Electrical quantities
Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)
Types of Electric Charge
- There are two types of electric charge: positive (+) and negative (−)
- Like charges repel
- two positive charges push apart, and two negative charges push apart
- Unlike charges attract
- a positive charge and a negative charge pull together
- Charge is measured in coulombs (C)
Conductors and Insulators
- An electrical conductor has free electrons that can move from atom to atom
- this lets charge flow easily
- An electrical insulator has electrons tightly bound to their own atoms
- charge cannot flow through it
| Good Conductors | Good Insulators |
|---|---|
| Metals (copper, aluminium, silver) | Plastics (polythene, PVC, Perspex) |
| Carbon (graphite) | Rubber, glass, dry air |
Experiment: Distinguishing Conductors from Insulators
- Set up a circuit with a cell and a lamp, with a gap left in the circuit for a test material
- Connect the test material across the gap
- Lamp lights → the material is a conductor; lamp stays off → the material is an insulator
What is Electric Current?
- An electric current is a flow of electric charge
- in metals, current is the flow of free electrons
Defining Current
Extended Only
- Electric current is the charge passing a point per unit time
- Formula:
- I = (Q/t)
- Where:
- I = current (A)
- Q = charge (C)
- t = time (s)
- Units:
- current is measured in amperes (A); 1 mA = 0.001 A = 10^-3 A
- charge is measured in coulombs (C); 1 C = 1 A × 1 s
Question: Museli measures a charge of 12 C passing a point in a circuit in 4 s, and wants to find the current.
write down the equation. I = (Q/t) substitute the values. I = (12/4) AnswerI = 3 A
Worked Example: Calculating ChargeQuestion: Muumbe has a current of 2.5 A flowing for 20 s, and wants to find the charge that passes a point in the circuit.
rearrange the equation. Q = I × t substitute the values. Q = 2.5 × 20 AnswerQ = 50 C
Electrical Conduction in Metals
- Each atom in a metal has one or more loosely held (free) electrons
- Applying a p.d. across the metal makes the free electrons drift from the negative terminal towards the positive terminal
- This drift of electrons is what makes up the electric current in a metal
Free electrons drift from the negative terminal towards the positive terminal
Conventional Current and Electron Flow
Extended Only
- Conventional current flows from positive to negative
- this is the direction used in circuit diagrams, and is a historical convention
- The actual flow of free electrons is from negative to positive
- this is the physical reality of what happens inside a metal
Using Ammeters
- An ammeter measures electric current
- It must be connected:
- in series with the component being tested
- with its + terminal connected towards the + terminal of the supply
| Analogue Ammeter | Digital Ammeter |
|---|---|
| Pointer moves across a scale | Numerical display |
| May have multiple scales/ranges to choose from | Select the range before measuring |
| Simple types read d.c. only | Can read both a.c. and d.c. |
Direct Current (d.c.) and Alternating Current (a.c.)
| Direct Current (d.c.) | Alternating Current (a.c.) |
|---|---|
| Electrons flow in one direction only | Direction of flow reverses regularly |
| Produced by batteries and cells | Produced by generators and the mains supply |
| Constant or varying magnitude | Varies sinusoidally with time |
Steady d.c., varying d.c., and a.c. all shown as current-time graphs
- The frequency of a.c. is the number of complete cycles per second, measured in hertz (Hz)
- mains supply frequency is 50 Hz in most countries
Electromotive Force (e.m.f.)
- Electromotive force (e.m.f.) is the electrical work done by a source in moving a unit charge around a complete circuit
- e.m.f. is measured in volts (V)
Calculating e.m.f.
Extended Only
- Formula:
- E = (W/Q)
- Where:
- E = e.m.f. (V)
- W = work done (J)
- Q = charge (C)
Potential Difference (p.d.)
- Potential difference (p.d.) is the work done by a unit charge passing through a component
- p.d. between two points is measured in volts (V)
- the p.d. is 1 volt if 1 joule of energy is transferred when 1 coulomb of charge passes between the points
- 1 V = 1 J/C
Calculating p.d.
Extended Only
- Formula:
- V = (W/Q)
- Where:
- V = p.d. (V)
- W = work done (J)
- Q = charge (C)
Question: Museli finds that 8 J of work is done when 2 C of charge passes through a lamp, and wants to find the p.d. across it.
write down the equation. V = (W/Q) substitute the values. V = (8/2) AnswerV = 4 V
Energy Transfers in a Circuit
- The source (battery or cell) supplies electrical energy to the charges passing through it
- Each component (lamp, motor, resistor) transfers energy from the charges to the surroundings, as heat, light, sound or kinetic energy
- The charge itself is not used up - the same amount of charge flows into and out of every component; it is the energy it carries that gets transferred around the circuit
A unit of charge picks up energy at the source and delivers it at each component
Using Voltmeters
- A voltmeter measures potential difference
- It must be connected:
- in parallel (across) the component being tested
- with its + terminal on the side where current enters the component
| Ammeter | Voltmeter |
|---|---|
| Measures current (A) | Measures p.d. (V) |
| Connected in series | Connected in parallel |
| Very low resistance | Very high resistance |
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