Current is defined as the rate of flow of charge. It is caused by the flow of charged particles.
Potential difference is energy transferred per unit charge.
Potential difference is the difference in energy per unit change from one point in a circuit to another. p.d. is measured in Volts (
Resistance is opposition to the flow of charge. It is measured in Ohms (
Power is defined as energy over time. It is measured in Watts (
The movement of electrons down a wire is obstructed by metal ions. Work is done as energy is transferred to these metal ions, causing the wire to heat up. This process, often called Joule heating, causes power to be dissipated in components.
In a conductor carrying a current, the electrons will experience a force from negative to positive. This causes an average displacement over time, which causes drift velocity.
Where:
Conductance is the amount of current that flows through a component for each volt of p.d. applied across it. It is the reciprocal of resistance, and is measured in Siemens (
For components in parallel, the voltage across each component is the same:
Giving the general rule that conductances in parallel sum. As conductance is the reciprocal of resistance, this gives the
For components in series, the current through each component is the same:
Considering equal currents gives:
By Ohm's law, this means the sum of reciprocals of conductances is equal to resistance. Thus, the conductance, which is the reciprocal of resistance, must be given by:
Conductances in series act like resistances in parallel. As conductance is the reciprocal of resistance, this gives the rule that resistances add in series.
Resistivity (
Conductivity (
Conductivity and resistivity are both bulk properties of a material that do not depend on the dimensions of a sample.
An ideal cell has no internal resistance, but a non-ideal cell can be modelled as an ideal cell and a resistor in series. The terminal difference of a cell is given by:
Where:
The internal resistance is the negative gradient of a V-I graph:
In a potential divider, the ratio of voltages across each series component is equal to the ratio of resistances:
Potential dividers are used in sensing circuits. By placing an LDR or thermistor in series with a fixed resistor, the output voltage changes in proportion to the sensor's resistance. Different situations require voltage to either decrease or increase according to conditions, which can be achieved by measuring the voltage across different sides of the potential divider.
In the left circuit, as light intensity increases, the resistance of the LDR decreases, causing the measured voltage to increase. In contrast, in the right circuit, as light intensity increases, the measured voltage decreases.
These sensing circuits require calibration so a voltage can be converted to a temperature or light intensity reading. This can be done by testing the output voltage for a range of input conditions and plotting a calibration curve (e.g. voltage against light intensity or temperature).
The sensitivity is the ratio of output (voltage) change to input change. It is the gradient of a calibration curve. For most circuits. For the above circuits, this would be measured in
IS: recognise standard circuit symbols.
The below circuit symbols are provided as a guide. It is not entirely clear which of these symbols are actually required.
Light dependent resistors (LDRs) are typically semiconductor devices. The energy provided by incident photons gives electrons sufficient activation energy to become mobile charge carriers, increasing conductivity. Thus, LDR resistance decreases as light intensity increases.
Thermistors can be either positive temperature coefficient (PTC) or negative temperature coefficient (NTC). These are described below in the section on I-V graphs. As temperature increases, PTC thermistors have increasing resistance, whilst NTC thermistors have decreasing resistance.
IS: graphs of current against potential difference and graphs of resistance or conductance against temperature for ohmic and non-ohmic devices or components.
Graphs of current against potential difference are below (same as GCSE). The filament lamp and diode are non-ohmic conductors, as their resistance varies.
The below graphs show how resistance and conductance vary against temperature for positive temperature coefficient and negative temperature coefficient materials.
Most conductors, including metals (and filament bulbs) have increasing resistance as temperature increases (positive temperature coefficient). This is because of Joule heating.
Some semiconductors have decreasing resistance as temperature increases. This is because increasing temperatures free more charge carriers, causing the conductivity to increase. Semiconductor conduction is an example of a process with an activation energy (described here, (IS)).