G482 - Electricity Flashcards
Charge
Definition
An inherent property of some particles leading to forces of attraction and repulsion
Measured in coulombs
Electric Current
Definition
A flow of charge
A vector quantity
Measured in Amperes, A
Charge
Formula
Charge (coulombs) = Current (A) x Time (s)
Q = It
Coulomb
Definition
A unit of electrical charge
One coulomb of charge is carried by one amp of current in one second
Ammeter
Used to measure electric current
Connected in series
Must have negligible resist have so that they don’t reduce the current reading
Charge Carriers
In Metals
Electrons
Electric current in a metal is the flow of electrons
Charge Carriers
In Electrolytes
Ions
Electric current in electrolytes is the flow of ions
Electrolyte
Definition
A fluid that contains ions that are free to move and therefore conduct electricity
Conventional Current
Definition
Current flow is taken to be the direction of flow from positive to negative
Electron Flow
Definition
The actual movement of electrons in a metal and the direction of current flow, from negative from positive
Elementary Charge
Definition
The charge on one electron
Elementary Charge
Value
1.6 x 10^-19
Amp
Definition
Unit of electric current
One amp of current means one coulomb of charge passes a point per second
Kirchoff’s First Law
Conservation of charge
The sum of the currents flowing into a point in a circuit is equal to the sum of the currents leaving that point
Drift Velocity
Definition
The average velocity of an electron as it travel through a wire due to potential difference
Drift Velocity
Formula
v = I / Ane
v = drift velocity (m/s) I = current (A) A = cross sectional area of wire (m²) n = number density e = elementary charge
Number Density
Conductor
Huge number of free conduction electrons
Large number density
Number Density
Insulators
An effective insulator has very few conduction electrons
Number density close to zero
Number Density
Semiconductors
Intermediate properties
Intermediate number density
Potential Difference
Definition
The electrical energy transferred per unit charge when electrical energy is converted into some other form of energy
Work done moving charges around the circuit, energy used within the circuit
Electromotive Force
Definition
The electrical energy transferred per unit charge when one form of energy is converted into electrical energy
Energy supplied to the circuit
Energy
Formula
Energy (J) = P.D. (V) x Charge (coulombs)
W = VQ
Volt
Definition
Unit of potential difference
One volt means one joule of energy transferred per coulomb of charge
Voltmeter
Measures potential difference
Connected in parallel across a component to measure the potential difference across it
Must have a high resistance so that current flows through the circuit rather than the voltmeter
Resistance
Definition
A property of a component that regulates the electric current through it
Measured in ohms, Ω
Resistance
Formula
Resistance (Ω) = Voltage (V) / Current (I)
R = V / I
Ohm’s Law
For some metallic conductors maintained at a constant temperature and with all other conditions constant, the potential difference divided by the current is a constant
Materials that obey Ohm’s Law are called ohmic conductors
Benefits of LEDs
Switch in instantly Very robust Very versatile Operate on low p.d.s Have a long working life
Ohm
Definition
Unit of resistance
One ohm means one volt per amp
Resistivity
Definition
A property of a material that regulates the flow of current rather than a property of a component
Resistivity
Formula
ρ = RA / l
ρ = resistivity (Ωm) R = resistance (Ω) A = cross sectional area (m²) l = length of wire (m)
Resistivity
Metals and Temperature
Increasing the temperature gives the atoms in the metal more kinetic energy
This causes the atoms to vibrate and move around making it more difficult for charge carrying electrons to move past them
The resistance has increased and therefore so has the resistivity as ρ = RA / l
The heat energy allows electrons to move within the conduction band but the gap between the valence band and the conduction band is too big for more electrons to pass from the valence band to the conduction band with this energy
Resistivity
Semi Conductors and Temperature
Increasing temperature increases the number of charge carriers as the heat energy allows electrons to move from the valence band across a gap to the conduction band, in semiconductors this gap is very small
There are more charge carriers available so resistance decreases as temperature increase
Resistivity
Negative Temperature Coefficient Thermistors (NTC)
NTCs are semiconductors
This means that as temperature increase their resistance decreases
This is what allows them to act as temperature sensors
Superconductivity
A superconductors resistance will suddenly drop to 0 at its critical temperature
This temperature is very low
Power
Definition
The rate of doing work or the rate of energy transfer
A scalar quantity
Measured in watts, W
Power
Formulae
P = VI = I²R = V² / R
P = power (W) V = voltage (V) I = current (A) R = resistance (Ω)
Watt
Definition
Unit of power
One watt means that one joule of energy is transferred per second
Work Done
Definition
The energy converted from one form to another
Or the product of a force and the distance moved in the direction of the force
A scalar quantity
Measured in Joules, J
Work Done or Energy
Formula
W = Pt = IVt
W = work done or energy (J) P = power (W) t = time (s) I = current (A) V = voltage (V)
Joule
Definition
Unit of energy
One joule is the energy transferred or work done by one watt in one second
Fuses
Purpose
Prevent overloading
Fuses
How they work
Consist of a copper wire in a ceramic casing
If a certain current passes through the wire it is so thin that it would melt, breaking the circuit
Different fuses melt at different currents
A fuse should be rated above the maximum current drawn from a device calculated using its power rating and the voltage through it, P=IV
Fuses
Most Common in Household Appliances
3A and 13A fuses
Kilowatt Hour
Definition
A unit of energy used by electricity companies as they deal in such large amounts
One kWh means 1000W used for 3600 seconds which is equivalent to 3600000 joules
Mains Voltage
Value
230V
Series Circuit
Definition
Components are connected end to end therefore only providing one oath for current to flow along
Parallel Circuit
Components are connected in two or more branches therefore providing multiple paths for current
Kirchoff’s Second Law
Conservation of energy
In any closed loop circuit the sum of the e.m.f.s (energy supplied to the circuit) is equal to the sum of the p.d.s (energy used in the circuit)
Internal Resistance
Definition
All sources of e.m.f have a small internal resistance, r, as they are made from materials with electrical resistance
Terminal P.D.
Definition
The e.m.f. of a cell of battery when it is not supplying a circuit
Lost Volts
Definition
Some energy is lost as heat in the battery, it behaves as if it has internal resistance
EMF
Formula
E = V + Ir = I (R + r)
E = e.m.f (V) V = voltage (V) I = current (A) R = resistance in the circuit (Ω) r = internal resistance (Ω)
Potential Divider
Definition
A type of circuit containing two components designed to divide up the p.d. in proportion to the resistances of the components
Potential Divider V1
Formula
V1 = (R1 / (R1+R2)) x Vin
V1 = voltage across R1 (V) R1 = resistance of first resistor (Ω) R2 = resistance of second resistor (Ω) Vin = input voltage (V)
Potential Divider V2
Formula
V2 = (R2 / (R1+R2)) x Vin
V2 = voltage across R2 (V) R1 = resistance of first resistor (Ω) R2 = resistance of second resistor (Ω) Vin = input voltage (V)
Light Dependent Resistor (LDR)
Light and Resistance
When the light level falls the resistance of the LDR increases
When the light level increases the resistance of the LDR decreases
Advantages of Data Loggers
Can take lots of readings in a short space of time
Removes human error
Can simultaneously record readings e.g. voltage and current
Can take readings at precise intervals
Can display data as a graph on a computer
