Electricity Study Guide

Higher: Fleming's Left-Hand Rule is a mnemonic used to determine the direction of the force (or thrust) on a current-carrying conductor placed within a magnetic field. It is crucial for understanding the operation of electric motors and other devices that rely on the motor effect. To apply the rule, extend the thumb, forefinger, and middle finger of your left hand so that each is at right angles to the other two. Each finger represents a specific direction:

• Thumb: Represents the direction of the Force (or motion/thrust) on the conductor.
• Forefinger (Index Finger): Represents the direction of the Magnetic Field (from North to South).
• Middle Finger: Represents the direction of the Current (from positive to negative).

It is vital to remember that these three quantities—force, magnetic field, and current—are always mutually perpendicular. If you know the direction of any two, you can use the rule to find the direction of the third. For example, if a current flows into a magnetic field, a force will be exerted on the conductor. This rule helps predict the exact direction of that force, which is fundamental to the motor effect principles where electrical energy is converted into mechanical energy.

I-V characteristic graphs, where 'I' represents current and 'V' represents potential difference (voltage), are essential tools for understanding how different electrical components behave in a circuit. These graphs plot the current flowing through a component against the potential difference across it. The shape of the curve reveals fundamental properties of the component, particularly its resistance.

For an ohmic resistor, the I-V graph is a straight line passing through the origin. This linearity demonstrates that current is directly proportional to potential difference, which is a statement of Ohm's Law (V = IR), assuming constant temperature. The resistance of the ohmic component can be calculated from the gradient of this line (R = V/I or 1/gradient if I is on the y-axis and V on the x-axis).

In contrast, non-ohmic components exhibit non-linear I-V graphs. For example, a filament lamp's I-V graph is a curve that flattens as the potential difference increases. This indicates that its resistance increases with temperature. As more current flows through the filament, it heats up, causing its resistance to rise.

Similarly, a diode is another example of a non-ohmic component with a distinct I-V characteristic. In the forward bias direction, a diode has very high resistance until a certain 'threshold voltage' is reached, after which its resistance drops sharply, allowing current to flow easily. In the reverse bias direction, a diode has extremely high resistance, allowing virtually no current to flow.

Understanding these characteristic curves allows us to predict how components will behave in different circuit conditions and to design circuits for specific applications. Practical investigations often involve constructing these graphs by measuring current and voltage at various settings.