Electricity Principles and Circuits

Overview

This lecture covers the key principles of electricity, including circuits, current, voltage, resistance, series and parallel circuits, electrical power, safety features, transformers, and static electricity.

Basics of Electricity and Circuits

• Electricity is the flow of charged particles (usually electrons) carrying energy from a source to a component.
• Electric circuits require a complete loop for charge to flow.
• A cell or battery stores chemical energy, providing energy to electrons in the circuit.
• Electrons flow from the negative to the positive terminal, but current is defined from positive to negative.
• A complete circuit allows continuous electron flow, keeping components like bulbs lit.

Charge, Current, and Potential Difference (Voltage)

• Charge is measured in coulombs (C); current is the rate of flow of charge.
• Potential difference (PD) or voltage (V) tells how much energy is transferred per coulomb (V = E/Q).
• Current (I) is measured in amperes (A) and calculated as I = Q/t (charge divided by time).
• Voltage is measured using a voltmeter in parallel; current is measured with an ammeter in series.

Resistance and Ohm's Law

• Resistance opposes the flow of current and is measured in ohms (Ω).
• Ohm's Law: V = IR (voltage = current × resistance).
• Ohmic resistors have constant resistance; V-I graph is a straight line.
• Bulbs/filaments are non-ohmic; resistance increases at higher voltages due to temperature.
• Diodes allow current only in one direction; LEDs emit light as well.

Series and Parallel Circuits

• Series: PD is shared, current is the same throughout, total resistance is the sum.
• Parallel: PD is the same across branches, current splits, adding resistors decreases total resistance.
• The greater the resistance, the greater the share of PD in series circuits.

Special Components: Thermistors and LDRs

• Thermistor: resistance decreases as temperature increases.
• LDR (Light Dependent Resistor): resistance decreases as light intensity increases.
• These can be used in temperature and light sensing circuits.

Electrical Power and Safety

• Electrical power: P = VI (power = voltage × current).
• Alternative: P = I²R when substituting Ohm's Law.
• Mains electricity: AC (alternating current), 230 V, 50 Hz.
• Plugs have live (brown), neutral (blue), and earth (yellow/green) wires; fuses protect against excessive current.

Transformers and the National Grid

• Transformers change voltage for efficient electricity transmission.
• Step-up transformers increase voltage, decrease current to reduce energy loss.
• Step-down transformers lower voltage for safe domestic use.
• Transformer formula: (Np/Ns) = (Vp/Vs), where N is number of turns, V is voltage.

Static Electricity and Electric Fields

• Rubbing insulators transfers electrons, creating static charge.
• Like charges repel, opposite charges attract.
• Charged objects produce electric fields; field direction is from positive to negative.

Key Terms & Definitions

• Current (I) — Rate of flow of charge, measured in amperes (A).
• Potential Difference (V) — Energy transferred per coulomb, measured in volts (V).
• Resistance (R) — Opposition to current, measured in ohms (Ω).
• Ohmic — A component with constant resistance.
• Thermistor — A resistor whose resistance decreases as temperature increases.
• LDR — A resistor whose resistance decreases as light increases.
• Transformer — Device that changes voltage using electromagnetic induction.

Action Items / Next Steps

• Practice calculating current, voltage, and resistance using Ohm's Law.
• Review series and parallel circuit calculations.
• Memorize key equations: V = IR, Q = It, P = VI, P = I²R.
• Read about transformer function and National Grid efficiency.