Overview
This lecture covers the fundamentals of electricity, electric circuits, circuit components, formulas, types of current, the National Grid, and electric fields.
Basics of Electricity and Circuits
- Electricity is the flow of charge (usually electrons) transferring energy through a circuit.
- A complete loop of components and wires is necessary for current to flow.
- Cells (single) and batteries (multiple cells) supply chemical potential energy.
Current, Voltage (Potential Difference), and Charge
- Current is the rate of flow of charge, measured in amperes (A).
- Charge is measured in coulombs (C); a current flows from positive to negative terminal.
- Potential difference (PD or voltage V) is energy per coulomb transferred, measured in volts (V).
- Formulas:
- ( V = \frac{E}{Q} ) (Voltage = Energy/Charge)
- ( I = \frac{Q}{t} ) (Current = Charge/Time)
Measuring Current and Voltage
- Voltmeters measure PD; always connected in parallel to components.
- Ammeters measure current; always connected in series with components.
Resistance and Ohm’s Law
- Resistance opposes current; measured in ohms (Ω).
- Ohm’s Law: ( V = IR ) (Voltage = Current × Resistance).
- For resistors, current and voltage are directly proportional (ohmic behavior).
- Resistance of filament lamps/metals increases with higher current and temperature (non-ohmic).
Circuit Components and Behavior
- Series circuits: total PD is shared, current is the same everywhere, total resistance is summed.
- Parallel circuits: PD is the same across branches, current splits, total resistance decreases with more branches.
- Diodes allow current in only one direction; LEDs emit light in that direction.
- Thermistors: resistance decreases as temperature increases.
- LDRs (Light Dependent Resistors): resistance decreases as light intensity increases.
Power in Electrical Circuits
- Power (P) is the rate of energy transfer: ( P = VI ) (Power = Voltage × Current).
- Alternate power formula using resistance: ( P = I^2R ).
Mains Electricity and Safety
- Direct current (DC) flows in one direction (from batteries); alternating current (AC) switches direction (mains).
- Mains voltage in the UK is 230 V AC at 50 Hz.
- Plugs have live (brown), neutral (blue), and earth (yellow/green) wires for safety.
- Fuses protect circuits by breaking if current exceeds a safe value.
- Use ( P = VI ) to select the appropriate fuse.
The National Grid & Transformers
- Step-up transformers increase voltage, reducing current and power lost as heat during transmission.
- Step-down transformers lower voltage for safe use in homes.
- Transformer formula: ( N_P / N_S = V_P / V_S ) (ratio of coil turns equals ratio of voltages).
- Transformers require AC to operate, as changing magnetic fields induce current.
Static Electricity and Electric Fields
- Rubbing insulators transfers electrons, causing static charge.
- Opposite charges attract; like charges repel.
- Charged objects create electric fields; field lines show force direction from positive to negative.
- Electric field strength decreases with distance from the charge.
Key Terms & Definitions
- Current (I) — Flow of charge per second (amperes, A).
- Charge (Q) — Quantity of electricity (coulombs, C).
- Potential Difference (V) — Energy transferred per unit charge (volts, V).
- Resistance (R) — Property opposing current (ohms, Ω).
- Ohmic — Component where current and voltage are directly proportional.
- Diode — Component allowing current in only one direction.
- Thermistor — Resistor with resistance decreasing as temperature increases.
- LDR — Resistor with resistance decreasing as light increases.
- Transformer — Device changing voltage using coils and magnetic fields.
- Electric Field — Region where electric forces act between charges.
Action Items / Next Steps
- Practice calculations using Ohm's Law (( V = IR )), charge (( Q = It )), and power (( P = VI )).
- Review circuit diagrams for series and parallel arrangements.
- Prepare to use transformer equations for Paper 2.
- Understand how to select appropriate fuses for electrical appliances.