LEARNING OBJECTIVES
What you will be able to do
- explain electrostatic charging and electric fields
- apply charge, current, voltage, resistance and power equations
- analyse series and parallel circuits
- describe component behaviour, mains safety and electrical energy use
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Connect charge, current, potential difference, resistance and energy in safe circuits.
Electric circuits transfer energy through moving charge. Current describes charge flow, potential difference describes energy transferred per charge, and resistance describes opposition to current.
Circuit rules follow from conservation of charge and energy. They also explain why homes use parallel circuits and why fuses, circuit breakers and earthing reduce danger.
SECTION 01
Charge, current and electric fields
Friction can transfer electrons between insulating materials. Gaining electrons makes an object negative; losing them makes it positive. Like charges repel and unlike charges attract.
An electric field is a region where a charge experiences a force. Field lines point from positive to negative and closer spacing represents a stronger field. Conventional current is defined from positive to negative, opposite electron flow in metals.
Charge through a lamp
- A current of 0.35 A flows for 4.0 min.
- Convert time: 4.0 min = 240 s.
- Q = 0.35 × 240.
Answer: 84 C of charge passes.
SECTION 02
Potential difference, resistance and components
Potential difference is energy transferred per unit charge; electromotive force is energy supplied per unit charge by a source. Resistance depends on material, length, cross-sectional area and temperature.
An ohmic conductor at constant temperature has current proportional to voltage. A filament lamp's resistance rises as it heats. An LDR has lower resistance in brighter light; a thermistor used here has lower resistance at higher temperature.
| Component | Input change | Resistance response |
|---|---|---|
| Filament lamp | temperature rises | increases |
| LDR | light intensity rises | decreases |
| NTC thermistor | temperature rises | decreases |
SECTION 03
Series and parallel circuits
In series, current is the same throughout, supply voltage is shared and resistances add. In parallel, voltage is the same across each branch and branch currents add to the supply current.
Adding a parallel branch lowers total resistance and increases total supply current. Potential dividers use series components to produce a variable output voltage, often with a sensor.
Two resistors in parallel
- 6 Ω and 3 Ω resistors are connected in parallel.
- 1/R = 1/6 + 1/3 = 3/6.
- R = 2 Ω.
Answer: Combined resistance = 2 Ω, less than either branch resistance.
SECTION 04
Electrical power, energy and safety
Electrical power is the rate of energy transfer. Kilowatt-hour is a unit of energy used for billing: one kilowatt-hour is the energy transferred by 1 kW in 1 hour.
A fuse melts or a circuit breaker opens when current is excessive. The live wire is dangerous because it is at high potential; the neutral completes the circuit; the earth wire keeps a metal case near zero potential. Double-insulated appliances do not require an earth wire.
Cost of using a heater
- A 2.0 kW heater runs for 3.5 h.
- Energy = 2.0 × 3.5 = 7.0 kWh.
- At $0.24 per kWh, cost = 7.0 × 0.24.
Answer: The cost is $1.68.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Current is charge flow; voltage is energy per charge.
- Series and parallel circuits obey different conservation rules.
- Component resistance can change with temperature or light.
- Safety devices interrupt dangerous currents or provide a low-resistance path to earth.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- explain electrostatic charging and electric fields
- apply charge, current, voltage, resistance and power equations
- analyse series and parallel circuits
- describe component behaviour, mains safety and electrical energy use