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Circuits and Symbols
Circuits and Symbols
An Electric circuit is one or more Electric components connected to a power supply in a closed loop. There are thousands of different Electric components, and millions of different combinations for connecting them together. Below is examples of symbols in common use:-
An Electric circuit is one or more Electric components connected to a power supply in a closed loop. There are thousands of different Electric components, and millions of different combinations for connecting them together. Below is examples of symbols in common use:-
Power Supplies:-
Power Supplies:-
Output Devices:-
Output Devices:-
Resistor Devices:-
Resistor Devices:-
Measuring Devices:-
Measuring Devices:-
Note - The above only show a small selection of possible symbols, in this course others will be used also.
Note - The above only show a small selection of possible symbols, in this course others will be used also.
Drawing Circuits
Drawing Circuits
In Physics, no circuit is ever drawn pictorially (as they actually look), they are always drawn in symbol form. Below is an example of a simple circuit shown in both ways for ease of understanding:-
In Physics, no circuit is ever drawn pictorially (as they actually look), they are always drawn in symbol form. Below is an example of a simple circuit shown in both ways for ease of understanding:-
The circuit above consists of a cell connected to two lamps.
The circuit above consists of a cell connected to two lamps.
Series and Parallel Circuits
Series and Parallel Circuits
There are two main ways to connect Electric components:-
There are two main ways to connect Electric components:-
Series circuit - All components connected in one loop one after the other.
Series circuit - All components connected in one loop one after the other.
Parallel circuit - Components connected in two or more branches.
Parallel circuit - Components connected in two or more branches.
The diagram below shows an example circuit for each type:-
The diagram below shows an example circuit for each type:-
As can be seen from the diagram above, each type looks very different. The different layouts cause the circuits to act in different ways.
As can be seen from the diagram above, each type looks very different. The different layouts cause the circuits to act in different ways.
For example:-
For example:-
In a Series circuit, if one component breaks, all stop working.
In a Series circuit, if one component breaks, all stop working.
In a Parallel circuit, if one component breaks, the rest keep working.
In a Parallel circuit, if one component breaks, the rest keep working.
This can be seen in the diagram below:-
This can be seen in the diagram below:-
Current in Series Circuits
Current in Series Circuits
In previous sections the concept of Current flow was discussed. Current is the flow of charge through a circuit and is measured using an Ammeter. The following diagram shows a simple Series circuit with several Ammeters placed throughout :-
In previous sections the concept of Current flow was discussed. Current is the flow of charge through a circuit and is measured using an Ammeter. The following diagram shows a simple Series circuit with several Ammeters placed throughout :-
In the above Circuit, there are two lamps in series with a switch. There are 3 Ammeters in the positions shown.
In the above Circuit, there are two lamps in series with a switch. There are 3 Ammeters in the positions shown.
As the flow of Charge cannot change throughout the circuit (it cannot 'leak out'), the reading on all three Ammeters will be the same.
As the flow of Charge cannot change throughout the circuit (it cannot 'leak out'), the reading on all three Ammeters will be the same.
For a Series Circuit, the Current will be the same at all points
For a Series Circuit, the Current will be the same at all points
A1 = A2 = A3
A1 = A2 = A3
Voltage in Series Circuits
Voltage in Series Circuits
In previous sections the concept of Voltage was discussed. Voltage is the Energy per unit Charge supplied by the battery and is measured using a Voltmeter. The following diagram shows a simple series circuit with several Voltmeters placed throughout :-
In previous sections the concept of Voltage was discussed. Voltage is the Energy per unit Charge supplied by the battery and is measured using a Voltmeter. The following diagram shows a simple series circuit with several Voltmeters placed throughout :-
In the above circuit, there are two lamps in series with a switch. There are three Voltmeters in the positions shown.
In the above circuit, there are two lamps in series with a switch. There are three Voltmeters in the positions shown.
The Energy supplied to each Coulomb of Charge must be enough for the Charge to make a full trip round the circuit. This means that each component will use part of this energy.
