Electrical Quantities and Safety Notes

Electric Charge

• Two types: positive and negative.
• Atoms: protons (+), electrons (-), neutrons (neutral).
• Normally, atoms are neutral (equal numbers of protons and electrons).
• Like charges repel; opposite charges attract.
• Electric charge measured in Coulombs (C).

Demonstrating Electric Charges

• Charging by friction: insulating solids rubbed together transfer electrons.
• One material gains electrons (negative charge), the other loses electrons (positive charge).
• Electrons are the particles that move during charging; positive charges do not move.

Experiments:

• Rod and cloth experiment: different materials gain or lose electrons.
• Gold-leaf electroscope: detects charge; leaf rises when charged, falls when discharged.

Electric Fields

• Region where an electric charge experiences a force.
• Direction: force on a positive charge.
• Electric field lines: from positive to negative.
• Point charge: radial field lines (outward for positive, inward for negative).
• Charged sphere: similar to a point charge.
• Parallel plates: uniform field, parallel lines from positive to negative.

Investigating Conductors & Insulators

• Conductors: allow charge flow easily (e.g., metals).
• Insulators: resist charge flow (e.g., rubber, plastic).
• Metals have delocalized electrons, enabling conductivity.
• Experiment: use electroscope to test conductivity; good conductors discharge the electroscope quickly.

Current

• Rate of flow of electric charge; measured in Amperes (A).
• Requires a closed circuit.
• Measured using an ammeter in series.
• Digital ammeters: more accurate; Analogue ammeters: parallax error.
• $I = frac{Q}{t}$, where $Q$ is charge (C) and $t$ is time (s).
• Conventional current: positive to negative; electron flow is opposite.

Direct & Alternating Current

• Direct current (DC): constant direction (e.g., batteries).
• Alternating current (AC): continuously changes direction (e.g., mains electricity).
• UK mains: 50 Hz, around 230 V.

Electromotive Force

• EMF: potential difference of a power source.
• Electrical work done by a source in moving a unit charge around a complete circuit.
• Measured in volts (V).
• $E = frac{W}{Q}$, where $W$ is energy transferred (J) and $Q$ is charge (C).

Potential Difference

• Work done by a unit charge passing through a component; measured in volts (V).
• Voltmeter in parallel.
• $V = frac{W}{Q}$, where $W$ is energy transferred (J) and $Q$ is charge (C).

Resistance

• Opposition to current; measured in Ohms ($\Omega$).
• Caused by collisions of electrons with metal ions.
• $R = frac{V}{I}$ (Ohm's Law).

Current-Voltage (I-V) Graphs:

• Ohmic resistors: linear I-V graph (constant resistance).
• Non-ohmic resistors: non-linear I-V graph (variable resistance).
• Filament lamp: resistance increases with temperature.
• Diode: allows current in one direction only.

Resistance of a Wire

• Longer wire: greater resistance.
• Thicker wire: smaller resistance.
• $R propto L$ (length), $R propto frac{1}{A}$ (cross-sectional area).

Electrical Energy

• Energy transferred by appliances; calculated by $E = VIt$.
• Measured in joules (J) or kilowatt-hours (kWh).

Electrical Power

• Rate of energy transfer; measured in watts (W).
• $P = frac{E}{t} = IV$.
• Energy usage: often measured in kWh for practical purposes.

Magnetism

• Poles: North and South.
• Like poles repel, opposite poles attract.
• Non-contact force.
• Permanent magnets: retain magnetism.
• Induced magnets: temporary magnetism in magnetic materials (iron, cobalt, nickel, steel).

Magnetic Fields

• Region where a magnetic pole experiences a force.
• Field lines: from North to South.
• Stronger field: lines closer together.
• Plotting: using iron filings or plotting compasses.

Electromagnetic Induction

• EMF induced when conductor and magnetic field have relative motion.
• Lenz's Law: induced EMF opposes the change causing it.
• Right-hand dynamo rule: determines the direction of induced EMF.

Demonstrations:

• Moving magnet through coil.
• Moving wire through a magnetic field.

Factors affecting EMF:

• Speed of movement, number of coil turns, strength of the magnetic field.

A.C. Generator

• Converts mechanical energy to electrical energy via a rotating coil in a magnetic field.
• Components: magnet, rotating coil, slip rings, carbon brushes.
• Produces alternating EMF.

Magnetic Effect of a Current

• Current-carrying wires produce magnetic fields.
• Straight wire: concentric circles around the wire (Right-hand grip rule).
• Solenoid: similar to a bar magnet.
• Electromagnets: solenoid with iron core.

Applications:

• Relay circuits.
• Loudspeakers.

Investigating the Field Around a Wire

Methods to investigate the magnetic field:

• Using iron filings to observe the field pattern.
• Using plotting compasses to map field direction.

Force on a Current-Carrying Conductor

• Current-carrying conductor in a magnetic field experiences a force (Motor effect).
• Fleming's left-hand rule: determines the direction of force, magnetic field, and current.
• Charged particles in a magnetic field are deflected.

Electric Motors

• D.C. motor: uses the motor effect to rotate a coil in a magnetic field.
• Components: coil, magnet, split-ring commutator, brushes.

Transformers

• Change the size of AC voltage or current.
• Components: primary coil, secondary coil, iron core.
• Step-up transformer: increases voltage.
• Step-down transformer: decreases voltage.

Calculations:

• $\frac{V<em>p}{V</em>s} = \frac{N<em>p}{N</em>s}$
• Ideal transformer equation: $I<em>p V</em>p = I<em>s V</em>s$
• High-voltage transmission: reduces current and energy loss.

Electric Circuits & Electrical Safety

Circuit Diagrams & Circuit Components:

Recognize and understand standard circuit symbols for circuit elements like cells, resistors, ammeters, voltmeters, switches, motors, lamps, diodes, LDRs and thermistors.

Current in Circuits:

• Series circuits: Current is constant throughout.
• Parallel circuits: Current splits at junctions (Kirchhoff's Current Law).

EMF & Potential Difference in Circuits:

• Series: EMFs add up; potential difference is divided among components.
• Parallel: Potential difference is the same across each branch.

Combined Resistance:

• Series: Resistances add up.
• Parallel: Combined resistance is less than individual resistances. Use $R<em>T = (\frac{1}{R</em>1} + \frac{1}{R_2})^{-1}$ to compute the total resistance.

Potential Dividers:

• Circuits that split voltage using series resistors: $frac{V<em>{out}}{V</em>{in}}=\tfrac{R<em>2}{R</em>1+R_2}$

Electrical Safety:

• Understand electrical hazards (damaged insulation, overheating cables, damp conditions, overloading).
• Know the functions and color codes of live, neutral, and earth wires.
• Safety devices: double insulation, earthing, fuses, circuit breakers.