National 5 Electricity Flashcards
1
Q
Label the following model of the atom, indicating the charge of each particle.
A
2
Q
How many types of electric charge are there, and what are their names?
A
There are two types of electric charge - positive and negative
3
Q
How do the types of electric charge interact?
A
Opposites attract - positive and negative
Like charges repel - positive and positive or negative and negative
4
Q
What is an electric field?
A
An electric field is the space around a charged particle where another charged particle will experience a force.
5
Q
Describe what would happen to a positive charge if it was placed in the field shown.
A
A positive charge would accelerate towards the negative plate.
6
Q
Describe what would happen to a negative charge if it was placed in the field shown.
A
A negative charge would accelerate towards the positive plate.
7
Q
In the diagram shown, what type of charge does the particle have?
Explain your answer.
A
The particle has a negative charge.
The particle is negative because it is attracted to the positive plate.
8
Q
What effect does an electric field have on a conductor?
A
An electric field will cause the free electrons in a conductor to drift towards the positive end of the battery.
9
Q
Explain in words what is meant by current.
A
An electric current if a flow of charge.
10
Q
What is the numerical definition of current?
A
Current is how much electrical charge flows (past a point) in one second.
11
Q
Q = It
(Define symbols and their units)
A
Q - charge (C)
I - current (A)
t - time (s)
12
Q
Example
Calculate the current flowing during a bolt of lightning which lasts for 10 ms and transfers 50 C of charge.
A
Q = 50 C
I = ?
t = 10 ms = 10 x 10-3 s
Q = It
50 = I x 10 x 10-3
I x 10 x 10-3 = 50
I = 50/10 x 10-3
I = 5000 A
13
Q
What does d.c. stand for and what does it mean?
A
d.c. stands for direct current, which means the electrons flow in one direction only (negative to positive).
14
Q
What does a.c. stand for, and what does it mean?
A
a.c. stands for alternating current, which means the electrons change direction regularly.
15
Q
What is the peak voltage of an AC supply?
A
The peak voltage of an AC supply is the maximum voltage it reaches in its cycle.
16
Q
What is the quoted voltage of an AC supply?
A
The quoted voltage is an average which enables fair comparison with a battery.
17
Q
How do quoted and peak voltage compare?
A
Peak voltage is greater than quoted voltage.
18
Q
What is the quoted voltage of the UK mains?
A
230 V
19
Q
What is the frequency of the UK mains?
A
50 Hz
20
Q
The voltage of a supply is a measure of the ………………. given to the charges in a circuit.
A
The voltage of a supply is a measure of the ENERGY given to the charges in a circuit.
21
Q
Describe how to use an ammeter to measure the current in a component.
A
An ammeter must be connected in series with a component.
22
Q
Describe how to use a voltmeter to measure the voltage across a component.
A
A voltmeter must be connected in parallel with a component.
23
Q
State the circuit rule for current in a series circuit.
A
The current in a series circuit is the same at all points OR
Is = I1 = I2 = …
24
Q
State the circuit rule for voltage in a series circuit.
A
The voltage in a series circuit adds up to the supply voltage OR
Vs = V1 + V2 + …
25
State the circuit rule for current in a parallel circuit.
The current in a parallel circuit adds up to the supply current OR
## Footnote
Is = I1 + I2 + ...
26
State the circuit rule for voltage in a parallel circuit.
The voltage in a parallel circuit is the same **as the supply voltage** across all branches OR
## Footnote
Vs = V1 = V2 = ...
27
What is the name of this component?
**MOSFET (transistor)**
28
What is the name of this component?
**L.D.R. (light-dependent resistor)**
29
What is the name of this component?
**Cell**
30
What is the name of this component?
**Resistor**
31
What is the name of this component?
**Motor**
32
What is the name of this component?
**Capacitor**
33
What is the name of this component?
**Battery**
34
What is the name of this component?
**Diode**
35
What is the name of this component?
**Variable resistor**
36
What is the name of this component?
**npn transistor**
37
What is the name of this component?
**Lamp**
38
What is the name of this component?
