1. A TV set, a stove and a flatiron are connected in series to a 220-V line. The resistance of the TV set is 20-ohms, the stove 50-ohms, and the flatiron 35-ohms. Find a) the total resistance, b) the amount of current flowing each device and c) the voltage drop across each device.

1. ELECTRICITY

2. CURRENT
•Current: the actual
“substance” that is flowing
through the wires of the
circuit (electrons!)

3. ELECTRIC CURRENT
• Is the movement of charged particles in a specific direction.
• The relationship between time, charge and current is given as :
current = charge / time
I = q / t
The unit is
ampere = coulomb / second
A = C / s

4. VOLTAGE
• Voltage: a force that
pushes the current
through the circuit (in
this picture it would be
equivalent to gravity)

5. VOLTAGE
• Is known as electromotive force or potential difference. It is the
electric pressure that causes current to flow.
• The relationship between charge, energy and voltage is
voltage = energy / charge or
V= W / q
The unit is
volt = joule / coulomb or
V= J/ C

6. RESISTANCE
•Resistance: friction that
impedes flow of current
through the circuit
(rocks in the river)

7. RESISTANCE
•Is the opposition a material offers
current.
•The symbol for resistance is R.

8. OHM’S LAW
• It is named by the German physicist Georg Simon Ohm
(1787 – 1854 )
• It is stated as ; current is directly proportional to the
voltage and inversely proportional to the resistance.
• Equation form:
• I= V/R
• From this equation, V = IR and R = V/I

9. SAMPLE PROBLEMS
• 1. How much current flows through a lamp with resistance 90 –
ohm when it is connected to a 220-V outlet?
• 2. What is the drop in voltage from the generator to the
auditorium when a current of 15 A is used and the resistance of
the wires is 0.3 – ohm?
• 3. What is the resistance of a lamp which allows 0.8 A current
when 110.0 V is applied to it?

10. Types of circuit
There are two types of electrical circuits;
SERIES CIRCUITS PARALLEL CIRCUITS

11. The components are connected end-to-end, one
after the other.
They make a simple loop for the current to flow
round.
SERIES CIRCUITS
If one bulb ‘blows’ it breaks the whole circuit and
all the bulbs go out.

12. RULES FOR SERIES CIRCUIT
1. The current that flows through each resistance is the
same as the total current throughout the circuit.
That is, IT= I1=I2=I3=…In
2. The sum of all the individual voltage drops is the same as
the applied electromotive force (emf) or voltage.
That is, VT = V1 + V2 + V3 +…+Vn
3. The total resistance is equal to the sum of the individual
resistances.
That is, RT= R1 + R2 + R3 +… +Rn

13. PARALLEL CIRCUITS
The current has a choice of routes.
The components are connected side by side.
If one bulb ‘blows’ there is still be a complete circuit to
the other bulb so it stays alight.

14. RULES FOR PARALLEL CIRCUIT
1.The total current that flows through the circuit
is equal to the sum of the currents in the
separate resistances.
That is, IT = I1 + I2 + I3 +…+In
2. The potential drop in each resistor is the same
as the magnitude of the applied electromotive
force.
That is, VT= V1=V2=V3=…Vn
3. The reciprocal of the total resistance is equal
to the sum of the reciprocals of the separate
resistances.
That is, 1/RT= 1/R1 + 1/R2 + 1/R3 +… +1/Rn

15. circuit diagrams
In circuit diagrams components are represented by
the following symbols;
cell battery switch lamp
motorammeter voltmeter
buzzer
resistor variable
resistor

16. SAMPLE PROBLEM
1. A TV set, a stove and a flatiron are connected in
series to a 220-V line. The resistance of the TV set
is 20-ohms, the stove 50-ohms, and the flatiron 35-
ohms. Find a) the total resistance, b) the amount
of current flowing each device and c) the voltage
drop across each device.