Electricity Study Guide

Higher. When an electric current flows through a conductor, it creates a magnetic field around that conductor. If this current-carrying conductor is placed within the magnetic field of another magnet (e.g., a permanent magnet or an electromagnet), the two magnetic fields interact. This interaction results in a force being exerted on the conductor. This phenomenon is known as the motor effect. The direction of this force is perpendicular to both the direction of the current and the direction of the external magnetic field. This can be determined using Fleming's Left-Hand Rule: the forefinger points in the direction of the magnetic field (North to South), the middle finger points in the direction of the current (positive to negative), and the thumb indicates the direction of the force experienced by the conductor. The magnitude of this force depends on several factors: the strength of the magnetic field, the magnitude of the current, and the length of the conductor within the magnetic field. Increased values for any of these factors will lead to a stronger force. For the force to be exerted, the current must flow perpendicular to the magnetic field. If the current flows parallel to the magnetic field, no force is experienced.

Electrical resistance is a measure of how much an object opposes the flow of electric current. It is defined by Ohm's Law as the ratio of the potential difference (voltage) across a component to the current flowing through it (R = V/I). Some resistors are called 'ohmic' resistors, and for these components, their resistance value remains constant regardless of the potential difference across them or the current flowing through them, assuming temperature and other physical conditions remain constant. This means that if you double the voltage, you double the current, and their R value stays the same. A graph of current against potential difference for an ohmic resistor is a straight line passing through the origin.

However, for other components, known as 'non-ohmic' conductors, the resistance changes as the current and potential difference change. A common example is a filament lamp. As current flows through the filament, it heats up significantly. The increased temperature causes the metal ions within the filament to vibrate with greater amplitude. This increased vibration makes it more difficult for the free electrons (which carry the current) to pass through the material, leading to more frequent collisions between electrons and ions. These collisions impede the flow of charge, effectively increasing the resistance of the filament lamp. Therefore, as the current through a filament lamp increases, its temperature rises, and its resistance increases. A graph of current against potential difference for a filament lamp would show a curve, indicating that resistance is not constant. Diodes are another example of non-ohmic components, exhibiting very high resistance in one direction and low resistance in the other, and their resistance also changes with voltage.