Students should be able to identify the action of common circuit components and explain the use of variable resistors, potential dividers, thermistors and light-dependent resistors in simple d.c. circuits.
An electric circuit is a complete conducting path that allows charge to flow. Each component is included for a purpose: to supply energy, control the circuit, convert electrical energy into another form, measure a quantity, or respond to the surroundings.
A cell supplies electrical energy by creating a potential difference between its terminals. A battery is made from two or more cells connected together. In circuit diagrams, the longer line represents the positive terminal and the shorter line represents the negative terminal.
A d.c. supply provides a steady potential difference in one direction. An a.c. supply provides a potential difference that repeatedly changes direction. In this topic, most circuit calculations use d.c. circuits, where current has a fixed direction from the positive terminal of the supply through the external circuit to the negative terminal.
A switch opens or closes a circuit. When the switch is open, the circuit is incomplete and no current flows. When it is closed, the circuit is complete and current can flow.
A fuse is a safety component. It contains a thin wire that melts when the current is too large, breaking the circuit and protecting the wires and components from overheating.
A lamp converts electrical energy mainly into light and thermal energy. A bell converts electrical energy into sound energy, usually using an electromagnet to make repeated motion. A light-emitting diode, or LED, emits light when current flows through it in the correct direction.
A fixed resistor has a constant resistance, provided its temperature remains steady. It reduces the current in a circuit and can be used to share potential difference between components.
A variable resistor allows its resistance to be changed. Increasing its resistance decreases the current in a simple circuit; decreasing its resistance increases the current. For example, a variable resistor can be used to control the brightness of a lamp.
A potentiometer is a variable potential divider. It has three terminals: two ends of a resistive track and a movable contact called a wiper. Moving the wiper changes the fraction of the supply potential difference taken as the output.
An input transducer changes a physical condition into an electrical effect. Thermistors and light-dependent resistors are common input transducers because their resistance changes with the environment.
| Component | How its resistance changes | Typical use |
|---|---|---|
| NTC thermistor | Resistance decreases as temperature increases. | Temperature sensing, such as fire alarms or thermostats. |
| Light-dependent resistor (LDR) | Resistance decreases as light intensity increases. | Light sensing, such as automatic night lights. |
A potential divider uses two resistors in series across a supply. The output potential difference is taken across one of the resistors. For two resistors \(R_1\) and \(R_2\) connected in series across a supply \(V_{\text{s}}\), the potential difference across \(R_2\) is:
\[V_{\text{out}} = V_{\text{s}}\frac{R_2}{R_1 + R_2}\]
If \(R_2\) is replaced by an NTC thermistor, \(V_{\text{out}}\) decreases when the temperature rises because the thermistor resistance decreases. If \(R_2\) is replaced by an LDR, \(V_{\text{out}}\) decreases when the light intensity increases because the LDR resistance decreases.
The placement of the sensor matters. If the sensor is the upper resistor instead, the output across the lower resistor changes in the opposite way. Always identify which component the output is measured across before deciding whether \(V_{\text{out}}\) increases or decreases.