Ch 18 - (Electrical Quantities ) Flashcards
1
Q
Define current.
A
- Amount of charge passing a point in a circuit every second
- charge per sec
2
Q
What is the equation in which change, current and time are related?
A
Charge = current x time
Q = I x t
Q = charge (coulombs, C)
I = current (amps, A)
3
Q
What is electric current?
A
flow of charge
4
Q
What are the 2 types of current?
A
- Direct current
- Alternating current
5
Q
Define direct current.
A
- electrons flow in one direction only from negative terminal to positive terminal
- produced when using dry cells and batteries
6
Q
Define alternating current.
A
- comes from main electricity generators
- direction of electron flow charge changes direction regularly
- typical frequency for the reversal of ac currents, in mains electricity is 50 Hz
7
Q
How is current measured?
A
Using ammeters
8
Q
How should ammeters connected?
A
In series with part of circuit
9
Q
What does ammeter do?
A
- Amount of charge passing through them per unit time
10
Q
What are the 2 types of ammeters?
A
- Digital (with an electronic read out)
- Analogue (with a needle and scale )
11
Q
What is the range of analogue ammeters?
A
1.0 - 5.0 A
12
Q
What are the two errors possible for analogue ammeters?
A
- Zero error
- Parallax error
13
Q
What do digital ammeters do?
A
- measure very small currents in mA
- displays accurate values
- easy to use
14
Q
What are the errors and issues with digital ammeters?
A
- May flicker
- zero error
15
Q
How do metals conduct electricity?
A
flow of electrons
16
Q
What is the charge of electrons
A
negative
17
Q
How do electrons flow?
A
Negative to positive
18
Q
How does conventional current flow?
A
Positive to negative
19
Q
What is electromotive force?
A
Name given to potential difference of power source in a circuit
20
Q
Define electromotive force.
A
The electrical work done by a source in moving a unit charge around a complete circuit.
21
Q
What is e.m.f. measured in?
A
Volts (v)
22
Q
What is the symbol equation of e.m.f?
A
E = W / Q
E = electromotive force (V)
W = charged from power source (J)
Q = charge on each carrier (C)
23
Q
What does potential difference help in?
A
- light bulb lights up
- related to amount of energy transferred between the 2 points
24
Q
Define potential difference.
A
the work done by a unit charge passing through a component
25
What is the unit of potential difference?
Volts ()V
26
What is the equation for p.d. ?
V = W / Q
V = potential difference (V)
W = energy from charge carriers (J)
Q = charge on each carrier (C)
27
How is potential difference measured?
Voltmeter
28
What are the 2 ways in which potential difference is measured?
1. Digital (with an electronic read out)
2. Analogue (with a needle and scale )
29
How are voltmeters connected?
- parallel to wires
- measure potential difference between 2 points
30
what are the errors likely to be found in analogue voltmeter?
- Parallax error
- zero error
0 - 5.0 V is range
31
What are the errors likely to be found in digital voltmeters?
zero error
32
What is Ohm's law?
Resistance is the opposition to current
33
What are resistors in circuits used for?
To control current
34
What is the unit of resistance?
- ohm
- represented by Greek symbol ohms
35
What is the equation for ohm's law?
R = V / I
R = Resistance (ohms)
V = potential difference (volts, V)
I = current (amperes, A)
36
What are resistors in circuits used to control?
- Current in branches
- potential difference across components
37
What are the consequences of Ohm's law?
- current in electrical conductors decreases as its resistance increases
- p.d. across an electrical conductor increases as resistance increases
38
What happens as potential difference across a component is increased?
Current component also increases
39
How is the relationship between voltage and current shown?
By IV graph
40
How is the IV graph of a resistor ?
the current is proportional to the potential difference
- resistor has a constant resistance
41
How is the IV relationship for the lamp?
The current increases at a proportionally slower rate than the potential difference
42
Why is the relationships s such for the IV of a graph?
- current causes filament in lamp to heat up
- as filament gets hot, its resistance increases
- this opposes the current, causing it to increase at a slower rate
43
what happens in reverse for diode?
- High resistance
- no current flaws
- reverse bias
43
What is a diode?
- Non - ohmic conductor
- flow only in 1 direction
- shown by triangular arrows of diode
- forward bias
44
How is the IV graph for a diode?
1. Forward bias: Sharp increase in voltage and current (on right side)
2. Reverse bias: Flat line with current and voltage is zero (on left side)
45
What happens when electrons pass through a wire?
- collide with metal ions
- ions get in way (resist flow)
46
What happens to resistance if wire is longer?
the longer the wire, the greater the resistance
47
What happens to resistance if wire is thick?
The thicker a wire, the smaller it's resistance
48
What is the relationship of proportionality of resistance and length?
Resistance is directly proportional to length
R is proportional to length
49
What is the relationship of proportionality of resistance and cross - sectional area ?
Resistance is inversely proportional to the cross sectional area
R is proportional to 1/A
50
What are the factors the amount of energy an appliances transfer deoends on?
- how long the appliance is switched on for
- the power of the appliance
51
What is the formula used to calculate electrical energy?
E = VIt
E = energy ( joules, J )
V = voltage (volts,V)
I = current (amps,A)
t = time (seconds, s)
52
What happens as electricity passes around a circuit?
- charge passes through the power supply it is given energy
- as it passes through a component, it loses energy
53
What do most household appliances transfer energy from?
AC mainz
Kinetic energy of electric motor
54
What are motors used in?
1. Vacuum cleaners - to create suction to suck in dust and dirt off carpets
2. Washing machines - to rotate drum or wash clothes
3. Refrigerators - compress refrigerant chemical into liquid to reduce the temp
55
What is heating used in?
1. Toasters - to toast bread
2. Kettles - to boil water
3. Radiators - hot water is pumped from boiler so radiator can heat up room.
56
Define power in terms of mechanics?
Rate of doing work
57
What is power in mechanics?
1. Potential difference = work done per unit time
2. Current = rate of flow of charge
58
Define electrical power.
Rate of change of work done
59
What is the formula for power dissipated by an electrical device?
P = IV
P = power (W)
I = current (A)
V = potential difference / voltage (V)
60
Substitute power equation with ohm's law.
Resistance:
P = I^2 R
- if current or voltage doubles, power will be 4 times as great.
Voltage:
P = V^2 / R
61
What is the rearranged power and energy equation?
E = VIt
E = energy transferred (J)
V = potential difference (V)
I = current (A)
t = time (s)
62
Define kilowatt hour.
A unit of energy equivalent to one kilowatt of power expended for one hour
63
What do power rating on appliances tell consumers? kW h
The amount if energy transferred (by electrical work) to the device every sec
64
What is the formula to calculate kilowatt hour?
E = Pt
E = Energy (kW h)
P = Power (kW)
t = time (h)
