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### Simple Circuits

Series and Parallel Circuits

Learning Intentions

- Understand the role of the energy source in a circuit.
- Understand how electricity flows through a circuit
- Be able to construct series and parallel circuits
- Understand what current and voltage is.
- Be able to measure current and voltage in simple series and parallel circuits.

Galvani and frog legs

Galvani was preparing frog legs for his wife who was ill. He hung them from an iron rail at the edge of a balustrade by a copper wire.

Each time the wind blew or some other force would cause the prepared legs to swing and touch the iron they would suddenly contract as if alive.

“Animal Electricity”

Galvani knew that electric shocks caused the convulsive movement but he did not know what caused the electricity to flow.

He developed a theory of animal electricity i.e. a life force within the muscles of the animal itself.

…back to Volta

At the University of Pavia, Galvani\'s colleague Alessandro Volta was able to reproduce the results, but was skeptical of Galvani’s explanation. By experiment Volta found that it was two dissimilar metals, not the frog’s leg that produced the electricity. The frog’s leg was just an indicator of the presence of electricity. i.e. current flowed through the frog’s legs.

+ve

-ve

Source of electrical energy e.g. battery

ENERGY

User of electrical energy e.g. light globe

= Charged particle/electron

KEY POINT:

For an electric current to flow there needs to be a path from the positive terminal of the battery into and out of the circuit components (e.g. light globe) and back to the negative terminal of the battery

“What\'s Happening?” Sheet

- An electric current is the overall movement of charged particles in one direction
- To obtain an electric current, there needs to be a continuous circuit from one terminal of a battery to another
- An electric current in a circuit transfers energy from the battery to the circuit components. No \'current\' is used up in this process.
- In circuits, the moving charged particles are negatively charged electrons that are always present in the wires and other components of the circuit.
- The battery pushes the electrons in a circuit.

Activity

- Circuits, Current and Voltage

Parallel Circuits

- when there is more than one path for the current to take. The current can go in different directions.

Fig 2: Parallel Circuit

Current

- Electric current is the measurement of the rate of flow of charge through a circuit.
- It is the amount of electric charge which passes a point in one second

Example - If a current of 5 A flows through a globe, this means 5 coulombs of electrons pass through one globe each second.

- Remember electrons flow from negative to positive, however,…
- Current direction is from positive to negative
- Current is measured in ampere (A).
- 1 milliamp (mA) is one thousandth of an amp.
- In different circuits charge flows at different rate.

Digital circuits

- Use http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

To create

- Series circuits with 1 light globe
- Series circuits with 1 light globe
- Parallel circuit with 2 light globe

What do you notice in each case

Charge

- Electrons flow around the circuit in groups not on their own.
- Charge is how the flow of electrons is measured.
- 1 coulomb of charge is equal to the charge carried by 6 X 1018 electrons.

Measuring Current in a series circuit

- When measuring current the ammeter is always placed in series.
- We can place an ammeter at any point in a series circuit and the current will be the same.

I

e

Measuring Current in a parallel circuit

- In a parallel circuit current splits at each path.
- The total of the currents from each path equals the total current in the circuit

- IT = I1 + I2 + I3

- If the resistance in each path is the same then current splits up evenly.
- Current flows more easily in the path with less resistance.

2

Role play demonstrating current used in a series and parallel circuit

Use http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

- To measure the current in each circuit?
- What is the same? What is different?

Energy in a circuit

- Energy supplied to a circuit i.e. via a battery
- This voltage of a battery refers to how much energy the battery supplies to each coulomb of charge that passes through it.
- A 12 V battery gives each coulomb of charge 12 joules.
- Energy used by load components (e.g. Light globe) in the circuit.
- Energy supplied to a circuit must be entirely used up by that circuit.

Voltage ...

- is equal to the work being done per unit of charge.
- represents the source of energy (e.g. a AA battery has a voltage of 1.5V

Or

- represents lost or stored energy i.e. the potential drop (e.g. light globe uses 2 volts of energy to work)

Voltmeter

Measures the voltage or potential drop between two points in a system

Use http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

- Add a voltmeter to series circuit with 2 bulbs. What is voltage in each component(battery, each light bulb, across 2 light globes)?
- Add a voltmeter to parallel circuit with 2 bulbs. What is voltage in each component(battery, each light bulb, across 2 light globes)?

Explaining Potential difference

Which horse has the greater potential energy?

Explaining Potential difference

Which horse has the lower potential energy?

Explaining Potential difference

Is there a difference in potential gravitational energy between horses A and B?

An analogy for potential difference

The horse at the top of the side has the potential to slide down to the bottom. In the process, the horse’s potential gravitational energy can be changed to kinetic, heat and sound energy.

The electron has the potential to move around the circuit. As it does so, its electrical potential energy will be changed to kinetic, light and heat energy.

An analogy for potential difference

Which horse on the slide has the greatest potential gravitational energy? Is their a potential difference between the two horses?

Which electron A or B , has the greatest electrical potential energy?

Is there a potential difference between the electrons?

An analogy for potential difference

The two slides are the same length. Which horse would move fastest?

Which electron would move fastest around the circuit?

An analogy for potential difference

On slide B thoughtless people have left their rubber mats. Which horse, A or B, will move fastest down the slide?

Which electron would meet the greatest resistance to go around the circuit?

An analogy for potential difference

In order to go around the circuit again, the electron must pass through the battery. In the battery chemical energy is changed into potential electric energy which can be used by the electron as it goes through the circuit.

In order to go down the slide again, a horse must climb up the steps. In the process, kinetic energy is changed into potential gravitational energy.

A definition: potential difference

The difference in electrical potential of a unit of charge at one point in a circuit, compared with a unit of charge at another point is called the potential difference (p.d.) or voltage drop, or more usually just voltage (V). It is measured in volts using a voltmeter. The largest potential difference in a circuit is between the terminals of the battery or power supply.

Voltage in load components in series

- In a series circuit the total voltage will be distributed across all components

- If a circuit has 3 identical resistors placed in series with a 12V battery what must the voltage be across each resistor?

- VT = V1 + V2 + V3

Voltage in parallel circuits

- If a circuit has 3 identical globes placed in parallel with a 12V battery each globe will have voltage of 12 Volts across it.
- VT = V1 = V2 =V3

If all load components were the same in which type of circuit series or parallel do you think the battery would go flat first? Why?

More on voltage

- When the load components(e.g. Globe) in a circuit have different resistance than the energy used in each component will differ.
- Load components with less resistance use less energy than components with more resistance i.e. You plasma TV uses more energy than your LCD screen or a light globe uses less energy than your heater.
- BUT the overall energy in any series circuit will equal the total energy from the battery (or energy source)

More on voltage

- If all load components were the same in which type of circuit series or parallel do you think the battery would go flat first?

Role play demonstrating energy used in a series and parallel circuit when components use same amount of energy different amounts of energy

Worksheets

- Complete the parallel sheet worksheets

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