Section 2 c) Summary

Series and parallel circuits

The current is the rate of flow of charge: I = Q/t
The voltage is the energy transferred per unit of charge: V = E/Q

In a series circuit, all of the components are connected in a line. This means the voltage is split between each of the components, but the current stays constant throughout, which is why ammeters are connected in series with components.

In a series circuit, the components are not individually controllable: they are either all on, or all off (e.g. Christmas lights). This can be disadvantageous as it means that if one of the components is broken, the whole circuit is disconnected and it can be difficult to find out which is faulty.


In a parallel circuit, the components are connected in 'branches'. Current is split between the 'branches', but each 'branch' receives the same voltage, which is why voltmeters are connected in parallel with components.

In a parallel circuit, the branches are individually controllable. A switch can be attached to each 'branch' and the components can be turned on and off as desired (e.g. different lights in a house). If one component breaks, the circuit will still work and the faulty component is easily located. 

Resistance and Resistors

Each component in a circuit has resistance; this can be thought of as like drag or friction opposing motion.
A component has greater resistance if it:

• Emits heat (e.g. toaster)
• Has a larger cross sectional area (of where the current flows)(e.g. thicker wires)
• Total length of wires
• Is made out of a less conductive material (e.g. copper has less resistance than carbon)

If there is more resistance in a circuit, less current will flow (or it will require more voltage to reach the same current). 

Variable resistors, fixed resistors, LDRs and thermistors are all components used to increase or decrease resistance in a circuit. 

Variable resistors allow the user to choose how much resistance they want, this is useful in dimmer switches for example. 

Fixed resistors are used to reduce the flow of electrons in a circuit, some appliances need a lower current to be able to work correctly. 

LDRs, or light dependent resistors, decrease in resistance when exposed to more intense light, and increase in dimmer light. These are used in burglar alarms and light intensity meters.

Thermistors are temperature dependent resistors, they increase in resistance when exposed to cooler temperatures. They are used in many appliances to maintain temperature, e.g. microwaves.

The resistance of a component can be determined by measuring the voltage and current across it in a circuit, and using this formula:

Voltage = Current x Resistance

The results can be graphed on an I-V graph (current on the y-axis, voltage on the x-axis), where the gradient is 1/R. These are the most important I-V graphs to know:

^ This is the I-V graph of a wire, or a resistor at a constant temperature. A steeper gradient means a lower resistance, and a shallower gradient means it has a higher resistance. This is according to Ohm's law: Electrical current is proportional to voltage and inversely proportional to resistance.

^ This is the I-V graph of a filament lamp. It is curved, because as the temperature of the metal filament increases so does the resistance.

^ This is the I-V graph of a diode. Diodes only allow current to flow in one direction; the resistance is very high in the opposite direction. The point at which the current increases dramatically is around 0.6 V.

To create a graph like this yourself, you must set up a circuit as shown:

Then, using the variable resistor to adjust the voltage, record the current across the component. Plot these points in an I-V graph, and draw a line of best fit.


Electrical symbols

Below is a guide on the basic components in an electrical circuit. There shouldn't be any other components that come up in the exam and most of these won't either.
The 10 most important are:

• Cell
• Battery
• Filament lamp
• Variable resistor, LDR, Thermistor
• Power supply
• Ammeter
• Voltmeter
• Diode
• Switches
• Fuse