1. Electricity for Gadgets
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Electricity for Gadgets
OCR Gateway
W Richards

2. 23/05/2018
P6a Resisting

3. Circuit Symbols Variable resistor Diode Switch Bulb A V Ammeter
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Variable resistor
Diode
Switch
Bulb A V
Ammeter
Voltmeter
LDR
Resistor
Cell
Fuse
Thermistor
Battery

4. Electric Current – A Definition
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Electric current is a flow of negatively charged particles (i.e. electrons).
Note that electrons go from negative to positive - + e-
By definition, current is “the rate of flow of charge” e-

5. Words: volts, amps, ohms, voltage, ammeter, voltmeter
Basic ideas…
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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

6. Words – faster, decrease, slower, increase, push, resistance
More basic ideas…
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If a battery is added the current will ________ because there is a greater _____ on the electrons so they move ______
If a bulb is added the current will _______ because there is greater ________ in the circuit, so the electrons move _____
Words – faster, decrease, slower, increase, push, resistance

7. Using Variable Resistors (Rheostats)
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What will happen to the ammeter reading when the resistance is increased?
How will this affect the brightness of the bulb?

8. Resistance V R I Resistance = Voltage (in V) (in ) Current (in A)
Georg Simon Ohm
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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)

9. An example question: Ammeter reads 2A A V
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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

10. What is the resistance of these bulbs?
More examples…
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12V 4V 2A 1A 2V 3A
What is the resistance of these bulbs?

11. Resistance Resistance is anything that opposes an electric current.
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Resistance is anything that opposes an electric current.
Resistance (Ohms, ) = Potential Difference (volts, V)
Current (amps, A)
What is the resistance of the following:
A bulb with a voltage of 3V and a current of 1A.
A resistor with a voltage of 12V and a current of 3A
A diode with a voltage of 240V and a current of 40A
A thermistor with a current of 0.5A and a voltage of 10V

12. Current-Voltage graphs for conductors
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1) An Ohmic conductor
2) A non-Ohmic conductor V I V I
High resistance
Low resistance
In Ohmic conductors the resistance of the conductor stays the same. The gradient gives us the resistance.
In non-Ohmic conductors (like filament bulbs) the resistance of the conductor increases as the current increases (when it gets hot).

13. Understanding Resistance
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When a voltage is applied it basically causes electrons to move towards the positive end of the battery:
Electrons
Ions
Negative
Positive
Notice that the ions were vibrating and getting in the way of the electrons – this is resistance. What would happen if we increased the temperature of the metal?

14. 23/05/2018
P6b Sharing

15. Potential Dividers (R2) VOUT VIN x (R1 + R2) VIN R1 VOUT R2 0V
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VIN
VOUT R1 R2
The Potential Divider equation:
(R2)
VOUT
VIN x
(R1 + R2)

16. Some example questions
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12V
VOUT
100  0V
50V
VOUT
10 
75  0V 3V
VOUT
75 
25  0V
1.5V
VOUT
50 
45 

17. Potential Dividers VIN R1 VOUT R2 0V Notice the following:
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VIN
VOUT R1 R2
Notice the following:
If R2 is much greater than R1 then the output voltage is more or less equal to VIN.
If R2 is much less than R1 then the output voltage is more or less zero.
If two variable resistors are used in place of R1 and R2 then the output voltage will have an adjustable threshold.

18. LDRs and Thermistors
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19. Two simple components:
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1) Light dependant resistor – resistance DECREASES when light intensity INCREASES
2) Thermistor – resistance DECREASES when temperature INCREASES
Resistance
Amount of light
Resistance
Temperature

20. Using Thermistors and LDRs in circuits
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What will happen to the resistance of the thermistor when it gets hotter?
How will this affect the brightness of the bulb and the reading on the ammeter?
Try designing your own circuit that warns you when an intruder enters your home, maybe by standing between a source of light and an LDR

21. Practical applications
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Vin
VOUT
Here’s a potential divider that is used to control light-activated switches…
When the light intensity on the LDR decreases its resistance will ________. This causes VOUT to _______ so the processor and output will probably turn _____. The variable resistor can be adjusted to change the ________ of the whole device.
Words – decrease, sensitivity, increase, off

22. Resistors in Series and Parallel
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The following equations can be used to calculate the total resistance in a circuit:
For resistors in series:
RT = R1 + R2
1 =
RT R R2
For resistors in parallel:

