Magnetic Effects of Electric Current

The One Thing

Electric current and magnetism are directly linked: current produces a magnetic field, a magnetic field can exert a force on a current-carrying conductor, and a changing magnetic field can induce current. These principles underlie electric motors, electric generators and the safe distribution of electrical energy in domestic circuits.

Reactions, Processes and Experiments

Key Terms

• Magnetic field: The region around a magnet or current-carrying conductor in which a magnetic force can be experienced.
• Magnetic field lines: Imaginary lines used to show the direction and strength of a magnetic field; the tangent at any point gives the field direction.
• Right-hand thumb rule: If a straight conductor is held in the right hand with the thumb pointing in the direction of current, the curled fingers show the direction of magnetic field lines.
• Solenoid: A long coil containing many closely spaced circular turns of insulated wire; it produces a magnetic field similar to that of a bar magnet when current flows through it.
• Electromagnet: A temporary magnet made by passing current through a coil, usually wound around a soft iron core.
• Force on a current-carrying conductor: The force experienced by a conductor carrying current when it is placed in a magnetic field.
• Fleming's left-hand rule: With the thumb, forefinger and middle finger of the left hand held mutually perpendicular, the forefinger shows magnetic field direction, the middle finger shows current direction and the thumb shows force or motion direction.
• Electric motor: A device that converts electrical energy into mechanical energy using the force on a current-carrying coil in a magnetic field.
• Electromagnetic induction: The production of an induced current in a coil when the magnetic field linked with the coil changes.
• Fleming's right-hand rule: With the thumb, forefinger and middle finger of the right hand mutually perpendicular, the thumb shows conductor motion, the forefinger shows magnetic field direction and the middle finger shows induced current direction.
• Electric generator: A device that converts mechanical energy into electrical energy through electromagnetic induction.
• Domestic electric circuit: The wiring system that supplies electrical energy to appliances in a building through live, neutral and earth wires.
• Fuse: A safety device containing a wire of suitable low melting point that melts and breaks the circuit when excessive current flows.
• Short circuit: A dangerous condition in which live and neutral wires come into direct contact, producing a very large current.
• Overloading: A condition in which too many appliances draw current from a circuit, causing excessive current and possible overheating.

Easily Confused

• Fleming's left-hand rule vs Fleming's right-hand rule: The left-hand rule gives the force or motion of a current-carrying conductor in a magnetic field; the right-hand rule gives the direction of induced current when a conductor moves in a magnetic field.
• AC generator vs DC generator: An AC generator uses slip rings and produces alternating current; a DC generator uses a split-ring commutator and produces direct current.
• Alternating current vs direct current: Alternating current periodically changes direction; direct current flows in one direction.
• Short circuit vs overloading: A short circuit occurs when live and neutral wires come into direct contact; overloading occurs when too many appliances draw current from a circuit.
• Fuse vs earth wire: A fuse breaks the circuit when excessive current flows; the earth wire provides a low-resistance path for leakage current and protection against electric shock.
• Magnetic field direction vs magnetic field strength: Field direction is shown by the tangent to a field line, whereas field strength is indicated by the closeness of the field lines.
• Series protective connection vs parallel appliance connection: A fuse or miniature circuit breaker is connected in series with the live wire, whereas domestic appliances are connected in parallel.

What Gets Asked

• Determining the magnetic-field direction around a straight conductor: Apply the right-hand thumb rule carefully; reversing the current reverses the field direction.
• Interpreting field-line diagrams: State that field lines emerge from the north pole and enter the south pole outside a magnet, go from south to north inside it, never intersect, and are closer together where the field is stronger.
• Calculating or comparing the force on a conductor: Use F = BIL sin theta and identify that the force is greatest when the conductor is perpendicular to the field and zero when it is parallel.
• Describing an electric motor or generator: Include the specific components and functions: the motor has a coil, magnet or electromagnet, split-ring commutator and carbon brushes; an AC generator uses slip rings, whereas a DC generator uses a split-ring commutator.
• Explaining domestic wiring and protection: Distinguish the functions of live, neutral and earth wires, state that appliances are connected in parallel, and identify overloading and short circuit as conditions requiring protective disconnection.
• Calculating electrical power or energy: Use the stated power equations and remember that 1 kWh is equal to 3.6 x 10 to the power 6 joules.