The Energy supplied to each Coulomb of Charge must be enough for the Charge to make a full trip round the circuit. This means that each component will use part of this energy.
For a Series circuit, the Voltage at each point will add up to the Supply Voltage.
For a Series circuit, the Voltage at each point will add up to the Supply Voltage.
Vs = V1 + V2
Vs = V1 + V2
Current in Parallel Circuits
Current in Parallel Circuits
In previous sections the concept of Current flow was discussed. Current is the flow of charge through a circuit and is measured using an Ammeter. The following diagram shows a simple Parallel circuit with several Ammeters placed throughout :-
In previous sections the concept of Current flow was discussed. Current is the flow of charge through a circuit and is measured using an Ammeter. The following diagram shows a simple Parallel circuit with several Ammeters placed throughout :-
In the above Circuit, there are two lamps in parallel with a switch. There are 4 Ammeters in the positions shown.
In the above Circuit, there are two lamps in parallel with a switch. There are 4 Ammeters in the positions shown.
As the total flow of Charge cannot change throughout the circuit (it cannot 'leak out'), the reading on A1 and A4 will be the same.
As the total flow of Charge cannot change throughout the circuit (it cannot 'leak out'), the reading on A1 and A4 will be the same.
When the Current enters the parallel branches, however, There is more than one path for it to follow, meaning that the Current will be shared between the branches.
When the Current enters the parallel branches, however, There is more than one path for it to follow, meaning that the Current will be shared between the branches.
For a Parallel Circuit, the Current in each branch will be the add up to the Supply Current.
For a Parallel Circuit, the Current in each branch will be the add up to the Supply Current.
A1 = A2 + A3 = A4
A1 = A2 + A3 = A4
Voltage in Parallel Circuits
Voltage in Parallel Circuits
In previous sections the concept of Voltage was discussed. Voltage is the Energy per unit Charge supplied by the battery and is measured using a Voltmeter. The following diagram shows a simple series circuit with several Voltmeters placed throughout :-
In previous sections the concept of Voltage was discussed. Voltage is the Energy per unit Charge supplied by the battery and is measured using a Voltmeter. The following diagram shows a simple series circuit with several Voltmeters placed throughout :-
In the above circuit, there are two lamps in series with a switch. There are three Voltmeters in the positions shown.
In the above circuit, there are two lamps in series with a switch. There are three Voltmeters in the positions shown.
As the Voltage is the amount of Energy given to each coulomb of Charge passing through the Circuit and each branch acts as an independent circuit, the Voltage across each branch is the same as the supply Voltage.
As the Voltage is the amount of Energy given to each coulomb of Charge passing through the Circuit and each branch acts as an independent circuit, the Voltage across each branch is the same as the supply Voltage.
For a Parallel Circuit, the Voltage across each branch is equal to the Supply Voltage.
For a Parallel Circuit, the Voltage across each branch is equal to the Supply Voltage.
Vs = V1 = V2
Vs = V1 = V2
Resistance
Resistance
In a previous section, the concept of Electrical Current was covered. The flow of Charge around a circuit can be seen as similar to the flow of water through pipes. Using this analogy, Resistance can be thought of as the size of the pipe; the wider the pipe, the easier the water can flow through the system. The diagram below shows another analogy for Resistance of traffic being reduced from 3 lanes to 1 (the 'Pinch Point' represents the Resistor):-
In a previous section, the concept of Electrical Current was covered. The flow of Charge around a circuit can be seen as similar to the flow of water through pipes. Using this analogy, Resistance can be thought of as the size of the pipe; the wider the pipe, the easier the water can flow through the system. The diagram below shows another analogy for Resistance of traffic being reduced from 3 lanes to 1 (the 'Pinch Point' represents the Resistor):-
Resistance is a measure of how hard it is for Current to flow. The larger the Resistance, the lower the Current. Resistance is caused by components not being perfect conductors; when current flows through them, heat is generated, causing energy to be lost.
