6.1 Electricity - Circuits

An electric circuit is a complete, closed loop that allows electric charge (current) to flow.

There are two main types of circuits:

• Series Circuit: The components are connected end-to-end, one after another. There is only one path for the current to flow.
• Parallel Circuit: The components are connected in separate branches. The current splits and has more than one path.

Below is an image showing some useful circuit symbols, you don't need to remember them all, but you should try to remember the basics such as cells, voltmeters, ammeters, switches,and resistors.

Source: Wikimedia Commons

Special components (LDR's and Thermistors)

Some components are designed to change their resistance based on their surroundings.

Light Dependent Resistor (LDR):

• In bright light, its resistance is low.
• In darkness, its resistance is high.
• Use: Automatic street lights, phone brightness sensors.

Thermistor (Temperature Dependent Resistor):

• In hot conditions, its resistance is low.
• In cold conditions, its resistance is high.
• Use: Thermostats (in ovens or central heating), digital thermometers.

Electric Current (I): This is the rate of flow of electric charge (usually electrons) in a circuit. It is measured in amperes (A) using an ammeter (which is always placed in series).

Charge = Current × Time

Q = I × t

Potential Difference (V): This is also called voltage. It is the "push" that makes the charge flow. It is a measure of the energy transferred per unit of charge. It is measured in volts (V) using a voltmeter (which is always placed in parallel).

Energy Transferred = Charge × Potential Difference

E = Q × V

Resistance (R): This is a measure of how much a component opposes the flow of current. A high resistance makes it hard for current to flow. It is measured in ohms (Ω).

Ohm's Law

Ohm's Law describes the relationship between these three quantities:

Potential Difference = Current × Resistance

V = I × R

For a component like a resistor, as the temperature increases, its resistance also increases.

Series Circuits

• Current: The current is the same everywhere in the circuit.
• Voltage: The total voltage from the power supply is shared between the components.
• Resistance: The total resistance is the sum of the individual resistances (R_total = R1 + R2 + ...).

Parallel Circuits

• Current: The total current from the power supply splits at the junctions.
• Voltage: The voltage is the same across each branch.
• Resistance: The total resistance is less than the smallest individual resistor.

Source: Wikimedia Commons

Above is an image showing components in parallel (left) and components in series (right). In series, all the components are connected by the same wire, theyre all on the same line. In parallel, the components are 'hugged' by the wires connecting to the other components.

I-V Graphs

An I-V graph shows how the current (I) through a component changes as the voltage (V) across it is changed.

• Resistor: A straight line through the origin. This is "ohmic" - it obeys Ohm's Law.
• Filament Lamp: A curve that gets flatter. As the bulb gets hotter, its resistance increases.
• Diode: Current only flows in one direction.The hotter it gets the less resistance
• Thermistor: More current means thermistor heats up, causing resistance to decrease.
• LDR: The resistance only depends on light intensity, if light is constant during experiment, the LDR behaves like a resistor.