Electricity and magnetism are fundamental topics in O Level Physics. This guide explains the key concepts clearly.
Static Electricity
Static charge:
- Caused by electron transfer
- – Positive = lost electrons
- – Negative = gained electrons
Like charges repel, unlike charges attract.
Electric fields:
- Region where a charge experiences a force
- – Field lines point from positive to negative
Current Electricity
Current (I):
- Rate of flow of charge
- – Unit: Ampere (A)
- – I = Q/t (current = charge/time)
Potential Difference (V):
- Energy transferred per unit charge
- – Unit: Volt (V)
- – V = W/Q (voltage = energy/charge)
Resistance (R):
- Opposition to current flow
- – Unit: Ohm (Ω)
- – R = V/I
Ohm’s Law
For ohmic conductors:
V = IR
Voltage = Current × Resistance
Ohmic conductors have constant resistance (e.g., wire at constant temperature).
Non-ohmic conductors have varying resistance (e.g., filament bulb, diode).
Circuits
Series Circuits:
- Components in a single loop
- – Same current through each component
- – Voltages add up: V(total) = V1 + V2 + V3
- – Resistances add up: R(total) = R1 + R2 + R3
Parallel Circuits:
- Components in separate branches
- – Same voltage across each branch
- – Currents add up: I(total) = I1 + I2 + I3
- – 1/R(total) = 1/R1 + 1/R2 + 1/R3
Electrical Energy and Power
Energy:
E = VIt
E = Pt
E = I²Rt
E = (V²/R)t
Power:
P = IV
P = I²R
P = V²/R
Units:
- Energy: Joule (J) or kilowatt-hour (kWh)
- – Power: Watt (W)
Cost of electricity:
Cost = Power (kW) × Time (hours) × Rate ($/kWh)
Electrical Safety
Hazards:
- Overheating
- – Short circuits
- – Electric shock
Safety devices:
- Fuses: melt if current too high
- – Circuit breakers: trip when overloaded
- – Earth wire: provides path for fault current
- – Switch: breaks circuit
Three-pin plug:
- Live (brown): carries current
- – Neutral (blue): return path
- – Earth (green/yellow): safety wire
Magnetism
Magnetic materials:
- Iron, nickel, cobalt
- – Attracted to magnets
- – Can be magnetised
Magnetic poles:
- North and South
- – Like poles repel, unlike poles attract
Magnetic fields:
- Region where magnetic force is felt
- – Field lines go from North to South outside magnet
Electromagnetism
Current-carrying conductors produce magnetic fields.
Right-hand grip rule:
- Thumb points in current direction
- – Fingers curl in field direction
Solenoids:
- Coil of wire
- – Strong magnetic field inside
- – Can be made into electromagnets
Factors affecting electromagnet strength:
- Number of turns
- – Current
- – Soft iron core
Uses of electromagnets:
- Electric bells
- – Relays
- – Maglev trains
- – Scrap metal lifting
The Motor Effect
A current-carrying wire in a magnetic field experiences a force.
Fleming’s Left-Hand Rule:
- First finger: Field (North to South)
- – Second finger: Current (positive to negative)
- – Thumb: Motion (force direction)
This is how electric motors work.
Electromagnetic Induction
A changing magnetic field can induce a voltage.
Faraday’s Law:
Induced voltage is proportional to rate of change of magnetic flux.
Ways to induce voltage:
- Move wire through magnetic field
- – Move magnet near wire
- – Change current in nearby wire
Generators:
- Convert mechanical energy to electrical
- – Use electromagnetic induction
- – Produce alternating current (AC)
Transformers:
- Change voltage levels
- – Only work with AC
- – Consist of primary and secondary coils
Transformer equation:
Vp/Vs = Np/Ns
Where:
Vp = primary voltage
Vs = secondary voltage
Np = primary turns
Ns = secondary turns
Step-up transformer: Vs > Vp (more secondary turns)
Step-down transformer: Vs < Vp (fewer secondary turns)
Power in transformers (ideal):
Pp = Ps
Vp × Ip = Vs × Is
Common Mistakes
- Confusing current and voltage
- 2. Wrong units
- 3. Not applying Fleming’s left-hand rule correctly
- 4. Forgetting transformers only work with AC
- 5. Not considering energy conservation
How Ace Scorers Helps
Our Physics programme covers:
- Electricity fundamentals
- – Circuit analysis
- – Electromagnetism
- – Exam techniques
Contact us for Physics tuition.
Ace Scorers – Nurturing Minds, Crafting Achievers
2A Maju Ave, Singapore 556680 | 6582934785