Ch 18 - (Electrical Quantities ) Flashcards
Define current.
- Amount of charge passing a point in a circuit every second
- charge per sec
What is the equation in which change, current and time are related?
Charge = current x time
Q = I x t
Q = charge (coulombs, C)
I = current (amps, A)
What is electric current?
flow of charge
What are the 2 types of current?
- Direct current
- Alternating current
Define direct current.
- electrons flow in one direction only from negative terminal to positive terminal
- produced when using dry cells and batteries
Define alternating current.
- 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
How is current measured?
Using ammeters
How should ammeters connected?
In series with part of circuit
What does ammeter do?
- Amount of charge passing through them per unit time
What are the 2 types of ammeters?
- Digital (with an electronic read out)
- Analogue (with a needle and scale )
What is the range of analogue ammeters?
1.0 - 5.0 A
What are the two errors possible for analogue ammeters?
- Zero error
- Parallax error
What do digital ammeters do?
- measure very small currents in mA
- displays accurate values
- easy to use
What are the errors and issues with digital ammeters?
- May flicker
- zero error
How do metals conduct electricity?
flow of electrons
What is the charge of electrons
negative
How do electrons flow?
Negative to positive
How does conventional current flow?
Positive to negative
What is electromotive force?
Name given to potential difference of power source in a circuit
Define electromotive force.
The electrical work done by a source in moving a unit charge around a complete circuit.
What is e.m.f. measured in?
Volts (v)
What is the symbol equation of e.m.f?
E = W / Q
E = electromotive force (V)
W = charged from power source (J)
Q = charge on each carrier (C)
What does potential difference help in?
- light bulb lights up
- related to amount of energy transferred between the 2 points
Define potential difference.
the work done by a unit charge passing through a component
What is the unit of potential difference?
Volts ()V
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)
How is potential difference measured?
Voltmeter
What are the 2 ways in which potential difference is measured?
- Digital (with an electronic read out)
- Analogue (with a needle and scale )
How are voltmeters connected?
- parallel to wires
- measure potential difference between 2 points
what are the errors likely to be found in analogue voltmeter?
- Parallax error
- zero error
0 - 5.0 V is range
What are the errors likely to be found in digital voltmeters?
zero error
What is Ohm’s law?
Resistance is the opposition to current
What are resistors in circuits used for?
To control current
What is the unit of resistance?
- ohm
- represented by Greek symbol ohms
What is the equation for ohm’s law?
R = V / I
R = Resistance (ohms)
V = potential difference (volts, V)
I = current (amperes, A)
What are resistors in circuits used to control?
- Current in branches
- potential difference across components
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
What happens as potential difference across a component is increased?
Current component also increases
How is the relationship between voltage and current shown?
By IV graph
How is the IV graph of a resistor ?
the current is proportional to the potential difference
- resistor has a constant resistance
How is the IV relationship for the lamp?
The current increases at a proportionally slower rate than the potential difference
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
what happens in reverse for diode?
- High resistance
- no current flaws
- reverse bias
What is a diode?
- Non - ohmic conductor
- flow only in 1 direction
- shown by triangular arrows of diode
- forward bias
How is the IV graph for a diode?
- Forward bias: Sharp increase in voltage and current (on right side)
- Reverse bias: Flat line with current and voltage is zero (on left side)
What happens when electrons pass through a wire?
- collide with metal ions
- ions get in way (resist flow)
What happens to resistance if wire is longer?
the longer the wire, the greater the resistance
What happens to resistance if wire is thick?
The thicker a wire, the smaller it’s resistance
What is the relationship of proportionality of resistance and length?
Resistance is directly proportional to length
R is proportional to length
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
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
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)
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
What do most household appliances transfer energy from?
AC mainz
Kinetic energy of electric motor
What are motors used in?
- Vacuum cleaners - to create suction to suck in dust and dirt off carpets
- Washing machines - to rotate drum or wash clothes
- Refrigerators - compress refrigerant chemical into liquid to reduce the temp
What is heating used in?
- Toasters - to toast bread
- Kettles - to boil water
- Radiators - hot water is pumped from boiler so radiator can heat up room.
Define power in terms of mechanics?
Rate of doing work
What is power in mechanics?
- Potential difference = work done per unit time
- Current = rate of flow of charge
Define electrical power.
Rate of change of work done
What is the formula for power dissipated by an electrical device?
P = IV
P = power (W)
I = current (A)
V = potential difference / voltage (V)
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
What is the rearranged power and energy equation?
E = VIt
E = energy transferred (J)
V = potential difference (V)
I = current (A)
t = time (s)
Define kilowatt hour.
A unit of energy equivalent to one kilowatt of power expended for one hour
What do power rating on appliances tell consumers? kW h
The amount if energy transferred (by electrical work) to the device every sec
What is the formula to calculate kilowatt hour?
E = Pt
E = Energy (kW h)
P = Power (kW)
t = time (h)
