiment, friction transfers electrons;
- polythene gains electrons to become negatively charged,
- while acetate loses electrons to become positively charged.
Detection of Static Charge Using a Gold-Leaf Electroscope
- When a charged object approaches the conductive metal plate of an electroscope, charge is induced down the stem and gold leaf;
- like charges repel,
- causing the leaf to diverge from the plate, confirming the presence of static voltage.
Electric Current
- Direct vs Alternating Flow — Electric current I is the rate of flow of electric charge, driven by voltage from a power supply; direct current (DC) flows continuously in one direction,
- (I = Q / t, in amperes, A)
- whereas alternating current (AC) periodically reverses its direction of flow.
Electroscope
- Instrument with a metal cap, stem and gold leaf used to detect charge.
Insulator
- Material that does not allow current to flow.
Alternating current
- Current that continually reverses direction.
Field lines
- Lines showing the direction and strength of a magnetic field.
Electromotive force
- Energy transferred from other forms to electrical energy per unit charge.
Gold-leaf electroscope
- A device in which a charged gold leaf falls when photoelectrons discharge the plate.
Mains supply
- Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
Electrical power
- Rate of energy transfer, P = I V.What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
Key terms
| Term | Meaning |
|---|---|
| Current | Rate of flow of charge, I = Q / t, in amperes. |
| Ampere | Current of one coulomb per second. |
| Conventional current | Current direction from the positive to the negative terminal. |
| Ammeter | Instrument that measures current, connected in series. |
| Charge carrier | Particle that carries charge, such as an electron or ion. |
Avoid these mistakes
1
Forgetting to convert minutes to seconds or mA to A before using I = Q / t.
2
Saying electrons move from positive to negative — conventional current does, electrons go from negative to positive.
3
Believing a lamp uses up current — the same current leaves as enters.
4
Connecting an ammeter in parallel with a component — it must be in series.
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