**Loudspeaker**
39
What is the name of this component?
**Fuse**
40
What is the name of this component?
**Switch**
41
What is the name of this component?
**Thermistor**
42
What is the name of this component?
**Photovoltaic Cell (Solar Cell)**
43
What is the function of a **cell** or **battery**?
(including the energy change)
A cell or battery supplies the energy for a circuit, converting **chemical** energy to **electrical** energy.
44
What is the function of a **lamp**?
(including the energy change)
A lamp supplies light, converting **electrical** energy to **light** (and heat) energy.
45
What is the function of a **diode**?
A diode only lets current flow in one direction.
46
What is the function of a **switch**?
A closed switch makes a circuit complete letting current flow.
An open switch makes a circuit incomplete meaning no current can flow.
47
What is the function of a **capacitor**?
A capacitor is a device that stores charge. It takes time to fill up so can be used as a time delay.
48
Define Ohmic and Non-Ohmic conductors, giving an example of each.
Ohmic conductors have a constant resistance meaning that a V-I graph is a straight line. e.g. resistor.
The resistance of non-ohmic conductors can change e.g. the resistance of a light bulb increases when it heats up (producing more light). This gives a curved V-I graph.
49
V = IR
## Footnote
(Define symbols and their units)
V - voltage (V)
I - current (A)
R - resistance (Ω)
50
Example
Calculate the current flowing when a 2 kΩ resistor is connected to a 6V battery.
V = 6 V
I = ?
R = 2 kΩ = 2 x 103 Ω
V = IR
6 = I x 2 x 103
I x 2 x 103 = 6
I = 6/2 x 103
I = 3 x 10-3 A
51
How could the **resistance** of a component be found from this graph?
The **resistance** could be found by
(a) calculating the **gradient** of the graph
(b) choosing a point on the graph and calculating **voltage / current**
52
What happens to the resistance of a conductor when it heats up?
The resistance of a conductor **increases** when it heats up.
53
What happens to the resistance of a thermistor when it heats up?
The resistance of a thermistor **decreases** when it heats up.
54
What happens to the resistance of an **LDR** when more light shines on it?
The resistance of an **LDR** **decreases** when more light shines on it.
55
RT = R1 + R2 + ....
(What type of circuit does this apply to?
Define symbols and units)
This is the formula for resistance in series.
RT - Total Resistance (Ω)
R1 - Resistance 1 (Ω)
R2 - Resistance 2 (Ω)
56
Example
Calculate the total resistance of the circuit shown.
```
RT = ?
R1 = 1·5 kΩ = 1500 Ω
R2 = 100 Ω
```
```
RT = R1 + R2 + ....
RT = 1500 + 100
RT = 1600 Ω
```
57
1/RT = 1/R1 + 1/R2 + ....
(What type of circuit does this apply to?
Define symbols and units)
This is the formula for resistance in parallel.
RT - Total Resistance (Ω)
R1 - Resistance 1 (Ω)
R2 - Resistance 2 (Ω)
58
Example
Calculate the total resistance of the circuit shown.
```
RT = ?
R1 = 5 Ω
R2 = 20 Ω
```
```
1/RT = 1/R1 + 1/R2 + ....
1/RT = 1/5 + 1/20
1/RT = 1/4
RT = 4/1 = 4 Ω
```
59
Example
Calculate the total resistance of the circuit shown.
25 & 25 in series
```
RT = ?
R1 = 25 Ω
R2 = 25 Ω
```
```
RT = R1 + R2 + ....
RT = 25 + 25
RT = 50 Ω
```
50 (above) in parallel with 100
```
RT = ?
R1 = 50 Ω
R2 = 100 Ω
```
```
1/RT = 1/R1 + 1/R2 + ....
1/RT = 1/50 + 1/100
1/RT = 3/100
RT = 100/3 = 33.3 Ω (to 3 sig fig)
```
60
In the circuit below calculate
a) the battery voltage
b) the voltage across the 10 Ω resistor.
a) First find the total resistance
Two 20 Ω resistors in parallel
RT = ?
