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Energy and ElectricityEnergy and Electricity
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Basic ideas…Basic ideas…Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.
Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.
Resistance is anything that resists an electric current. It is measured in _____.
Words: volts, amps, ohms, voltage, ammeter, voltmeter
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Current in a series circuitCurrent in a series circuit
If the current here is 2 amps…
The current here will be…
The current here will be…
And the current here will be…
In other words, the current in a series circuit is THE SAME at any
point
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Current in a parallel circuitCurrent in a parallel circuit
A PARALLEL circuit is one where the current has a “choice of routes”
Here comes the current…
And the rest will go down here…
Half of the current will go down here (assuming the bulbs are the same)…
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Current in a parallel circuitCurrent in a parallel circuit
If the current here is 6 amps
The current here will be…
The current here will be…
The current here will be…
And the current here will be…
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Voltage in a series circuitVoltage in a series circuit
V
V V
If the voltage across the battery is 6V…
…and these bulbs are all identical…
…what will the voltage across each bulb be? 2V
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Voltage in a series circuitVoltage in a series circuit
V
V
If the voltage across the battery is 6V…
…what will the voltage across two bulbs be?
4V
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Voltage in a parallel circuitVoltage in a parallel circuit
If the voltage across the batteries is 4V…
What is the voltage here?
And here?
V
V4V
4V
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SummarySummary
In a SERIES circuit:
Current is THE SAME at any point
Voltage SPLITS UP over each component
In a PARALLEL circuit:
Current SPLITS UP down each “strand”
Voltage is THE SAME across each”strand”
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An example question:An example question:
V1
V2
6V
3A
A1
A2
V3
A3
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Advantages of parallel circuits…Advantages of parallel circuits…
There are two main reasons why parallel circuits are used more commonly than series circuits:
1) Extra appliances (like bulbs) can be added without affecting the output of the others
2) If one appliance breaks it won’t affect the others either
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Georg Simon Ohm 1789-1854
ResistanceResistance
Resistance is anything that will RESIST a current. It is
measured in Ohms, a unit named after me.
The resistance of a component can be calculated using Ohm’s Law:
Resistance = Voltage (in V)
(in ) Current (in A)
V
RI
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An example question:An example question:
V
A
1) What is the resistance across this bulb?
2) Assuming all the bulbs are the same what is the total resistance in this circuit?
Voltmeter reads 10V
Ammeter reads 2A
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More examples…More examples…
12V
3A
3A
6V
4V
2A
1A
2V
What is the resistance of these bulbs?
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Two simple components:Two simple components:
2) Thermistor – resistance DECREASES when temperature INCREASES
1) Light dependant resistor – resistance DECREASES when light intensity INCREASES
Resistance
Amount of light
Resistance
Temperature
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Current-voltage graphsCurrent-voltage graphsI
VI
V
I
V
1. Resistor 3. Diode
2. BulbCurrent increases in proportion to voltage
As voltage increases the bulb gets hotter and resistance increases
A diode only lets current go in one direction
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DC and ACDC and AC
DC stands for “Direct Current” – the current only flows in one direction:
AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)
1/50th s
240V
V
V
Time
T
+
+
-
-
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FusesFuses
Fuses are _______ devices. If there is a fault in an appliance which causes the ____ and neutral (or earth) wire to cross then a ______ current will flow through the _____ and cause it to _____. This will break the _______ and protect the appliance and user from further _____.
Words – large, damage, safety, melt, live, circuit, fuse
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Other safety devices…Other safety devices…1) Insulation and double insulation
2) Residual Current Circuit Breaker
Insulating a wire prevents someone from touching the ____ wire.
A device can also be double insulated – this is when the metal parts inside the device are insulated from the outer case.
A double insulated device does not need an _______ wire but they still have _____.
This circuit breaker detects any difference in current between the live and neutral connectors and switches off the current when needed. They can be easily _______.
Words – earth, reset, live, fuses
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The Cost of ElectricityThe Cost of ElectricityElectricity is measured in units called “kilowatt hours” (kWh). For example…
A 3kW fire left on for 1 hour uses 3kWh of energy
A 1kW toaster left on for 2 hours uses 2kWh
•A 0.5kW Vacuum Cleaner left on for 4 hours uses __kWh
•A 200W TV left on for 5 hours uses __kWh
•A 2kW kettle left on for 15 minutes uses __kWh
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The Cost of ElectricityThe Cost of ElectricityTo work out how much a device costs we do the following:
Cost of electricity = Power (kW) x time (h) x cost per kWh (p)
For example, if electricity costs 8baht per unit calculate the cost of the following…
1) A 2kW fire left on for 3 hours
2) A 0.2kW TV left on for 5 hours
3) A 0.1kW light bulb left on for 10 hours
4) A 0.5kW hoover left on for 1 hour
48B
8B
8B
4B