Electricity transfers energy
Electrical circuits transfer energy from one form to another. When they are connected to a battery, they transfer stored chemical energy into electrical energy. Components in the circuit transfer the electrical energy into other forms of energy, for example a light bulb will transfer the electrical energy into light.
In this unit we will be looking at electricity in the home. This will include the different types of electrical current (hint: there’s two!), and the uses of these. We will also be looking at the energy transfers that occur around the home.
You should already know that there are different stores of energy (or energy can be stored in different ways). One of these is electrical energy. You can find out more about this here: https://www.bbc.co.uk/bitesize/guides/z99jq6f/revision/1
You should be familiar with the concept of electrical current and have practised setting up some simple electrical circuits. You should also be able to describe what is meant by potential difference and resistance. You should be able to draw simple circuit diagrams and use circuit symbols for the different components. You can find out more about this here: https://www.bbc.co.uk/bitesize/guides/zsfgr82/revision/1
P1.4, P2.4, P4 L
1. Which component does this symbol represent? £ A. cell £ B. wire £ C. battery
2. Which of the following is true about parallel circuits? £ A. There is more than one path through which the current can flow £ B. There is only one path through which the current can flow £ C. There are always two paths through which the current can flow
3. What is current measured in? £ A. Volts £ B. Ammeter £ C. Amps
4. In a series circuit containing one bulb, the voltage of the battery was increased from 1.5 V to 3 V. What happens to the brightness of the bulb? £ A. it doesn’t change £ B. it increases £ C. it decreases
5. Some students make a model of a series circuit by holding a loop of rope in a circle. The teacher pushes the rope along so it moves at a steady speed, whilst five other students hold the rope loosely in their hands. What does the speed of the moving rope represent? £ A. potential difference £ B. energy £ C. current
6. The ammeters in this circuit showed the following readings. A1 = 2 A, A2 = 1 A. What are the readings on A3 and A4? £ A. A3 = 2 A, A4 = 2 A £ B. A3 = 1 A, A4 = 1 A £ C. A3 = 1 A, A4 = 2 A
7. Bulb 1 breaks in this circuit. Now bulb 2 does not light. The bulb is not broken. What is the best explanation for this? Bulb 1 Bulb 2 £ A. Bulb 1 has used up all the current in the circuit. £ B. The circuit is now incomplete so current can’t flow. £ C. Bulb 2 must need more energy to light up.
8. Complete the sentence. The potential difference across a battery is.. £ A. split between two bulbs in a series circuit £ B. the same as the potential difference across two bulbs in a series circuit £ C. is split between two bulbs each in a branch of a parallel circuit
9. Which is the best definition of potential difference? £ A. The flow of charge around the circuit £ B. The amount of energy shifted from the charge to the component £ C. How many cells or batteries are in the circuit
10. Which is not one of the two types of circuit? £ A. series £ B. complete £ C. parallel
Answering A suggests a common error of confusing a single cell with multiple cells, which is a battery. Answering B suggests a lack of knowledge about C circuit symbols. To address both issues, draw out all circuit symbols to practice
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Answering B suggests a confusion with series circuits; to resolve this draw 6 circuit diagrams; 2 which are series and 2 which are parallel circuits. Answering C suggests a A misconception that parallel circuits are always just two branches, as many diagrams show, so recap the definition of a parallel circuit and draw a parallel circuit with 4 possible paths for current to flow.
Answering A suggests a gap in knowledge about the units of current and potential difference. Answering B suggests a gap in knowledge about units vs. equipment used C to measure current/voltage. To resolve either issue, complete a table about current and voltage that includes definition, how to measure (i.e. ammeter/voltmeter), units.
Answering A suggests that there is a gap in knowledge, that increasing the potential difference across the bulb doesn’t affect current and therefore the brightness of the bulb. Answering C suggests a B misconception about the relationship between current and potential difference. Remember that increasing potential difference increases the current, so that more energy is transferred to the bulb, so it looks brighter. Answering A or B suggests a confusion with the definitions of these key terms. To resolve this, C explain the rope model again, and how it’s similar to a circuit.
Answering A or B suggests confusion about current splitting at the branch point or current after the branches have re-joined. Remember that current is the same C everywhere in a series circuit, but it splits between branches in a parallel circuit. Draw some example circuits with ammeter readings to practice applying this.
Answering A shows the misconception that components ‘use up’ current. Current is a flow of charge that flows through components in the circuit and never gets ‘used up’. To improve, list B 3 ways that current in a wire is the same as current in a river. Answering C suggests that you have a gap in knowledge about complete vs. incomplete circuits. To improve, look up and draw some examples of complete and incomplete circuits.
Answering B or C shows a confusion about how potential difference acts in series and parallel A circuits; to resolve this write out rules using their notes and/or knowledge organiser about how current and p.d. acts in series and parallel circuits.
Answering A shows a confusion with the definition of current. To improve, define current. Answering C suggests some understanding that a cell/battery provides a potential difference, B but it is not the definition of potential difference. To improve, define potential difference.
There are only two types of circuit; series and parallel. If you answered A or C, draw out 2 B examples of series circuits and 2 examples of parallel circuits. B
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Electricity in the Home
AC and DC
AC electricity - Alternating current
If the current constantly changes direction it is called alternating current, or AC. Mains electricity is an AC supply. The UK mains supply is about 230 V. It has a frequency of 50 Hz, which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an AC supply. Power stations sometimes produce electricity using magnets. This provides an alternating current (ac).