R1 = 20 Ω
R2 = 20 Ω
```
1/RT = 1/R1 + 1/R2
1/RT = 1/20 + 1/20
1/RT = 1/10
RT = 10 Ω
```
Now combine this 10 Ω resistor with the other one in series
RT = ?
R1 = 10 Ω
R2 = 10 Ω
```
RT = R1 + R2
RT = 10 + 10
RT = 20 Ω
```
Use this total resistance along with the total current to get the total voltage
V = ?
I = 450 mA = 450 x 10-3 A
R = 20 Ω
V = IR
V = 450 x 10-3 x 20
V = 9 V
The battery voltage is 9 V
b) The ammeter and 10 Ω resistor are in series so will have the same current.
```
V = ?
I = 450 mA = 450 x 10-3 A
R = 10 Ω
```
V = IR
V = 450 x 10-3 x 10
V = 4·5 V
The voltage across the 10 Ω resistor is 4·5 V
61
Describe the three parts of an **electronic system.**
Input ⇒ Process ⇒ Output
62
What is the function of a **motor**?
(including the energy change)
A motor makes things move, converting **electrical** energy to **kinetic** energy.
63
What is the function of a **loudspeaker**?
(including the energy change)
A loudspeaker produces sound, converting **electrical** energy to **sound** energy.
64
What is the function of a **relay**?
(including the energy change)
A relay uses a small current to switch on a larger current, converting **electrical** energy to **kinetic** energy.
65
What is the function of an **LED**?
(including the energy change)
An LED supplies light, converting **electrical** energy to **light** energy.
66
Will the **LED** in this circuit switch on?
Explain your answer.
**No** - the LED will not switch on.
The positive terminal of the LED is **not** connected to the positive terminal of the battery.
67
Will the **LED** in this circuit switch on?
Explain your answer.
**Yes** - the LED will switch on.
The positive terminal of the LED is connected to the positive terminal of the battery.
68
Why does an **LED** always have a resistor connected in series with it?
An LED always has a resistor connected in series with it to **protect the LED**
The resistor will **limit the current in the LED**
69
Example
Calculate the value of the protective resistor for an LED rated at 4V, 20 mA being used with a 9V cell.
First calculate the voltage across the resistor using 'voltage in series' rule
Vs = 9 V
V1 (LED) = 4 V
V2 (R) = ?
```
Vs = V1 + V2
9 = 4 + V2
V2 = 9-4 = 5 V
```
The current is the same through LED and R using 'current in series' rule = 20 mA
For the resistor
V = 5 V
I = 20 mA = 20 x 10-3 A
R = ?
V = IR
5 = 20 x 10-3 x R
20 x 10-3 x R = 5
R = 5/20 x 10-3
R = 250 Ω
The protective resistor should have a resistance of 250 Ω
70
What is the function of a **photovoltaic cell**?
(i.e. the energy change)
A photovoltaic cell converts **light** energy to **electrical** energy.
71
What is the function of a **thermocouple**?
(i.e. the energy change)
A thermocouple converts **heat** energy to **electrical** energy.
72
What is the function of a **microphone**?
(i.e. the energy change)
A microphone converts **sound** energy to **electrical** energy.
73
What is the following circuit usually called?
A **potential divider** or **voltage divider**.
74
Explain the function of a potential divider circuit such as the example shown.
A **potential divider** or **voltage divider** splits the battery voltage between the resistors in proportion to their resistance i.e. larger resistors will take a larger share of the voltage.
75
V2 = R2/(R1+R2) x Vs
| (Define symbols and units)
V2 - Voltage across resistor 2 (V)
R1 - Resistance of resistor 1 (Ω)
R2 - Resistance of resistor 2 (Ω)
Vs - Supply Voltage (V)
(NB it doesn't matter which resistors you choose to call 1 and 2 as long as you are consistent in labelling the voltages in the same way i.e. V1 across R1, V2 across R2)
76
Example
Calculate V1 in this circuit.
```
V1 = ?