23. Example questions Calculate the equivalent resistance: 40Ω 1) 10Ω 2)
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Calculate the equivalent resistance:
40Ω 1)
10Ω 2)
20Ω
10Ω
20Ω
100Ω
50Ω
100Ω 3) 4)
20Ω
100Ω
50Ω

24. 23/05/2018
P6c It’s Logical

25. Computers
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IBM 5100 portable PC (1975), 25kg, 1.9MHz processor, 64K RAM, 200K internal tape storage, cost up to $20,000
Apple 1 (1976) – 1Mhz processor, 4K RAM, cost $670
Apple Macintosh (1984) – 8Mhz processor, 512K RAM, cost $2500
Commodore 64 (1982) – 1Mhz processor, 64K RAM, 16 colours, cost $600, 17m sold
All of these computers (and modern ones) are based on transistors, which have enabled computers to get smaller over the years.

26. Computers before Transistors
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The “Harwell Dekatron” a relay-based computer (first run in 1951):

27. Transistors Ie = Ib + Ic A transistor acts like a switch: Collector
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A transistor acts like a switch:
Collector
When a SMALL current flows through the base-emitter part of the transistor a different current is switched on through the collector-emitter part.
Base
Emitter
Ie = Ib + Ic

28. Drawing electronic circuits
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1) Two resistors in series
2) Two resistors in series with one bulb in parallel
VIN 0V
VIN 0V
3) A voltmeter measuring the voltage across two bulbs in series
4) An ammeter measuring the current through a resistor
VIN 0V V
VIN 0V A

29. Using a transistor as a switch for an LED
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Basically, a small current through the base-emitter part of the transistor will “switch on” a larger current through the LED and cause it to emit light.
A high resistor is placed here – why? 0V

30. Combining Transistors
Out 6V
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Consider this circuit:
Input A
Input B
Output
Off On
Off
Off
Off On
The output is only on if A AND B are on. This is called an “AND” gate is a basic logic gate. Other logic gates can be made from transistor combinations and they usually take voltages of 5V (“on”) or 0V (“off”)

31. Logic gate symbols
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NOT gate:
AND gate:
OR gate:

32. Logic gates NOT gate – “the output is NOT what the input is”
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NOT gate – “the output is NOT what the input is”
Logic gates are the basics behind any kind of processor. Here are the three basic ones:
Input
Output 1
Input A
Input B
Output 1
AND – “the output is on if A AND B are both on”
Input A
Input B
Output 1
OR – “the output is on if A OR B are on”

33. NAND and NOR gates NAND gate: NOR gate: Input A Input B Output 1
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NAND gate:
Input A
Input B
Output 1
Input A
Input B
Output 1
NOR gate:

34. 23/05/2018
P6d Even More Logical

35. Complex Logic Gate problems
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Work out the truth tables for the following arrangements: A B J O A B J O 1

36. Complex Logic Gate problems
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Work out the truth tables for the following arrangements: A B J O A B J O 1

37. Complex Logic Gate problems
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Work out the truth tables for the following arrangements: A B C J K O A B C J K O 1

38. Complex Logic Gate problems
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Work out the truth tables for the following arrangements: A B C J K O A B C J K O 1

39. Complex Logic Gate problems
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Work out the truth tables for the following arrangements: A B C D J K L O 1 A B C D J K L O

40. Complex Logic Gate problems
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Work out the truth tables for the following arrangements: A B C D J K L O 1 A B C D J K L O

41. Using LDRs and Thermistors as inputs
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Output
Thermistors and LDRs can be used as the input to a logic gate:
When the light intensity on the LDR decreases its resistance will ________. This causes the input to the ____ gate to turn ___ so the output will turn on. The variable resistor can be adjusted to change the ________ of the whole device.
Words – AND, sensitivity, increase, on

42. Some problems to solve
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The pump on a central heating system is switched on at room temperature if the system is switched on (with the slide switch). When the temperature rises the pump needs to be switched off.
Design a circuit that will sound a buzzer if the temperature of a hot radiator falls during the day ONLY. Include a test switch to check the operation of the buzzer.
Design a circuit for a gardener that will warn them of cold conditions at night. The alarm should be able to be switched off.