DC electricity - Direct current
If the current flows in only one direction it is called direct current, or DC. Batteries and solar cells supply DC electricity. A typical battery may supply 1.5 V. The diagram shows an oscilloscope screen displaying the signal from a DC supply.
Plugs
A plug connects a device to the mains electricity supply. The cable between the device and the three-pin plug contains three copper wires that are coated with plastic.
• copper wires are good conductors • plastic is a good insulator Each part of the plug has a function.
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Earthing
Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the case. The next person who uses the appliance could get electrocuted. The earth wire is therefore connected to the case and is attached to a metal plate or water pipe underground. As the wire is made of copper, the earth wire provides a low resistance path to the ground. In the event of a fault, the live current passing through the case will follow this path to the ground instead of passing through a person.
Fuses
A fuse provides a built-in fail-safe to the electrical circuit for a device. The fuse contains a thin wire that will melt if the current gets too high. If there is a fault that causes the casing of the device to become live, a large current will flow through the low-resistance earth wire. This high current will cause the fuse to melt.
1. What type of current is mains electricity? £ A. dc £ B. ad £ C. ac
2. What is the potential difference of the UK’s mains supply? £ A. 60 V £ B. 100 V £ C. 230 V
3. What is the correct colour coding for a UK mains cable? £ A. live wire = red, neutral wire = black, earth wire = green and yellow stripes £ B. live wire = red, neutral wire = blue, earth wire = green £ C. live wire = brown, neutral wire = blue, earth wire = green and yellow stripes
Q1. There are two types of current, AC and DC. Mains electricity is AC (alternative current). If you answered A or B, then write down the two different types of current and describe the difference between them. Q2. Answer = C Q3. Answer = C
Power and Energy Transferred
Appliances, power and energy
All electrical appliances transfer energy from one store to another, for example chemical energy in the fuel in power stations. This is transferred into kinetic energy in a fan or heat energy in a cooker.
The amount of energy transferred depends on the power (the energy transferred each second) and the amount of time the appliance is switched on for. The energy transferred by an appliance can be calculated using the equation:
energy = power × time
\[E = P \times t\]
This is when:
• energy (E) is measured in joules (J) • power (P) is measured in watts (W) • time (t) is measured in seconds (s) One watt is the power when one joule of energy is transferred in one second.
Time should be converted from minutes into seconds - this is done by multiplying the number of minutes by 60.
Example
How much energy is transferred by a 1,500 W hair dryer in 15 minutes?
E=P × t
E=1,500 × (15×60)
E=1,500 × (900)
E=1,350,000 J or 1.35 mega joules (MJ)
Power can also be calculated using the equation:
power = potential difference × current
P=V × I
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This is when:
• Power (P) is measured in watts (W) • potential difference (V) is measured in volts (V) • current (I) is measured in amps (A)
When working with mains electricity and appliances, the potential difference is 230 V.
Example 1
What is the power of an electric heater that draws a current of 8 amps (A) when plugged into the mains?
P=V×I
P=230×8
P=1,840 W
Example 2
If the electric heater is used for 20 minutes, how much energy is transferred in that time?
E=P×t
E=1,840×(20×60)
E=1,840×(1,200)
E=2,208,000 J or 2.21 MJ
1. Calculate the current drawn by a 24 W bulb connected to a 12 V battery. £ A. 288 A £ B. 2 A £ C. 0.5 A
2. Calculate the voltage needed to get a current of 10 A to flow through a 200 W heating element. £ A. 20 V £ B. 200 V £ C. 0.05 V
3. What is the unit of electrical power? £ A. Volt £ B. Amp £ C. Watt
Q1. To calculate current, you need to divide power by potential difference (voltage). In this question, this means dividing 24 by 12, which is 2 A. If you answered A or C, calculate the current drawn by a 15 W bulb connected to a 10 V battery. Q2. Answer = A Q3. Answer = C
The National Grid
The National Grid distributes electricity across the country. The National Grid connects power stations to homes, workplaces and public buildings all around the country. The electricity may be produced by a conventional power station turning a generator or by another method.
Transformers
Transformers are used to change voltages and currents in transmission lines. A transformer is formed from two coils of wire around a magnetic core. The number of coils determines whether the transformers will step-up or step-down the voltage.
As the power transferred must stay the same:
• increasing voltage decreases current • decreasing voltage increases current In the National Grid, a step-up transformer is used to increase the voltage and reduce the current. The voltage is increased from about 25,000 Volts (V) to 400,000 V causing the current to decrease. Less current means less energy is lost through heating the wire. To keep people safe from these high voltage wires, pylons are used to support transmission lines above the ground. Before reaching the end user, a step-down transformer, reduces the voltage from the transmission voltage to the safer voltage of 230 V for home use.
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1. What type of transformer is used to increase the potential difference? £ A. Positive £ B. Additive £ C. Step-up
2. What type of transformer is used to connect the power station to the transmission cables? £ A. Step-down £ B. Additive £ C. Step-up
3. What is the point of the transformers used by the National Grid? £ A. So power is transmitted at low voltages and high currents increasing efficiency. £ B. So power is transmitted at high voltages and high currents increasing efficiency. £ C. So power is transmitted at high voltages and low currents increasing efficiency.
Q1: Step-up transformers are used to increase the potential difference in the National Grid. If you answered A or B, state what step-up and step-down transformers are for in the National Grid. Q2: A Q3: C