Electricity Part 1 - Electric Current Flashcards
Describe electric current.
One of the 5 base quantities.
Base unit is Ampere (I)
Conventional current: positive to negative
Electron movement: negative to positive
Describe charge.
Charge is Q, measured in coulombs.
Q = It
Charge is quantised -> means it exists in packets whose value are a multiple of the elementary charge (1.6x10 -19 C)
Describe the equation I = nAve
n - number density wave whose units are m -3
n tells us the number of electrons per unit volume of a conductor.
A - cross-sectional area
v - average drift velocity
e - elementary charge
I - electric current
Derived from Q = nALe:
n x Ve = Q (nv = total number of electrons)
V = AL
But we know Q = It
Therefore It = nALe
I = nALe / t
Observe that L/t = v, where v is the average drift velocity
Therefore I = nAve
From this equation, we can deduce:
V is directly proportional to I
V is inversely proportional to A
V is inversely proportional to the diameter of the conductor squared
What are the practical skills for electricity?
Electricity flows in a complete circuit.
Current is measured by an ammeter connected in series with the part of the circuit that is being measured.
A voltmeter is always connected in parallel to the electrical component whose voltage is measured.
When connecting an electrical circuit, start by laying all the electrical components on desk, as they are in the given circuit diagram.
Start by connecting one loop at a time, from one side of the cell to the other side.
Define voltage/potential difference.
Energy per unit charge converted from electricity to other forms of energy in a closed circuit.
Define E.M.F.
Electromotive force is the energy per unit charge converted from other forms of energy to electrical in a closed circuit.
What are the similarities between E.M.F and potential difference?
Both measured with a voltmeter
Both are energy/charge; E.M.F = total energy / charge, potential difference = energy through the component / charge
What is the formula for energy in a circuit?
V = W/Q
Describe power.
In general, power is the rate of energy transferred.
P = W/t
P = QV/t
P = IV
What are the Ohm’s Law equations?
R = V/I
P = IV
V = IR
P = I2 R
I = V/R
P = V2/R
Define resistance.
Electrical resistance is the ratio of voltage to electric current through an electrical component.
Ohm is the unit of resistance when 1 volt is across a component with a current of 1 A.
Ohm = V/A
Describe Ohmic conductors.
Obey Ohm’s Law. It states that, when voltage is directly proportional to current, provided temperature remains constant/ the same.
V is directly proportional to I
V = RI (R is a constant)
Ohmic resistor in a I-V graph is constant positive gradient and goes through the origin.
Non-ohmic resistors have a curve that also goes through the origin.
What are the I-V characteristics of an ohmic resistor?
Ratio V/I is constant as R is constant
Any metal at constant temperature
What are the I-V characteristics of a filament lamp?
Curves upwards before V=0 then curves downwards after V=0.
Ratio V/I increases as R increases.
Fill in: A ___ wire has ____ resistance than a ____ one of the ____ type.
A HOT wire has MORE resistance than a COLD one of the SAME type.
What are the I-V characteristics of a semi-metal?
Curves downwards before V=0 then curves upwards after V=0.
What are the I-V characteristics of a diode?
I = 0 until switching on the voltage, at which point the line increases at a constant rate upwards.
Define resistivity.
Resistance R of a conductor is directly proportional to its length l and inversely proportional to its cross-sectional area A.
Unit for resistivity is ohm m.
R = (resistivity x length) / Area
Define potential divider.
A potential divider is made up of 2 or more resistors connected in series.
It allows for part of the e.m.f of the circuit to be supplied to a secondary circuit.
What is the formula for the current in a potential divider circuit?
I = E / R1 + R2
What is the formula for the p.d across R1?
R1 = Vout
and Vout = IR1
= (E/ R1+R2) x R1
Vout = (R1 / R1+R2) x E
This is called the potential divider formula.
Vout is directly proportional to R1.
Define LDR.
Light-Dependent Resistors.
If in high light intensity, resistance is low.
If in low light intensity, resistance is high.
Define thermistor.
If in high temperature, resistance is low.
If in low temperature, resistance is high.
