Every component has its own agreed symbol. A circuit diagram shows how components are connected together. A battery consists of two or more cells connected together.
Lamp – Cell – Battery – Voltmeter –
Resistor – Fuse – Ammeter -
Variable resistor – Thermistor –
Light Dependent Resistor (LDR) –
Light Emitting Diode (LED) – Diode -
Current-potential difference graphs are used to show how the current through a component varies with the potential difference across it.
The current is measured with an ammeter. Ammeters are always placed in series with the component. The unit of current is the ampere, A.
The potential difference is measured with a voltmeter. Voltmeters are always placed in parallel with the component. The unit of potential difference is the volt, V.
If the resistor is kept at a constant temperature the graph shows a straight line passing through the origin. This means the current is directly proportional to the potential difference across the resistor (Ohm's law).
Wire is an ohmic conductor because its resistance is constant.
Current-Potential Difference Graphs - Filament Lam
Filament lamp is a common type of light bulb which contains a thin coil of wire (filament). This heats up when an electric current passes through it, and produces light.
The resistance of a lamp increases as the temperature of its filament increases. The current flowing through a filament lamp is not directly proportional to the voltage across it.
An electric current flows when electrons move through a conductor, such as a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance. As the temperature increases, the metal ions vibrate more. There are more collisions with the electrons and so the resistance increases.
Current-Potential Difference Graphs - Filament Lam
The ‘reverse’ measurements are plotted on the negative sections of each axis.
- Line curves away from the current axis - the current is not directly proportional to the potential difference - filament lamp is a non-ohmic conductor
- Resistance (=potential difference/current) increases as the current increases, so the resistance of a filament lamp increases as the filament temperature increases
- Reversing potential difference makes no difference to the shape of curve, resistance is the same for the same current, regardless of its direction
Current-Potential Difference Graphs - Diode
Diodes are electronic components which can be used to regulate the potential difference in circuits and to make logic gates. They have very high resistance in one direction, current can only flow in the other direction.
- In the 'forward' direction, the line curves towards the current axis, current is not directly proportional to the potential difference - diode is not an ohmic conductor
- In the 'reverse' direction, current is negligible, resistance in the reverse direction is much higher than in the forward direction
Current-Potential Difference Graphs - Thermistor
Thermistors are used as temperature sensors, e.g. in fire alarms. Their resistance decreases as the temperature increases:
- At low temperatures, the resistance of a thermistor is high and little current can flow through them
- At high temperatures, the resistance of a thermistor is low and more current can flow through them
- At constant temperature, the line is straight so its resistance is constant
Current-Potential Difference Graphs - LDR
LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:
- In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it
- In bright light, the resistance of an LDR is low and more current can flow through it
- The same current passes through components in series with each other.
- The total potential difference of the voltage supply in a series circuit is shared between the components.
- The total potential difference of cells in series is the sum of the potential difference of each cell.
- The total resistance of components in series is equal to the sum of their separate resistances.
In the circuit, the lamps, cell and switch are connected in series with each other. Each electron is pushed through each lamp by the cell. The potential difference (voltage) of the cell is a measure of the energy transferred from the cell by each electron that passes through it.
- The total current through the whole circuit is the sum of the currents through the separate components.
- For components in parallel, the potential difference across each component is the same.
Calculations on Parallel Circuits
Components in a parallel have the same potential difference across them. The current through each component depends on the resistance of the component.
- The bigger the resistance of the component, the smaller the current through it. The resistor which has the largest resistance passes the smallest current.
Potential Difference (volts) = Current (amperes) x Resistance (ohms)