1.0 Electricity: The Basics
Electricity is a form of energy resulting from the existence of charged particles. In this chapter, we transition from static charges to Current Electricity, which is the flow of electrons through a conductor.
Electric Current
Electric Current ($I$) is defined as the rate of flow of electric charge through a cross-section of a conductor.
SI Unit: Ampere ($A$)
Instrument: Ammeter (always connected in series).
Current Formula
$$I = \frac{Q}{t}$$
Where: $I$ = Current, $Q$ = Charge (Coulombs), $t$ = Time (seconds)
1.1 Components of an Electric Circuit
A continuous path for the flow of electricity is called an Electric Circuit. To draw these circuits easily, we use standard symbols:
| Component | Symbol Description |
|---|---|
| Cell | One long vertical line (+) and one short thick line (-) |
| Battery | A combination of two or more cells |
| Switch (Key) | Open (no flow) or Closed (flow) |
| Resistor/Bulb | Zig-zag line or a looped filament |
Electrons (negative charges) flow from the negative terminal to the positive terminal. However, by convention, we say Electric Current flows from positive to negative.
If 20 Coulombs of charge flows through a wire in 4 seconds, find the current in the wire.
Solution:
1. Given Charge ($Q$): $20\,C$
2. Given Time ($t$): $4\,s$
3. Formula: $I = Q / t$
4. Calculation: $20 / 4 = 5\,A$
Final Answer: The current is $5\,Amperes$.
A Fuse is a safety device in a circuit. It has a low melting point so that if too much current flows, the wire melts and breaks the circuit, protecting your appliances from damage.
2.0 Series and Parallel Circuits
In an electric circuit, components like bulbs or resistors can be connected in two primary ways. These arrangements determine how the current and voltage are distributed across the components.
2.1 Series Circuit
In a Series Circuit, components are connected end-to-end, forming a single path for the current to flow. The same current passes through every component.
- Current: Remains the same throughout the circuit ($I = I_1 = I_2$).
- Disadvantage: If one component (like a bulb) breaks or is switched off, the entire circuit is broken, and all other components stop working.
2.2 Parallel Circuit
In a Parallel Circuit, components are connected across the same two points, providing multiple paths for the current. The total current is divided among the branches.
- Voltage: Remains the same across each branch.
- Advantage: If one bulb fuses, the others continue to glow. Each appliance can be operated independently. This is why household wiring is done in parallel.
Key Differences
| Feature | Series | Parallel |
|---|---|---|
| Pathways | One | Many |
| Current | Same | Divided |
If our home was wired in series, turning off the kitchen light would turn off the refrigerator, the fans, and every other device in the house! Parallel wiring ensures each device gets the full 220V (or 110V) it needs to operate correctly.
Two bulbs are connected in series. If the current flowing through the first bulb is 2 A, what is the current flowing through the second bulb?
Solution:
1. Concept: In a series circuit, there is only one path for the flow of electricity.
2. Rule: The current remains the same at all points in a series circuit.
Final Answer: The current through the second bulb will also be $2\,Amperes$.
Decorative "fairy lights" used during festivals were traditionally connected in series. That's why if a single small bulb burned out, the entire string of hundreds of lights would go dark!
3.0 Magnetic Effect of Electric Current
In 1820, a scientist named Hans Christian Oersted discovered that electricity and magnetism are linked. He noticed that a compass needle deflected when placed near a wire carrying electric current. This proved that an electric current produces a magnetic field around it.
3.1 Electromagnets
An electromagnet is a temporary magnet made by winding a coil of insulated copper wire around a soft iron core. It only acts as a magnet as long as the electric current flows through the coil.
Factors affecting Magnetic Strength
You can make an electromagnet stronger by:
- Increasing the amount of current flowing through the coil.
- Increasing the number of turns in the coil.
- Using a soft iron core (it magnetizes and demagnetizes quickly).
3.2 Electric Bell
The electric bell is a common application of the magnetic effect of current. It consists of an electromagnet, an armature, a striker (gong), and a contact screw.
How it works:
- When the switch is pressed, current flows and the electromagnet pulls the iron armature.
- The striker hits the gong, producing sound.
- As the armature moves, it breaks the connection at the contact screw.
- The electromagnet loses its magnetism, the armature springs back, and the circuit is completed again. This cycle repeats rapidly!
We use Soft Iron for electromagnets because it loses its magnetism instantly when the current is cut off. If we used Steel, it would become a permanent magnet, and the bell striker would get stuck to the magnet and never spring back!
State two differences between a permanent magnet and an electromagnet.
Solution:
1. Permanence: A permanent magnet retains magnetism forever; an electromagnet is temporary (on/off with current).
2. Strength: Strength of a permanent magnet is fixed; strength of an electromagnet can be changed by adjusting current or turns.
Maglev Trains (Magnetic Levitation) use powerful electromagnets to float above the tracks! Since there is no friction with the ground, they can travel at incredibly high speeds of over 600 km/h.