R1 = 100 Ω (This is R1 because it has been marked as V1 in the question)
R2 = 150 Ω
Vs = 10 V
```
```
V1 = R1/(R1+R2) x Vs
V1 = 100/(100+150) x 10
V1 = 4 V
```
(NB The formula sheet uses **V2** = **R2**/(R1+R2) x Vs but this can be adapted as above as long as the voltage and resistor in bold agree with each other i.e.
**V1** = **R1**/(R1+R2) x Vs
or
**V2** = **R2**/(R1+R2) x Vs
The choice of which resistor is labelled 1 or 2 doesn't matter unless it has already been marked in the Q)
77
V1/V2 = R1/R2
| (Define symbols and units)
V1 - Voltage across resistor 1 (V)
V2 - Voltage across resistor 2 (V)
R1 - Resistance of resistor 1 (Ω)
R2 - Resistance of resistor 2 (Ω)
(NB it doesn't matter which resistors you choose to call 1 and 2 as long as you are consistent in labelling the voltages in the same way i.e. V1 across R1, V2 across R2)
78
Example
Calculate V1 in this circuit.
V1 = ?
V2 = 3 V
R1 = 12 kΩ
R2 = 6 kΩ
(Can leave R1 and R2 in kΩ since the formula is a ratio.)
```
V1/V2 = R1/R2
V1/3 = 12/6
V1 = 3x12/6
V1 = 6 V
```
79
Explain the function of the following voltage divider circuit when temperature increases.
* As temperature rises, the resistance of the thermistor decreases.
* This means the voltage across the thermistor decreases.
* This means the voltage across the resistor, which is the output voltage, increases.
The circuit acts as a high temperature sensor i.e. its output voltage increases as temperature increases.
80
Explain the function of the following voltage divider circuit when temperature decreases.
* As temperature decreases, the resistance of the thermistor increases.
* This means the voltage across the thermistor, which is the output voltage, increases.
The circuit acts as a low temperature sensor i.e. its output voltage increases as temperature decreases.
81
Explain the function of the following voltage divider circuit when light levels increase.
* As light levels rise, the resistance of the LDR decreases.
* This means the voltage across the LDR decreases.
* This means the voltage across the resistor, which is the output voltage, increases.
The circuit acts as a light sensor i.e. its output voltage increases as light levels increase.
82
Explain the function of the following voltage divider circuit when light levels fall.
* As light levels fall, the resistance of the LDR increases.
* This means the voltage across the LDR, which is the output voltage, increases.
The circuit acts as a low light (darkness) sensor i.e. its output voltage increases as light levels fall.
83
Explain the function of the following voltage divider circuit when the switch is open.
* An open switch has a very high resistance.
* This means the voltage across the switch, which is the output voltage, will be very high.
The output voltage of this circuit is high when the switch is open. This could be used in a burglar alarm to detect a door or window being opened.
84
Explain how the time delay in following voltage divider circuit could be increased.
* A capacitor with larger capacitance takes longer to fill with charge, so the time delay would be longer.
* A larger resistance reduces the current flowing so that the capacitor takes longer to fill with charge, so the time delay would be longer.
85
Explain how the following voltage divider circuit acts as a time delay.
* The voltage across a capacitor builds up slowly when it is switched on.
* This means the output voltage, will build up slowly, taking time to reach Vs or a specific value.
This circuit acts as a time delay.
86
Describe what happens to the output voltage when the potentiometer is varied from position B to A.
* When the potentiometer is close to B the resistance below the contact is low, meaning the output voltage is low.
* When the potentiometer is close to A, the resistance below the contact is high, meaning the output voltage is high.
A potentiometer allows the voltage to be varied continuously between 0V (at B) and Vs (at A).
87
What is the purpose of a **transistor**?
A **transistor** acts as an **electronic switch**.
(it switches **on** above a certain voltage)
88
Label the connections of an NPN transistor.
89
Label the connections of a MOSFET transistor.
90
Explain how the circuit shown switches on the motor when light levels increase.
• As light levels rise, the resistance of the LDR decreases.
• This means the voltage across the LDR decreases.
• This means the voltage across the variable resistor, which is the base voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the motor.
91
Explain how the circuit shown switches on the light when it gets dark.
