topic 10 electricity

Created by

Pranitaa Manicassamy

Cards (27)

circuit diagrams
materials to conduct electricity :
must contain charged particles
those charged particles must be able to move when a potential difference is provided
circuits
these electrons are negatively charged so they move towards positive terminal of battery
when working with circuits , however the conventional direction of current is used .
current goes from positive to negative terminal
series circuit
in a series circuit , there is just one route from the current to flow round the circuit
parallel circuit
in a parallel circuit , there are multiple different routes the current can flow round the circuit
these points where the current split is called a junction
circuit symbols
A) cell
B) battery
C) open switch
D) closed switch
E) filament bulb
F) fixed resistor
G) variable resistor
H) voltmeter
I) ammeter
J) fuse
K) diode
L) LED
M) LDR
N) Thermistor
current
electric current is measured in units called ampetes ( often shortened to amps , A )
use ammeter to measure
must be connected in series to measure current flowing through circuit or its branch
electrical current = Rate of flow of charge in a circuit
(charge per unit time , flowing past a particular point )
current in series
current in series circuit , through single loop is the same
current before junction must be equal to current after a junction
current at junction will split depending on resistance of branch
charge must be conserved ( based on the conservation of charge law)
hence , if charge is always conserved , so must current
potential difference
also called voltage
energy per unit charge - measured in volts using voltmeter connected in parallel
rules of potential difference
P.D across all components in a series circuities' equal to the P.D provided by power source
P.D across each component in a series circuit is shared and adds up tp total P.D provided by power source $V^t =$ $V^1 +$ $V^2 +$ $V^3 +$ $....$
The P.D across each branch of a parallel circuit has the same P.D as the power source - branches in parallel have the same P.D
Potential difference
energy must be conserved ( based on conservation of energy law )
energy cannot be created or destroyed
hence energy provided by a power source must equal to the total energy given to components in a circuit
Current , charge and energy
in metals , current is a flow of electrons , each electron has a very small charge , only -1.6 x 10 ^19 C
millions and millions of electrons flow through metal wire to produce 1 C
electric charge is measured in coulombs ( C )
one coulomb is the charge that passes a point in a circuit when their current is of 1 amp for 1 second
charge = current x time
Q = I x T
Q= coulombs
I = amperes
T = seconds
electrical resistance
a resistor is a component or device that adds resistance to a circuit
resistance is defined as the restriction of a flow of current in an electrical circuit
it is measured in units called ohms , Ω
potential difference = current x resistance
V = I x R
V - potential difference
I - current
R - resistance
resistance rules in series
voltage is shared
depending on the resistance of each component
current is the same
depending on the total resistance of each component
resistance is addative
resistance rule in parallel
voltage is the same
each branch gets the same amount of voltage
current is shared
split across each branch and depending on the resistance
in parallel, total resistance is less than the individual components
ohms law
the P.D through a compound is directly proportional to the current flowing through it
the constant of proportionality is called resistance
V = IR -> for an ohmic conductor , I-V graph is a straight line
I-V characteristics - resistors
is a straight line graph ,
resistors is an ohmic conductor ( P.D is directly proportional to current)
I-V characteristics - filament lamps
current and voltage not proportional
resistance changes as the potential difference and current varies
bulb does not follow ohms law
as voltage increases , the resistance of the filament lamp increases
I = V / R , so the gradient of the graph is 1/R , and the gradient is decreasing
hence resistance will increase
i-v characteristics filament light
as current flows through the circuit , the electrons collide with the metal ions , transferring some of their kinetic energy to the metal ions
metal ions vibrate more as they gain K.E
as the average K.E increases , so does their temperature
as temp increase :
-> metal ions vibrate more
-> more collisions
=> more restriction in the flow of current
I-V characteristics - Diode
the current in a diode is not proportional to the voltage
when the negative voltage is applied current is zero
a small voltage is required in the positive direction to cause a current ( threshold voltage )
for a negative V : the gradient is 1/R
gradient is equal to 0 - therefore resistance is very large
I-V characteristics - Light dependent resistors - LDR
made of a semiconductor material
an LDR's resistance depends on the light intensity incident on it . As light intensity increases , the resistance of the LDR will decrease
they commonly used in light sensing circuits such as those in street lamps or cameras
as the light intensity increase the resistance decreases
at a decreasing rate
I-V characteristics - Thermistor
thermistors are also made of semiconductor materials
their resistance decreases as temperature decreases
electrical power
power = energy transferred per second
E = VIT
P = IV , v = IR
power (W) = current (A) x potential difference (V)
substitute
P = I^2R (sub V = IR into P = IV)
Mains Safety
mains means principle - hence primary source of electricity for household
the electricity that comes from the main is 230 v with a frequency of 50Hz , its an alternating current (A.C)
electricity comes in two forms, alternating current ( A.C ) and direct current ( D.C)
in A.C , the voltage and current change with time
in D.C , the voltage and current remain constant with time
A.C -> magnitude varies in time between maximum and minimum value
D.C -> magnitude is constant
frequency = number of waves / time = 1/T
fuses - main safety
fuses or circuit breakers are connected to the live wire entering the house and breaking the current if it becomes too large
a large current will transfer a lot of power which can become very dangerous
they are connected to the live wire so that the whole circuit stays at 0V if there is a blown fuse
if not all sockets in the house remain dangerous
fuses main safety
each plug has a smaller fuse
a live wires is considered dangerous because there P.D is 230v between it and other wires
if you make the connection between these wires the electrons will flow through your body as it is grounded and having a lower resistance , transferring electrical energy into you which causes burns and potentially stop the heart
fuses are used to prevent a large current passing through as they have a range of rating 3A, 5A , 13A which prevents a larger current passing as it melts the fuse breaking the circuit
uk mains plugs
three main wires
live wire ( brown ) - voltage changes between 230v and -230 v at 50Hz - A.C current
neutral wire ( blue ) -connects back to the power station and should remain at 0v
earth wire ( green ) - connected to a large metal spike pushed into the ground . attached to any metal casing so that charge cannot build up and become dangerous