1. 21/09/2018
Electricity

2. (Words: volts, amps, ohms, voltage, ammeter, voltmeter)
Basic ideas…
21/09/2018
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)

3. More basic ideas…
21/09/2018
If a battery is added the current will ________ because there is a greater _____ on the electrons
If a bulb is added the current will _______ because there is greater ________ in the circuit

4. Current in a series circuit
21/09/2018
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

5. Current in a parallel circuit
21/09/2018
A PARALLEL circuit is one where the current has a “choice of routes”
Here comes the current…
Half of the current will go down here (assuming the bulbs are the same)…
And the rest will go down here…

6. Current in a parallel circuit
21/09/2018
If the current here is 6 amps
And the current here will be…
The current here will be…
The current here will be…
The current here will be…

7. Voltage in a series circuit
21/09/2018
If the voltage across the battery is 6V… V
…and these bulbs are all identical… V V
…what will the voltage across each bulb be? 2V

8. Voltage in a series circuit
21/09/2018
If the voltage across the battery is 6V… V V
…what will the voltage across two bulbs be? 4V

9. Voltage in a parallel circuit
21/09/2018
If the voltage across the batteries is 4V…
What is the voltage here? V 4V
And here? V 4V

10. Summary Current is THE SAME at any point
21/09/2018
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”

11. An example question:
21/09/2018 6V A3 3A A1 V1 A2 V2 V3

12. Advantages of parallel circuits…
21/09/2018
There are two main reasons why parallel circuits are used more commonly than series circuits:
Extra appliances (like bulbs) can be added without affecting the output of the others
If one appliance breaks it won’t affect the others either

13. Resistance V R I Resistance = Voltage (in V) (in ) Current (in A)
Georg Simon Ohm
21/09/2018
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: V R I
Resistance = Voltage (in V)
(in ) Current (in A)

14. An example question: Ammeter reads 2A A V
21/09/2018
Ammeter reads 2A V A
What is the resistance across this bulb?
Assuming all the bulbs are the same what is the total resistance in this circuit?
Voltmeter reads 10V

15. What is the resistance of these bulbs?
More examples…
21/09/2018 3A 6V
12V 4V 2A 1A 2V 3A
What is the resistance of these bulbs?

16. DC and AC
21/09/2018 V
DC stands for “Direct Current” – the current only flows in one direction:
Time
1/50th s
AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)
240V T V

17. Words – large, harm, safety, melt, live, circuit, fuse
Fuses
21/09/2018
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, harm, safety, melt, live, circuit, fuse

18. Power and fuses
21/09/2018
Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: P I V
Power = voltage x current
in W in V in A
Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A).

19. Power and fuses Copy and complete the following table: Appliance
21/09/2018
Copy and complete the following table:
Appliance
Power rating (W)
Voltage (V)
Current needed (A)
Fuse needed (3, 5 or 13A)
Toaster
960
240
Fire
2000
Hairdryer
300
Hoover
1000
Computer
100
Stereo 80

20. Power is measured in Watts or kilowatts Power = Work done / Time taken
Power is a measure of how fast the energy changes from one form to another. It is the rate at which work is done
Power is measured in Watts or kilowatts
Power = Work done / Time taken
1 Watt means that 1 Joule of energy was changed in 1 second
Power

21. Power
Power

22. Power
Power

23. Energy transferred = Power x time
21/09/2018
Energy transferred = Power x time
(in J) (watts) (s) E P T

24. (The poles can be reversed by reversing the direction of the current)
Electromagnets
21/09/2018
The strength of an electromagnet can be increased by doing three things:
Increasing the voltage
Increasing the number of coils
Inserting an iron core
(The poles can be reversed by reversing the direction of the current)

25. Electromagnetic induction
21/09/2018
Electromagnetic induction
The direction of the induced current is reversed if…
The magnet is moved in the opposite direction
The other pole is inserted first

26. Electromagnetic induction
21/09/2018
Electromagnetic induction
The size of the induced current can be increased by:
Increasing the speed of movement
Increasing the magnet strength
Increasing the number of turns on the coil

27. AC Generator Induced current can be increased in 4 ways:
21/09/2018
Induced current can be increased in 4 ways:
Increasing the speed of movement
Increasing the magnetic field strength
Increasing the number of turns on the coil
Increasing the area of the coil

28. Words – step down, high, power, low, high
The National Grid
21/09/2018
Electricity reaches our homes from power stations through the National Grid:
Power station
Step up transformer
Step down transformer
Homes
If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much ______ loss by the time electricity reaches our homes. This is because the current is ___. To overcome this they use devices called transformers to “step up” the voltage onto the power lines. They then “____ ____” the voltage at the end of the power lines before it reaches our homes. This way the voltage is _____ and the current and power loss are both ____.
Words – step down, high, power, low, high

29. Words – alternating, magnetic field, induce, step up, voltage
Transformers
21/09/2018
Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil.
Words – alternating, magnetic field, induce, step up, voltage
We can work out how much a transformer will step up or step down a voltage:
Voltage across primary (Vp)
No. of turns on secondary (Ns)
Voltage across secondary (Vs)
No. of turns on primary (Np)

30. Some transformer questions
21/09/2018
Primary voltage Vp
Secondary voltage Vs
No. of turns on primary Np
No. of turns on secondary Ns
Step up or step down?
12V
24V
100 ?
400V
200V 20
25,000V
50,000V
1,000
23V
230V
150

31. Some example questions
21/09/2018
Primary voltage Vp
Secondary voltage Vs
No. of turns on primary Np
No. of turns on secondary Ns
Step up or step down? 6V
24V
100 ?
400,000V
200V
1,000
25,000V
20,000 20
230V
150
1,500
A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil?
A step-down transformer has twice as many turns on the primary coil than on the secondary coil. What will be the output (secondary) voltage if the input voltage is 50V?

32. Static electricity
21/09/2018
Static electricity is when charge “build up” on an object and doesn’t move, e.g. rubbing a rod:

33. 21/09/2018
Two different rods will attract each other if they have different charge:
Two rods made of the same material will repel each other due to having the same charge:

34. Van de Graff generators
21/09/2018
A charge builds up on the dome due to electrons being “______ off” by the belt. If a big enough ______ is built up then the voltage becomes high enough to _____ the air molecules and the electrons “___” down to Earth – this is an electric _________.
Words – charge, jump, current, rubbed, ionise

35. Use of static 1 - Photocopiers
21/09/2018
Photocopiers use static electricity. They work by: 1) Copying an ______ of the page onto a ______ plate, 2) Light then causes the charge to ____ away, leaving an “electrostatic impression” of the page, 3) The charges left on the plate ______ small drops of black powder, 4) The powder is transferred from the plate onto the _____, 5) The paper is _____ to “fix” the powder. P
Words – heated, leak, paper, image, charged, attract

36. Use of static 2 - Printers
21/09/2018
Inkjet printers work by spraying charged drops of ink onto a page. The droplets can be directed using two oppositely charged plates. The voltage on these plates can be easily swapped or varied. The inkjet cartridge can also be moved across the page by the printer.

37. Dangers of static
21/09/2018
During refuelling the fuel gains electrons from the pipe, making the pipe positive and the fuel negative. The resulting voltage may cause a spark – bad news!
Solution: Either earth the fuel tank with a copper rod or connect the tanker to the plane by a copper conductor.