Resistance is a measure of how hard it is for Current to flow. The larger the Resistance, the lower the Current. Resistance is caused by components not being perfect conductors; when current flows through them, heat is generated, causing energy to be lost.
Resistance has the symbol R and is measured in Ohms (Ω).
Resistance has the symbol R and is measured in Ohms (Ω).
Measuring Current and Voltage
Measuring Current and Voltage
In order to measure Current and Voltage within a circuit, the meters must be placed within the circuit in the correct positions:-
In order to measure Current and Voltage within a circuit, the meters must be placed within the circuit in the correct positions:-
Measuring Resistance of a fixed Resistor
Measuring Resistance of a fixed Resistor
The experiment below shows a method for calculating the Resistance of a Resistor:-
The experiment below shows a method for calculating the Resistance of a Resistor:-
By varying the power supply and measuring the Current and Voltage the following graph could be obtained:-
By varying the power supply and measuring the Current and Voltage the following graph could be obtained:-
The gradient of the graph above gives the Resistance of the Resistor.
The gradient of the graph above gives the Resistance of the Resistor.
Ohm's Law
Ohm's Law
In 1827, Georg Ohm published his complete theory of Electricity, part of which focussed on Electrical Resistance. By performing the above experiment using the newly invented Galvanic Cell, Ohm concluded that the Current flowing through the circuit was directly proportional to the Voltage applied across it.
In 1827, Georg Ohm published his complete theory of Electricity, part of which focussed on Electrical Resistance. By performing the above experiment using the newly invented Galvanic Cell, Ohm concluded that the Current flowing through the circuit was directly proportional to the Voltage applied across it.
This is written more formally as Ohm's Law:-
This is written more formally as Ohm's Law:-
V = I R
V = I R
Where :-
Where :-
V = Voltage in Volts (V).
V = Voltage in Volts (V).
I = Current in Amperes (A).
I = Current in Amperes (A).
R = Resistance in Ohms (Ω).
R = Resistance in Ohms (Ω).
Example 1 -
Example 1 -
A circuit is set up as shown below:-
A circuit is set up as shown below:-
If the readings on the meters are as shown, what is the value of the unknown Resistor?
If the readings on the meters are as shown, what is the value of the unknown Resistor?
V = I R
V = I R
R = V / I
R = V / I
R = 10 / 0.1
R = 10 / 0.1
R = 100 Ω
R = 100 Ω
Electrical Power
Electrical Power
The rate of transfer of Energy is known as the Power. The units for Power are Watts (W) and, the Power of a system can be found using the following formula:-
The rate of transfer of Energy is known as the Power. The units for Power are Watts (W) and, the Power of a system can be found using the following formula:-
P = E / t
P = E / t
Where:-
Where:-
P = Power (W)
P = Power (W)
E = Energy (J)
E = Energy (J)
t = Time (s)
t = Time (s)
Example 1 -
Example 1 -
A light bulb has a Power rating of 100 W. If the light bulb is used for 10 minutes, what is the total Energy used by the bulb?
A light bulb has a Power rating of 100 W. If the light bulb is used for 10 minutes, what is the total Energy used by the bulb?
P = E / t
P = E / t
E = P x t
E = P x t
E = 100 x ( 10 x 60 )
E = 100 x ( 10 x 60 )
E = 60000 J
E = 60000 J
E = 60 kJ
E = 60 kJ
Power in Electrical Circuits
Power in Electrical Circuits
When a current flows through an Electrical Circuit, Energy is transferred from the battery to the components. In the components, this Electrical Energy is converted into other forms, such as heat or light. This flow of Energy around the circuit depends the Voltage and Current within the circuit:-
When a current flows through an Electrical Circuit, Energy is transferred from the battery to the components. In the components, this Electrical Energy is converted into other forms, such as heat or light. This flow of Energy around the circuit depends the Voltage and Current within the circuit:-
1. Increasing Current - Faster flow of Charge and therefore faster Energy flow.
1. Increasing Current - Faster flow of Charge and therefore faster Energy flow.
2. Increasing Voltage - Each unit of Charge has more Energy and therefore more Energy flows through the circuit.
2. Increasing Voltage - Each unit of Charge has more Energy and therefore more Energy flows through the circuit.
This faster Energy flow means a higher rate of Energy flow - a larger Power.
