21.1. Magnetic effect of a current.
When charged particles move, a magnetic field is induced around the charged particles.
A current-carrying conductor produces a magnetic field around it.
a) Magnetic field pattern around a straight wire.
- A straight conductor carrying an electric current produces a magnetic field as shown below:
- The magnetic field lines are concentric circles around the conductor.
- The magnetic field lines near the conductor are closer together. This means that the magnetic field is stronger closer to the conductor.
- The right-hand grip rule shows the direction of the magnetic field around a current-carrying straight conductor. The thumb points in the direction of the current while the fingers give the direction of the magnetic field around the straight conductor.
- The strength of the magnetic field also depends on the magnitude of the current passing through the wire. The larger the current, the greater the magnetic field strength.
- A circle with a dot in the centre represents a wire carrying a conventional current that flows out of the plane of the paper.
- A circle with a cross in the centre represents a wire carrying a conventional current that flows into the plane of the paper.
- The direction of the magnetic field is reversed when the direction of the current is reversed.
b) Magnetic field pattern around a flat coil.
- A current-carrying flat coil is a combination of 2 conductors carrying current in opposite directions.
- There are two ways to increase the magnetic field strength at the centre of the flat coil:
- Increase the current.
- Increase the number of turns of the coil.
c) Magnetic field pattern of a solenoid.
- A single wire that is coiled many times is known as a solenoid. Due to the numerous coils in a solenoid, the magnetic field is concentrated inside the coil.
- The polarity of a solenoid is easily determined in the following ways:
- Grip the solenoid with the right hand with the fingers pointing in the direction of the current flow. The end of the solenoid where the thumb points to is the N pole.
- The other end is the S pole.
- The magnetic field strength in a solenoid can be increased by:
- Increasing the current.
- Increasing the number of turns per unit length of the solenoid.
- Placing a soft iron core within the solenoid. The soft iron core concentrates the magnetic field lines, thereby increasing the magnetic field strength.
d) Uses of magnetism.
Electromagnets are temporary magnets created from the flow of an electric current. They are used in circuit breakers, electric bells and medical imaging machines such a magnetic resonance imaging (MRI) machines.
- Circuit Breaker:
- Safety device that switches off the electric supply when excessive current flows through the circuit.
- Works as a result of an electromagnet in the circuit.
- When there is a surge in current, solenoid becomes a very strong electromagnet due to the larger current. It then attracts the soft iron latch, and pushes a safety bar outwards resulting in an open circuit.
- Magnetic relay
- Electric bell
- Magnetic Resonance Imaging (MRO)
21.2. Force on current-carrying conductors.
A current-carrying conductor experiences a magnetic force when placed in a magnetic field unless the magnetic field is parallel to that of the current.
a) Force on current-carrying conductor in a magnetic field (motor effect).
Fleming’s left-hand rule can be used to deduce the direction of the magnetic force when a current-carrying conductor is in a magnetic field.
- Position the thumb, the forefinger and the second finger of the left hand such that they are at right angles to one another.
- Point the forefinger in the direction of the magnetic field.
- Point the second finger in the direction of the current.
- The thumb will then point in the direction of the magnetic force.
According to Fleming’s left-hang rule, when the direction of the magnetic field or the conventional current reverses, the direction of the magnetic force is also reversed.
b) Why does a current-carrying conductor experience a force when placed in a magnetic field?
An electric current is a moving stream of electric charges. Like a current, a charged particle experience a magnetic force when moving in a magnetic field. In order to induce a magnetic force, the charged particles must be in a magnetic field that is not parallel to the direction of the particle’s motion.
c) Force on a moving charge in a magnetic field.
When a charged particle moves in a magnetic field perpendicular to its direction of motion, It will experience a magnetic force, causing it to deflect in a circular path as shown in the diagrams below. Note that the direction of magnetic force F continually changes as the charged particle moves in a circular path.
d) Forces between two parallel current-carrying wires.
- When two current-carrying conductors are placed parallel to each other, they will induce magnetic fields around themselves and exert magnetic forces on each other. Using Fleming’s left-hand rule, we can deduce that the current carrying conductors will attract each other if both currents are in the same direction, and repel each other if both currents are in opposite directions.
- Magnetic forces exerted on the current-carrying conductors are equal in magnitude but opposite in direction.
- The combined magnetic field pattern due to the 2 parallel current carrying conductors are shown in the diagram below.
21.3. Force on a current-carrying rectangular coil in a magnetic field.
A current-carrying rectangular coil is equivalent to four current-carrying straight conductors (corresponding to each side of the coil). The net force acting on the coil is equal to the vector sum of the 4 magnetic forces acting on each straight conductor.
The magnetic forces acting on different parts of a coil can give rise to a net moment (also known as torque) that causes the coil to turn as shown in the diagram below:
The net moment of the coil can be increased by increasing the number of turns on the coil, the current or the magnetic field strength.
a) DC Motor.
- The simple DC motor consists of a rectangular coil turning between 2 opposite poles of a magnet, a split-ring commutator and a current-supplying circuit.
- The simple DC motor uses the rotating rectangular coil to convert electrical energy to kinetic energy.
- Coil ABCD rotates about its axis PQ while in contact with a split ring commutator.
- The split-ring commutator reverses the direction of the current in the coil in every half revolution, i.e. when the coil is in the vertical position, to ensure that the coil always turns in the same clockwise or anticlockwise direction.
- The speed of rotation of the DC motor can be increased by:
- Increasing the strength of the magnets.
- Increasing the number of turns on the coil.
- Increasing the magnitude of the current.
- Placing a soft iron core in the coil.