Electrical Equipment & Circuits
Practical Skills - OCR A-Level Physics
Constructing Circuits
- To measure current, an ammeterAn instrument connected in series with a component to measure the current flowing through it. must be connected in series with the component.
- To measure potential difference, a voltmeterAn instrument connected in parallel across a component to measure the voltage drop across it. must be connected in parallel across the component.
- Switches are placed in series to control which path current takes (e.g. charging and discharging a capacitor).
- A diode must always face the direction of conventional current flow -- if placed backwards, it blocks the current completely.
- Every component has a maximum current it can safely handle. Exceeding this can blow internal fuses or cause permanent damage.
Polarity
Key Definition
Polarity
Whether an electrical component functions differently depending on the direction it is connected. Polarised components only work correctly in one specific orientation.
Whether an electrical component functions differently depending on the direction it is connected. Polarised components only work correctly in one specific orientation.
- Non-polarised components (resistors, lamps, wires) work equally well in either direction.
- Polarised components must be connected with the correct orientation:
- Cells and batteries -- have positive (+) and negative (-) terminals.
- Diodes -- allow current in one direction only (anode to cathode).
- LEDs -- the longer leg is the positive terminal (anode). Remember: "long = live".
- Electrolytic capacitors -- can be damaged or explode if reversed.
- Use a multimeter to check polarity: in forward bias, resistance is low; in reverse bias, resistance is very high.
Signal Generators
- A signal generatorAn electronic instrument that creates repeating waveforms of various frequencies and amplitudes for testing circuits. creates electrical waveforms that can be adjusted for different shapes, frequencies, and amplitudes.
- Often used alongside oscilloscopes to test and analyse circuit behaviour.
Cathode-Ray Oscilloscope (CRO)
Key Definition
Time-base
The horizontal scale setting on an oscilloscope that determines how much time each division represents, measured in s div$^{-1}$ or s cm$^{-1}$.
The horizontal scale setting on an oscilloscope that determines how much time each division represents, measured in s div$^{-1}$ or s cm$^{-1}$.
- A CRO displays, measures, and analyses waveforms. The x-axis represents time and the y-axis represents voltage.
- An a.c. voltage appears as a transverse wave; a d.c. voltage appears as a horizontal line.
- The time-base controls horizontal deflection. When switched off, only a vertical line is displayed.
- The voltage-gain (y-gain) controls vertical deflection. The peak voltageThe maximum vertical displacement from the time axis. Peak-to-peak voltage is the full range from trough to crest. is the maximum displacement from the time axis.
- To find frequency: measure the time period $T$ from the display, then use $f = \frac{1}{T}$.
- For accurate readings, adjust the time-base and voltage-gain so the waveform fills most of the screen.
Common MistakeHIGH
Wrong: Connecting a voltmeter in series or an ammeter in parallel.
Right: Ammeters go in series (current flows through them); voltmeters go in parallel (they measure the voltage drop across a component). An ammeter in parallel can cause a short circuit.
Right: Ammeters go in series (current flows through them); voltmeters go in parallel (they measure the voltage drop across a component). An ammeter in parallel can cause a short circuit.