This faster Energy flow means a higher rate of Energy flow - a larger Power.
Power in an Electrical Circuit can be found using the following formula :-
Power in an Electrical Circuit can be found using the following formula :-
P = I x V
P = I x V
Where:-
Where:-
P = Power (W)
P = Power (W)
I = Current (A)
I = Current (A)
V = Voltage (V)
V = Voltage (V)
Example 2 -
Example 2 -
How much Power is used by an Iron if it draws a Current of 5.6 A from a mains supply?
How much Power is used by an Iron if it draws a Current of 5.6 A from a mains supply?
P = I x V
P = I x V
P = 5.6 x 230
P = 5.6 x 230
P = 1280 W
P = 1280 W
Mains Electricity
Mains Electricity
The most common source of Electrical Energy in the home is mains electricity. Mains electricity provides an alternating Current with a stated Voltage of 230 V, alternating at 50 Hz. This is accessed by using the appropriate plug type for that region. In the UK, the standard plug is a 3-Pin plug (type G) as shown below:-
The most common source of Electrical Energy in the home is mains electricity. Mains electricity provides an alternating Current with a stated Voltage of 230 V, alternating at 50 Hz. This is accessed by using the appropriate plug type for that region. In the UK, the standard plug is a 3-Pin plug (type G) as shown below:-
Note - There are 15 different types of plug in use across the World, each with different Voltage and Current limits. Just because an adaptor allows a plug to connect to a foreign socket does not mean it is safe to use - always check that the device is compatible with the country's mains values.
Note - There are 15 different types of plug in use across the World, each with different Voltage and Current limits. Just because an adaptor allows a plug to connect to a foreign socket does not mean it is safe to use - always check that the device is compatible with the country's mains values.
The UK plug is designed with several important safety features, which can be seen in the diagram below:-
The UK plug is designed with several important safety features, which can be seen in the diagram below:-
Safety features of a 3-Pin plug:-
Safety features of a 3-Pin plug:-
1. Plastic coated - Electrical insulator to prevent Electrical shocks.
1. Plastic coated - Electrical insulator to prevent Electrical shocks.
2. Earth wire - Connected to any exposed metal, conducts Current safely to Earth if there is a fault preventing Electrical shocks (see below).
2. Earth wire - Connected to any exposed metal, conducts Current safely to Earth if there is a fault preventing Electrical shocks (see below).
3. Fuse - Breaks flow of Current if Current becomes too high, prevent risk of overheating or fire (see below).
3. Fuse - Breaks flow of Current if Current becomes too high, prevent risk of overheating or fire (see below).
4. Cable grip - Holds flex (cable containing the three wires) in place to prevent wires being exposed.
4. Cable grip - Holds flex (cable containing the three wires) in place to prevent wires being exposed.
Fuses
Fuses
A Fuse in a 3 Pin plug is a device used to prevent too high Current flow within a device. A fuse consists of a thin wire within a ceramic or glass cartridge, as shown in the images below:-
A Fuse in a 3 Pin plug is a device used to prevent too high Current flow within a device. A fuse consists of a thin wire within a ceramic or glass cartridge, as shown in the images below:-
The fuse acts as simply a piece of wire, as long as the Current is at the operating value. If the Current is too high, the wire within the fuse melts, breaking the circuit. This prevents the device from overheating and possibly from starting a fire.
The fuse acts as simply a piece of wire, as long as the Current is at the operating value. If the Current is too high, the wire within the fuse melts, breaking the circuit. This prevents the device from overheating and possibly from starting a fire.
Note - In everyday language, we say the fuse has 'blown'. Do not use the word 'blown' as this implies an explosion. The fuse wire 'melts'.
