Physics 05: Electric Currents
BY HEI MAN KWOK 12N03S
5.1 ELECTRIC POTENTIAL DIFFERENCE,
CURRENT AND RESISTANCE
Model of electric conduction in a
metal – energy transfer
1.
2.
3.
4.
5.
Charge carriers have kinetic energy
These collide with lattice ions
Increasing amplitude of vibrations
This is seen in an increase in temperature
Electrons loses it energy and transfers it to
thermal energy
Speed of electron through a conductor = drift
speed
Charge (Q)
• Measured in Coulombs (C)
• Charge of 1 electron = 1.6 𝑥 10−19 𝐶
• 1 Coulomb =
1
1.6𝑥 10−19
= 6.2 x 1018 electrons
Electric Potential Difference
• Energy taken per unit charge
𝐸 (𝑒𝑛𝑒𝑟𝑔𝑦)
𝑉(𝑣𝑜𝑙𝑡𝑎𝑔𝑒) =
𝑄 (𝐶ℎ𝑎𝑟𝑔𝑒)
1 volt = wd per unit charge, 1 joule per coulomb
Change in potential energy when a
charge moves between two points at
different potentials
• Potential energy (J) = Voltage (V) x Charge
(C)
𝐸𝑝 = 𝑉𝑞
Insert diagram 
Electronvolt (eV)
• Amount of energy (work done) one electron
would gain by moving through a potential
difference of 1 volt
• eV = 1.6 𝑥 10−19 𝐽
• Charge of a electron = −1.6 𝑥 10−19 𝐶
Electric Current (in a conductor)
The rate flow of electric charge (from positive to
negative potential)
𝑄 (𝐶ℎ𝑎𝑟𝑔𝑒)
𝐼 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 =
𝑡 (𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛 𝑖𝑛 𝑠𝑒𝑐𝑜𝑛𝑑𝑠)
Ampere = force per unit length between parallel
current-carrying conductors why? 2x 10^7 N
Resistance
• Ratio of Voltage to Current
• How easily does current flow through
𝑉 (𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒)
𝑅 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 =
𝐼 (𝑐𝑢𝑟𝑟𝑒𝑛𝑡)
Resistor = a component with a known resistance
Resistivity
𝐿 (𝑙𝑒𝑛𝑔𝑡ℎ)
𝑅(𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒) = 𝜌
𝐴 (𝑐𝑟𝑜𝑠𝑠 − 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎)
𝜌 = 𝑟𝑒𝑠𝑖𝑠𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
More current flows through a short fat conductor
than a long thin one
Ohm’s Law
• The ratio of potential difference to current are
proportional at constant temperature
• V = IR
• Eg. Wires and fixed resistors
Non-Ohmic – Filament lamp
• As current increases, temperature increases,
atoms vibrate more, collisions between
electrons and metal atoms are more frequent
so resistance increases, and graph flattens
• The current is not directly proportional to the
voltage – disobeys ohm’s law
• Graph = symmetrical
Diode
• One way value for electrons
• No current when negative V
• Positive = current flows easily as diode has low
resistance above about 0.7V
• LEDs also one way :D
Component’s Potential Difference
Cannot be equal to the p.d. of the battery:
• Some voltage will be lost to internal resistance
and/ or the resistance in wires
Cannot equal to zero
• Low voltage requires very high resistance,
max. resistance of the variable resistor cannot
be infinite, there will always be some
resistance from the component
5.2 ELECTRIC CIRCUITS
Electromotive Force (emf)
• Work done per unit charge made available by
the energy source (cell or battery)
• Power supplied by the cell per unit current
from the cell
Law of Conservation of Energy
• Energy cannot be created or destroyed – the
energy converted from chemical to electrical
in the cell must be equal to the amount
converted from electrical to heat in the
resistor
𝐹𝑟𝑜𝑚 𝑂ℎ𝑚′ 𝑠 𝐿𝑎𝑤: 𝑉 = 𝐼𝑅
Internal Resistance
• The resistance of the cell is ^
𝜀 = 𝐼𝑅 + 𝐼𝑟
• As current flows through the internal resistance –
some energy is converted from electrical to heat
inside the cell (so the cell gets hot)
• This means that there is less energy to be
converted to heat in the resistor – p.d. across the
resistor is therefore less than the emf of the cell
Finding Internal Resistance
Experimentally
• V = E – Ir
• By recording values of current and terminal pd as the
external resistance changes you can plot the graph and find
the internal resistance and the emf of the cell.
• If there is more than one cell in series the internal
resistances of the cells must be added.
Power Dissipated
𝑃 = 𝐼𝑉
𝑃 = 𝐼2 𝑅
𝑉2
𝑃=
𝑅
P = Power; I = Current; V = Voltage; R =
Resistance
Power Delivered
𝑃 = 𝜀𝐼
In a real battery – actual power delivered will be
a bit less, since there will be some power
dissipated in the internal resistance
V, I, R in Series
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 = 𝑉1 + 𝑉2 + 𝑉3
𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑠 𝑠𝑎𝑚𝑒
𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 𝑅1 + 𝑅2 + 𝑅3
V, I, R in Parallel
𝐶𝑢𝑟𝑟𝑒𝑛𝑡 = 𝐼1 + 𝐼2 + 𝐼3
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑖𝑠 𝑠𝑎𝑚𝑒
1
1
1
1
=
+
+
𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑅1 𝑅2 𝑅3
Ideal Ammeter
• Zero resistance so it does not change the
current in the circuit
• Connected in series so the current will flow
through the ammeter as it flows through a
component
Ideal Voltmeter
• Infinite resistance so it does not take any
current from the circuit
• Connected in parallel to see the difference in
potential energy between two points
Thermistor and LDR
• Made of semi-conducting material
• Heat and light frees more charge carriers: as the
temperature/ light intensity increases, the
resistance decreases
• The current is not directly proportional to the
voltage
LDR
Thermistor
Strain Gauge
• Think metal wire
• If stretch – length increases and crosssectional area decreases = increase in
resistance
Potential Divider
𝑉𝑜𝑢𝑡
𝑅2
= 𝑉𝑖𝑛
𝑅1 + 𝑅2