• When it gets dark, the resistance of the LDR increases.
• This means the voltage across the LDR, which is the base voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the lamp.
92
Explain how the circuit shown switches on the heater when the temperature falls.
• As temperature falls, the resistance of the thermistor increases.
• This means the voltage across the thermistor, which is the gate voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the heater.
93
Explain how the circuit shown switches on the motor (air conditioning pump) when the temperature rises.
• As temperature rises, the resistance of the thermistor decreases.
• This means the voltage across the thermistor decreases.
• This means the voltage across the variable resistor, which is the base voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the relay.
• This switches on the circuit with the motor (air conditioning pump)
94
How could the **air-conditioning** circuit (shown) be changed so that it performs the **opposite job** (i.e. switch on at low temperatures instead of high temperatures)?
The circuit can be changed to perform the **opposite job** by **swapping the thermistor with the variable resistor**.
95
How could the **air-conditioning** circuit (shown) be **changed** so that it switches on at a **different temperature**?
To switch the circuit on at a different temperature, the **variable resistor** could be adjusted.
96
Complete the sentence:
When an electric current flows in a component, there is ...
When an electric current flows in a component, there is **an energy transformation.**
97
What is the definition of the term **power**?
**Power** is defined as **the amount of energy transferred per second**.
98
P = E/t
| (Define symbols and units)
P - Power (W)
E - Energy (J)
t - time (s)
99
Example
Horse Power is an historical unit of power, used to compare engines to the work of a horse. 1 Horse Power is taken to be 735 W. How much energy can a horse supply when it works with this power for one hour?
```
P = 735 W
E = ?
t = 1 h = 60 x 60 = 3600 s
```
```
P = E/t
735 = E/3600
E/3600 = 735
E = 735 x 3600
E = 2,650,000 J (to 3 sig fig)
```
100
P = IV
| (Define symbols and units)
P - Power (W)
I - Current (A)
V - Voltage (V)
101
Example
Calculate the current drawn when a 2 kW kettle is plugged into the mains.
```
P = 2 kW = 2000 W
I = ?
V = 230 V (UK Mains Voltage - see previous LO)
```
P = IV
2000 = I x 230
I x 230 = 2000
I = 2000/230
I = 8.70 A (to 3 sig fig)
102
P = I2R
| (Define symbols and units)
P - Power (W)
I - Current (A)
R - Resistance (Ω)
103
Example
Calculate the current flowing when 3500 W of power are supplied to a 100 Ω resistor.
```
P = 3500 W
I = ?
R = 100 Ω
```
P = I2R
3500 = I2 x 100
I2 x 100 = 3500
I2 = 3500/100
I2 = 35
I = √35
I = 5.92 A (to 3 sig fig)
104
P = V2/R
| (Define symbols and units)
P - Power (W)
V - Voltage (V)
R - Resistance (Ω)
105
Example
A 100 Ω resistor is attached to a 9 V battery. Calculate the power supplied.
```
P = ?
V = 9 V
R = 100 Ω
```
```
P = V2/R
P = 92/100
P = 81/100
P = 0·81 W
```
106
State the function of a fuse in domestic circuits.
Explain how they work.
Fuses protect the wires in a house from overheating and possibly going on fire.
They do this by breaking the circuit if the current gets too large.
107
A hairdryer has an operating **current** of **6.5 amperes**.
What **value of fuse** should be used in the plug of the hairdryer? Choose from 3A and 13A.
The **fuse rating** has to be **higher than the operating current**.
The current is 6.5A, so a **13A fuse** should be used (next highest fuse rating available).
108
A microwave cooker has a power rating of **650 W**.
What **value of fuse** should be used in the plug of the microwave cooker? Choose from 3A, 5A and 13A.
The **fuse rating** has to be **higher than the operating current**. Calculate the current:
```
P = 650 W
I = ?
V = 230 V (UK Mains Voltage)
```
P = IV
650 = I x 230
I x 230 = 650
I = 650/230
I = 2.83 A
A **3A fuse** should be used (next highest fuse rating available).
