4.2 Electrical Quantities
Electric Charge & Electric Field
- Two kinds of charge: positive and negative — like charges repel, unlike attract.
- Charge is measured in coulombs (C).
- Electric field direction at a point is the direction of force on a positive test charge. :contentReference[oaicite:0]{index=0}
Typical Field Patterns
- Single positive charge: field lines are radially outward.
- Single negative charge: field lines are radially inward.
- Unlike charges: lines go from + to –; like charges: lines spread apart with a region of weak field between them.
- Parallel plates: uniform field from the positive plate to negative plate.
Electrostatics (Friction & Detection)
- Rubbing insulators transfers electrons → objects become charged (electrons move; protons don’t).
- Use a gold-leaf electroscope to detect charge (by contact or induction). :contentReference[oaicite:1]{index=1}
Conductors vs Insulators
| Type | Properties | Examples |
|---|---|---|
| Conductors | Allow current; many free electrons; low resistance | Metals (Cu, Al), graphite, human body |
| Insulators | Do not allow current; very few free electrons; high resistance | Plastic, rubber, glass, dry air |
Electric Current
- Definition: rate of flow of charge,
I = Q / t. - Conventional current: + to – ; electron flow: – to +.
- Measured with an ammeter in series (analogue or digital). :contentReference[oaicite:2]{index=2}
e.m.f. and Potential Difference
- e.m.f. (E): work done per unit charge supplied by a source,
E = W / Q. - p.d. (V): work done per unit charge across a component,
V = W / Q. - Measured in volts (V) using a voltmeter in parallel. :contentReference[oaicite:3]{index=3}
Resistance
- Ohm’s law: at constant temperature,
V ∝ I(ohmic conductors). - Factors for a uniform wire: material, length ↑ → R ↑, area ↑ → R ↓, temperature ↑ → R ↑.
- Unit: ohm (Ω); measured with an ohmmeter. :contentReference[oaicite:4]{index=4}
I–V Characteristics (sketch & ideas)
- Ohmic resistor: straight line through origin (constant R).
- Filament lamp: curve flattening (hotter → R increases → non-ohmic).
- Diode: conducts in forward bias; almost no current in reverse. :contentReference[oaicite:5]{index=5}
Power & Energy
P = IV = I^2R = V^2/RE = IVt(kWh is a unit of energy). :contentReference[oaicite:6]{index=6}