Science 8 Lecture and Problem Solving

1. ELECTRICAL POWER
AND ELECTRICAL
ENERGY
SCIENCE 8
Prepared by: Ms. Louie Jane T. Eleccion

2. ELECTRICAL POWER and ELECTRICAL ENERGY
•ELECTRICAL POWER (Watts or W)
P = IV
I – current (A)
V – Voltage (v)

3. Sample Problems (Electrical Power)
1. An oven toaster is plugged into an outlet that provides a
voltage difference of 120 V. What power does the oven use
if the current is 10 A?
2. Sarah’s reading lamp uses 60 W power with a current 1.05
A. What is the voltage of the lamp?
3. A VCR that is not playing still uses a 10 W power. What is
the current if the VCR is plugged into 120 V electric outlet?

4. ELECTRICAL POWER and ELECTRICAL ENERGY
•ELECTRICAL ENERGY (kilowatt-hour or kWh)
E = Pt
P – power (W)
t – time (h)
or E = IVt

5. Sample Problems (Electrical Energy)
1. A microwave oven requires 700 W of power to operate.
What is the energy the device uses in two hours.
2. A smartphone with a power rating of 40 W uses 0.5 kWh a
day. For how many hours will the smartphone during this
day?
3. If it takes 45 s for the toast to be made, how much energy
is consumed if the electric current is 3.0 A and the voltage
is 120 V?

6. ELECTRICAL POWER
AND ENERGY QUIZ

7. 1. A calculator uses 9 V battery and draws 0.1 ampere. How much
power does it use?
2. The current flowing in an appliance connected to a 120 V source
is 12 A. How much kilowatt hours of electrical energy does the
appliance use in 4 hours?
3. A refrigerator operates on the average for 10.0 hours a day. If the
power rating of the refrigerator is 700 W, how much electrical
energy does the refrigerator use in 1 day? (make sure to convert
to kW).
4. A television with a power rating of 200 W uses 0.8 kWh in one
day. For how many hours was the TV on during this day?
5. A rice cooker is plugged into an outlet with a voltage of 90. What
power does the rice cooker use if the current is 12 A?

8. ELECTRICAL HAZARDS
and SAFETY DEVICES

9. ELECTRICAL HAZARDS
• Damaged Insulation – causes electric shock upon contact
and can result to serious and fatal injuries.
• Overheating of Cables – large current flowing through the
conducting wires generates more heat that could melt the
insulation, causing to emit dangerous fumes or even catch
fire.
• Damp Conditions – appliance exposed on damped or wet
conditions.

10. ELECTRICAL HAZARDS
• Overloading – Lines that carry more than the safe amount of
current are said to be overloaded. The resulting heat may be
sufficient to melt the insulation and start a fire.
• Short Circuit – when insulation that separates the wires in a
circuit melts and the wires are allowed to touch

11. Electrical Safety Devices and Features
• Circuit breakers – safety devices that can switch off the
electrical supply in a circuit automatically when there is an
overflow of current.
• Fuse – device included in an electrical circuit to prevent
excessive electrical flow. It consists of a thin piece of wire
which becomes hot when heated and melts when current
flowing through it is greater than its rated capacity.

12. Electrical Safety Devices and Features
• Double Insulation – safety feature in electrical wires that provides
two levels of insulation:
a. The electric cable insulated from the internal components of the
appliance.
b. The internal components are also insulated from the external
casing.
• Earthing – safety feature that is meant to protect the user from
electric shock. The ground wire, which is usually connected to the
metal casing of the appliance.

13. CIRCUIT BREAKER FUSE
DOUBLE INSULATION EARTHING