43. Using LEDs with Logic Gates
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LEDs work on low power so they can be used to indicate the status of the output from a logic gate: A B J O

44. Words – motor, larger, electromagnet, small
Relays
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Circuits such as those containing logic gates only take very ______ currents. These circuits may be needed to operate a device that takes a much _____ current, e.g. a _________. To do this the circuit would need a RELAY switch, a device made of an _______ that can operate a switch. A relay switch is activated by the small current and the switch part is placed in the circuit needing a large current: M
Symbol for relay:
Words – motor, larger, electromagnet, small

45. 23/05/2018
P6e Motoring

46. Comparing magnets and solenoids
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Magnet:
Solenoid:

47. Magnetic Field around a current-carrying wire
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Magnetic Field around a current-carrying wire
“Right hand corkscrew” or “right hand grip” rule

48. Magnetic field around a rectangular coil
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49. The Motor Effect N S What will happen to this wire?
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What will happen to this wire?
How can you make it move faster?
How can you make it move in a different direction?

50. Current-carrying wire in a magnetic field
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F = Force
B = Magnetic field
I = Current S
Q. Where will this wire go?

51. Revision of DC and AC
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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)
230V T V

52. 23/05/2018
Electric Motors

53. Electric Motors
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The “split ring commutator” changes the current every half rotation (hence the red-blue colour change)
Notice the curved poles – these will give a greater magnetic field strength near the coil

54. 23/05/2018
P6f Generating

55. Electromagnetic Induction
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The direction of the induced current is reversed if…
The wire is moved in the opposite direction
The field is reversed
The size of the induced current can be increased by:
Increasing the speed of movement
Increasing the magnet strength

56. Electromagnetic induction
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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
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

57. AC Generators
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Slip rings and brushes
Magnetic Field

58. AC Generators
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Voltage
Time

59. Questions on the AC Generator
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How does the generator work?
How would you increase its output? Give two answers
How would you increase the frequency?

60. 23/05/2018
P6g Transforming

61. Transformers
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A transformer is basically a device made of two coils of wire around an iron core:
The circuit diagram for a transformer:

62. Transformers Time Time Time Current through primary Magnetic field
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Current through primary
Time
Magnetic field
Time
Voltage induced in secondary
Time

63. Words – alternating, magnetic field, induce, step up, voltage
Transformers
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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)
…and the current changes using this formula:
VPIP = VSIS

64. Simple transformer questions
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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

65. More example questions
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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 current of 0.5A is supplied to a transformer that steps down a voltage from 230V to 12V. What is the current from the secondary coil?

66. Isolating Transformers
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An isolating transformer is used in some mains circuits (for example, a bathroom _____ socket). Isolating transformers do not change the _____; they simply consist of two ______ that don’t make _____ with each other. This stops the user from getting ___________ from the mains supply.
Words – electrocuted, shaver, voltage, contact, coils

67. Transformers in the National Grid
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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

68. Power through the National Grid
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The following equation can be used to calculate the amount of power loss in the National Grid’s cables:
Power = I2R
A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor.
A motor takes a current of 10A. If its resistance is 2.2MΩ calculate the power dissipated by the motor.
A 2KW heater has a resistance of 20 Ω. Calculate the current through it.

69. Transformers and Power
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If this transformer is 100% efficient then the power output from the secondary coil is the same as the power input to the primary coil.
Power = voltage x current
in W in V in A
In previous units we came across this equation:
If we apply this equation to transformers and assume 100% efficiency we conclude:
Vp x Ip = Vs x Is

70. Example questions
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A transformer connected to the mains supply in the UK runs on a voltage of 230V and a current of 0.1A. If its output voltage is 12V what is its output current?
Another step down transformer outputs 120V and 2A. If it runs from the mains what is its input current?
A step up transformer near a power station would convert 20,000V and 10A into 300,000V and what current?
1.9A
1.04A
0.07A

71. 23/05/2018
P6h Charging

72. Diodes
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The current flows easily in the forwards direction but there is very high resistance to the reverse current.
A diode is a device that only allows current to flow in one direction:
If alternating current is passed through a diode it becomes “half-wave rectified”: V T V T

73. How Diodes work
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Diodes work by having areas of extra electrons and areas of extra “holes”: - +
Holes
Electrons
When the battery is connected like this the electrons move towards the holes and jump from one hole to another, which makes it seem like the holes are moving.

74. Full-Wave Rectification
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A group of 4 diodes can be used to make a “bridge rectifier” circuit to make full-wave rectification:
AC supply
Load V T V T

75. The Capacitor
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A capacitor is a device that can store charge (it has a “capacity”). Here’s how they work: e

76. Charging and discharging a capacitor
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P.d. across capacitor
Time
Current in circuit
Time
P.d. across capacitor
Current in circuit
Time
Time

77. Output without capacitor
Smoothing
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Many devices need a constant voltage supplied to them, rather than AC. Capacitors can be used in “smoothing” circuits to “smooth” out a supply:
AC supply
Output without capacitor
Output with capacitor
Input