Magnetism and magnetic fields Study Guide

Transformers are electrical devices that change the potential difference (voltage) of an alternating current. They consist of two coils of wire, called the primary coil and the secondary coil, wrapped around a soft iron core. When an alternating current flows through the primary coil, it creates a continuously changing magnetic field in the soft iron core. This changing magnetic field induces an alternating potential difference in the secondary coil. The relationship between the potential differences and the number of turns in each coil is fundamental to how transformers operate.

For an ideal transformer (one with 100% efficiency, meaning no energy loss), the ratio of the potential differences is equal to the ratio of the number of turns in the coils. This can be expressed by the equation:

Potential difference across primary coil (Vp) / Potential difference across secondary coil (Vs) = Number of turns in primary coil (Np) / Number of turns in secondary coil (Ns)

This equation allows us to calculate an unknown potential difference or number of turns if the other three values are known. Stepping up the voltage (Vs > Vp) requires more turns in the secondary coil than in the primary coil (Ns > Np), creating a 'step-up transformer'. Conversely, stepping down the voltage (Vs < Vp) requires fewer turns in the secondary coil than in the primary coil (Ns < Np), leading to a 'step-down transformer'. It is crucial to remember that these equations apply only to ideal transformers and alternating current, and practical transformers always have some energy losses.

Electric motors convert electrical energy into kinetic energy (motion). This is achieved through the motor effect, which describes the force experienced by a current-carrying conductor when placed in a magnetic field. In a simple DC electric motor, a coil of wire (the armature) is placed between the poles of a permanent magnet. When a current flows through the coil, each side of the coil experiences a force due to the magnetic field. According to Fleming's Left-Hand Rule, if the current flows in opposite directions in the two vertical sides of the coil, the forces on these sides will be in opposite directions. These opposing forces create a turning effect, or torque, which causes the coil to rotate. To ensure continuous rotation in the same direction, a component called a commutator is used. The commutator is a split-ring device that reverses the direction of the current in the coil every half rotation. This reversal ensures that the forces on the sides of the coil always work together to produce rotation in a single, consistent direction. Brushes, typically made of carbon, maintain electrical contact between the stationary power supply and the rotating commutator. The speed and strength of the motor's rotation can be increased by increasing the current, using a stronger magnetic field, or increasing the number of turns in the coil.