Energy and Voltage in Circuits

An electrical circuit is a loop in which electricity can flow. You’ve probably put a simple one together in your science class, but they can be much more complex and are responsible for powering everything from the kettle to your Christmas lights.

Circuit symbols

Circuit symbols are simple representations of the components commonly used in electrical circuits. You need to be able to recognise and draw the following symbols:

These circuit components carry out different functions:

• Switch - turns a circuit on and off

• Filament lamp - can indicate the presence of current in a circuit

• Fuse - melts to break the circuit if the current is too high

• Diode - ensures the current travels in one direction only

• Voltmeter - measures the potential difference (voltage) in a circuit

• Ammeter - measures the current in a circuit

• Fixed resistor - restricts the flow of electric current

• Variable resistor - restricts the flow of electric current to varying degrees

• Light-dependent resistor (LDR) - restricts the flow of electric current depending on light intensity

• Thermistor - restricts the flow of electric current depending on temperature

• Cell - converts chemical energy into electrical energy to provide the current

• Battery - a collection of cells

Lamps and light-emitting diodes (LEDs) can be used to indicate the presence of current in a circuit. If they light up, we know that the circuit is working and current is moving through the wires.

Variable resistance

The resistance of LDRs and thermistors changes depending on the amount of light intensity and temperature respectively. For a light-dependent resistor (LDR), its resistance decreases with increasing light intensity. It is used for things like sensors and street lighting. For thermistors, the resistance decreases with increasing temperature, making thermistors useful for things like thermostats and fire alarms.

Series vs parallel circuits

Components that are connected one after another in the same loop of a circuit are connected in series. There is only one route for electricity to flow, so if a break occurs due to a faulty component, this means that the current will stop and the whole circuit will not function. In addition, adding more components in series increases the resistance so that a higher voltage needs to be used to get the same output.

• In a series circuit, the current is the same everywhere. Let’s say we have 7 A of current at point A - this means we will also have 7 A of current at points B, C and D.

• The voltage (or potential difference) of the battery is shared between the components. For example, if our battery supplies a total voltage of 20 V, this will be divided between the two resistors. How much voltage each component receives depends on its resistance.

• The total resistance in a series circuit is the sum of the resistance of each component. If resistor 1 in our diagram has a resistance of 5 Ohms and resistor 2 has a resistance of 3 Ohms, the total resistance in the circuit = R1 + R2 = 8 Ohms.

Components that are connected in separate loops are connected in parallel. The current from the battery is shared between each component. The advantage of parallel circuits are that if one component fails, the rest of the circuit will keep working. Another benefit to parallel circuits is that more components can be added without the need for more voltage. Mains socket outlets and mains lights in homes are connected in parallel.

• In a parallel circuit, a junction is where a wire branches off to form a separate loop. It’s important to remember that the current is shared between the different loops in a parallel circuit. The total amount of current flowing into the junction (i.e. the current at point A in the diagram) is equal to the total current flowing out (point D). This is because current is conserved as no charge has been gained or lost. The current at points B and C will be half of the total current, since it has been split between the two branches. Therefore, if we have a current of 10 A at point A, we know there will be 5 A of current running through points B and C and will reach 10 A once the branches rejoin at point D.

• The voltage across two components connected in parallel is the same. Look again at the diagram on the right - if we connected a voltmeter around bulb B and recorded a voltage of 8 V, then we know the voltage at point C will also be 8 V.

Current, voltage and resistance in a series circuit

• Current is the rate of flow of charge around a circuit. If electrons are moving through a wire quicker, then we have a higher current. Current is measured in amps using an ammeter which is placed in series with the other components.

• Voltage, also known as potential difference, is a measure of how much energy is transferred between two points in a circuit. It is measured in volts using a voltmeter. Voltmeters are always placed in parallel around the component to be measured.

• Resistance is anything which slows the current down. If you add more components to a circuit (in series), there will be a higher resistance. Resistance is measured in Ohms.

Current, voltage and resistance are linked in the following equation:

When resistance is kept the same, voltage and current are directly proportional. This means that increasing the current should increase the voltage by the same amount.

Worked example: calculating potential differences

What is the potential difference when a current of 10 A flows through a circuit with a total resistance of 60 Ohms?

• Voltage = current x resistance

• Voltage = 10 x 60 = 600 V

Charge, current & time

When we talk about current, we’re talking about how fast electrons are moving (i.e. how much negative charge passes through a wire in a certain amount of time). We can see the relationship between charge, current and time in the following equation:

Worked example: calculating charge

A current of 5 A passes through an electrical circuit over a period of 3.5 hours. How many coulombs of charge pass through the circuit?

• Remember to convert the time to seconds! 3.5 x 60 x 60 = 12,600 s

• Charge = 5 x 12,600

• Charge = 63,000 C

Energy transferred

Potential difference, or voltage, is the amount of energy transferred per unit charge passed. In other words, it is the amount of energy given to the components in a circuit from the electrons passing through the wire. It is measured in volts - one volt is equivalent to one joule per coulomb.

The energy transferred by the electrons in a circuit to its component can be calculated using the following equation:

We already know that voltage can be calculated by multiplying the current by the resistance (V = IR), therefore this equation can also be written as:

Worked example: calculating energy transferred

How much energy is transferred in a circuit when 5000 C of charge passes through a component which has a potential difference of 150 V?

• In this question we have been given the charge and voltage, so we use the equation E = QV

• Energy transferred = 5000 x 150

• Energy transferred = 750,000 J or 750 kJ