Note - In everyday language, we say the fuse has 'blown'. Do not use the word 'blown' as this implies an explosion. The fuse wire 'melts'.
Fuses come in a range of values and every mains appliance will have a correct fuse to use. The correct fuse can be found by using information on the rating plate. The image below shows the rating plate of an electric drill:-
Fuses come in a range of values and every mains appliance will have a correct fuse to use. The correct fuse can be found by using information on the rating plate. The image below shows the rating plate of an electric drill:-
The correct value of the fuse is not given, but can be found using one of the above Power equations to calculate the operating Current. Once the operating Current is known, a fuse that has a slightly larger value should be used. A general rule for this is as follows:-
The correct value of the fuse is not given, but can be found using one of the above Power equations to calculate the operating Current. Once the operating Current is known, a fuse that has a slightly larger value should be used. A general rule for this is as follows:-
1. Under 720 W - Use a 3 A fuse.
1. Under 720 W - Use a 3 A fuse.
2. Over 720 W - Use a 13 A fuse.
2. Over 720 W - Use a 13 A fuse.
The Earth Wire
The Earth Wire
The Earth wire is an additional wire within the 3 Pin plug that, if the system is working correctly, does not carry any Current. The Earth wire's function is to provide a short, low Resistance path to 'Earth'.
The Earth wire is an additional wire within the 3 Pin plug that, if the system is working correctly, does not carry any Current. The Earth wire's function is to provide a short, low Resistance path to 'Earth'.
If a fault occurs by the Live wire coming into contact with the metal casing of a device, anyone touching that device will receive a dangerous electrical shock. This is because the person touching the casing will give the Current a path to 'Earth' through.
If a fault occurs by the Live wire coming into contact with the metal casing of a device, anyone touching that device will receive a dangerous electrical shock. This is because the person touching the casing will give the Current a path to 'Earth' through.
The Earth wire works by providing a lower Resistance path which allows the current to flow easily to 'Earth'. This high Current flow causes the fuse to melt, breaking the circuit. This renders the device safe.
The Earth wire works by providing a lower Resistance path which allows the current to flow easily to 'Earth'. This high Current flow causes the fuse to melt, breaking the circuit. This renders the device safe.
Cost of Electricity
Cost of Electricity
Measuring Power (in Watts) is useful to find a fuse value, but in terms of understanding how expensive a Electricity bill is, it is not a useful unit.
Measuring Power (in Watts) is useful to find a fuse value, but in terms of understanding how expensive a Electricity bill is, it is not a useful unit.
All Electricity bills are calculated using another unit of Energy, the kWh (kiloWatt-Hour)
All Electricity bills are calculated using another unit of Energy, the kWh (kiloWatt-Hour)
This unit of Energy allows you to calculate the cost of using any mains component for any length of time.
This unit of Energy allows you to calculate the cost of using any mains component for any length of time.
How to Calculate using kWh
How to Calculate using kWh
Power rating of Kettle = 1440W = 1.44 kW
Power rating of Kettle = 1440W = 1.44 kW
Time of use = 3 mins = 3/60 Hours = 0.05 Hours
Time of use = 3 mins = 3/60 Hours = 0.05 Hours
Energy consumed in use = P x t
Energy consumed in use = P x t
= 1.44 x 0.05
= 1.44 x 0.05
= 0.072 kWh
= 0.072 kWh
But how much does that cost?
But how much does that cost?
Knowing how many kWh used is still not enough to find out the cost of running components, we need to convert this to a price.
Knowing how many kWh used is still not enough to find out the cost of running components, we need to convert this to a price.
1 kWh = 18p (Scottish Power October 2020)
1 kWh = 18p (Scottish Power October 2020)
To run our kettle once, it then costs:-
To run our kettle once, it then costs:-
E = 0.072 kWh
E = 0.072 kWh
Cost = 0.072 x 18
Cost = 0.072 x 18
= 1.3 p each use
= 1.3 p each use
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