Electric Current

What you'll learn

• Basic electric circuit components and symbols.
• Heating effect of electric current — applications and safety.
• Magnetic effect of electric current — electromagnets and uses.

Key concepts

Electric current and circuits

• Electric current: flow of electric charge (electrons) through a conductor.
• Unit: Ampere (A) — measured with an ammeter (connected in series).
• Voltage (potential difference): force that drives current; unit: Volt (V) — measured with a voltmeter (connected in parallel).
• Resistance: opposition to flow of current; unit: Ohm (Ω).

A complete electric circuit needs:

1. A source of electricity (cell/battery).
2. Connecting wires (conductors).
3. A load/device (bulb, motor, etc.).
4. A switch (to open/close circuit).

Circuit symbols

Types of circuits:

• Series circuit: components connected end-to-end; same current through all; one break stops all.
• Parallel circuit: components connected across same two points; same voltage across all; one break doesn't stop others. (Household wiring uses parallel circuits.)

Heating effect of electric current

When current flows through a conductor with resistance, electrical energy converts to heat energy.

Factors affecting heating:

• More resistance → more heat.
• More current → much more heat (heat ∝ I²).
• Longer time current flows → more heat.

H = I²Rt (Joule's Law of Heating): H = heat in joules; I = current (A); R = resistance (Ω); t = time (s).

Applications:

Nichrome (nickel + chromium alloy) is used in heating elements because:

• High melting point.
• High resistivity → produces more heat.
• Does not oxidise/corrode at high temperatures.

Tungsten is used in bulb filaments because:

• Extremely high melting point (~3422°C) — can glow white-hot without melting.
• High electrical resistance at high temperature → good emitter of light.

Electric fuse:

• Made of a material with low melting point (tin-lead alloy).
• Carries a rated current (e.g., 5A, 15A).
• If current exceeds rating (short circuit, overload), fuse wire melts → breaks circuit → protects appliances/wiring.
• Modern alternative: MCB (Miniature Circuit Breaker) — switches off automatically; can be reset.

Safety precautions:

• Never touch electrical equipment with wet hands (water conducts electricity).
• Use proper fuses; don't bypass fuses.
• Use earthing (grounding) — protects against electric shock; excess current flows safely to earth.

Magnetic effect of electric current

A current-carrying conductor produces a magnetic field around it.

• Discovered by Hans Christian Oersted (1820) — a compass needle deflected when current flowed through a nearby wire.

Rules for direction of magnetic field:

• Right-hand thumb rule: Point thumb in direction of current; fingers curl in direction of magnetic field lines (circles around wire).

Electromagnet:

• A coil of wire (solenoid) wound around a soft iron core; when current flows → magnetic field around solenoid acts like a bar magnet.
• North pole at one end, South pole at other (determined by right-hand rule for solenoid).

Strength of an electromagnet depends on:

1. Number of turns of coil (more turns → stronger magnet).
2. Amount of current (more current → stronger magnet).
3. Nature of core material (soft iron = good; air core = weaker).

Advantages of electromagnets over permanent magnets:

• Can be switched on and off.
• Strength can be varied by changing current.
• Can be made much stronger.

Applications:

Quick check

• Name the components of a complete electric circuit.
• What is the heating effect of current? Name three devices that use it.
• Why is nichrome used in electric heaters and not copper?
• What is an electric fuse? Why does it use a low-melting-point material?
• What is an electromagnet? State two advantages over a permanent magnet.

Open the Practice tab for graded questions on Electric Current.