Circuits

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 Learning Objectives:
• Circuit symbols
• Conventional current flow
• Series and parallel circuits
• Emf and potential difference
• Ohm’s law and resistance
• Resistors in series and parallel
• Potential divider
• Current : voltage graphs
• Energy transfers in a circuit
• Solving circuit problems
 What is Electricity?
• Electricity is a flow of electron
• An electric current flows when
electrons move through a
conductor, such as a metal wire.
Metals are good conductors of
electricity
 What is a Coulomb?

• A coulomb is a unit of electrical charge with the symbol Q.

• Electrons are tiny and have a very small charge.
• In the physics of electricity, we take a very large number of
electrons as 1 unit of charge called a coulomb.
• 1 coulomb = 6·2e18 electrons.
• This is 6·2 million million million electrons.
• Such a large number of electrons can do useful things like light
a lamp.
 Calculating current
Calculating current

The size of an electric current shows the rate of flow of

electric charge. You can calculate the size of a current using
this equation:
𝐶ℎ𝑎𝑟𝑔𝑒 𝑖𝑛 𝑐𝑜𝑢𝑙𝑜𝑚𝑏𝑠
Current in amps =
𝑡𝑖𝑚𝑒 𝑖𝑛 𝑠𝑒𝑐𝑜𝑛𝑑𝑠
𝑄 Current is measured
using an ammeter. To
Or: I= measure the current
𝑡 through a component,
Where:
the ammeter must be
I is the current in amperes (amps), A placed in series with that
Q is the charge in coulombs, C component.
t is the time in seconds, s
 Conventional Current VS
Electron Flow
We now know that electrons
flow from negative to positive.
This is called electron flow. By
the time this was
discovered, a large number of
electrical circuits had
already been drawn and since it
makes no
practical difference, it was
decided to keep the
conventional direction of
current flow for circuit
diagrams.
 What is Potential Difference or Voltage?
• The potential difference (voltage) of a power supply is an
electrical pressure that causes a current to flow in a circuit.
Doubling the supply voltage will double the current.
Unit for Potential Difference or Voltage?
• What is a Volt?
The power supply (the cell or battery) gives an amount
of energy to each coulomb of charge in an electric
circuit.
1 volt = 1 joule per coulomb.
Energy is measured in joules.
A 6 volt cell gives 6 joules of energy to each coulomb.
The circuit on the left shows
the potential difference of
the cell. This is called the
supply voltage.
The circuit on the right has the
voltmeter connected across
a lamp. This will tell you how
many joules of energy are being
transferred from electrical
energy into light energy (+heat)
for each coulomb of charge
that passes through the lamp.

A reading of 6 volts tells you that 6 joules of energy are being

transferred for each coulomb passing through the lamp.
Resistance: A resistor is a component that decreases the current in
a circuit. A resistor transfers electrical energy into
heat energy
 Resistance: Resistance is the opposition that a substance
offers to the flow of electric current.
• What is the Cause of Electrical Resistance?

• When electrons move through the wire, some electrons

collide with the ions of the material in the wire.
It is these collisions that cause electrical resistance.
• With each collision, some energy is lost to the wire
as heat. This explains the heating effect of current.
• When the current through a wire increases, the number
of collisions between the electrons and the ions
increases and the wire gets hotter.
 Factors which affect resistance
• Thickness ( cross-sectional area)
• Length
• Temperature
• Conductivity of the material
 Factors affecting resistance of a wire
• Length: Resistance is proportional to length.
 Factors affecting resistance of a wire
• Resistance is inversely proportional to cross-sectional area
 Factors affecting resistance of a wire
• Resistance depends on the material the wire is made of.

 The electronic configuration of an atom determines how willing

the atom will be to allow an electron to leave and wander through
the lattice.
 If a shell is almost full the atom is reluctant to let its electrons
wander and the material is an insulator.
 If the outermost shell (or sub-shell with transition metals) is less
than half full then the atom is willing to let those electrons
wander and the material is a conductor.
 Factors affecting resistance of a wire
• Resistance increases with the temperature of the wire.

 When a material gets hotter the atoms in the lattice vibrate more.
This makes it difficult for the electrons to move without
interaction with an atom and increases resistance.
 Types of Circuits

 Devices within a circuit can be connected in one of two

ways:
 In Series: Devices form a single pathway for
electrons to flow
 In Parallel: Devices form branches, each of which is
a separate path for the flow of electrons
 Series Circuits
• When components are connected one after another in a ring, the components are
to be in series with each other and the circuit is called a series circuit.

• An example of a series circuit is shown above with three different resistors.

• The current has only one single path to flow through. Compare this with a
parallel circuit.
 Parallel Circuits
• When components are connected in parallel, each component provides its own path
for the current to flow through back to the cell.
Each path is called a branch of the circuit.

• A parallel circuit is shown above with three

branches and a different resistor in each branch.
• Compare this with a series circuit.
• Series Circuits • Parallel Circuits
 Ohm’s Law

• The CURRENT flowing

through a METAL wire is
PROPORTIONAL to the
POTENTIAL
DIFFERENCE across it.
Providing
TEMPERATURE
remains CONSTANT.
• A 2 V accumulator is connected to a wire of
resistance 20 ohms. What current flows in the
circuit?
1. An electronic device has a resistance of 20 ohms and a current of 15 A.
What is the voltage across the device?
Ans:V=IR=15×20=300V​

2.A 3−V potential difference is applied across a 6Ω resistor. What is the

current that flows into the resistor?
Ans: I=V/R​=3/6​=0.5A​
3.In a circuit, the potential drop across the 10 kΩ -resistor is 100
V. What is the current through the resistor?
Ans: I=V/R​=100V​/10000Ω=0.01A​

4.An alarm clock draws 0.5 A current when connected to a 120 V

circuit. Find its resistance.
Ans: R=V/I​=120/0.5​=240Ω​
 Exercise:
• A 6 V battery passes a
current of 1 A through a
lamp of 1 minute. How
much energy is
transferred from the
battery to the lamp?
Practice: Calculate the Voltage & Current
 Practice problems:
1. A 20 Ohm lamp and a 5 Ohm lamp are connected in series and placed
across a potential difference of 50 V. What is the equivalent resistance of the
circuit? What is the voltage drop across each lamp?

2. A 16 Ohm and a 20 Ohm resistor are connected in parallel. A difference

of potential of 40 V is applied to the combination. Compute the equivalent
resistance of the parallel circuit. What is the current in the circuit? How
large is the current through the 16 Ohm resistor?
• Solution:

1.R = 25 ohm,
V (20) = 40 v,
V (5) = 10 v

2.R = 20/9 ohm,

I = 4.5 amp,
I( 16ohm) = 2.5 amp