OHM’S LAW AND POWER

OHM’S LAW

For a given resistance, the potential difference is directly proportional to the intensity of the current.

Ohm’s law establishes that for a given resistance the potential difference in an electrical circuit is directly proportional to the intensity of the current.

V = RI V represents the potential difference R represents the resistance (in Ω)I represents the intensity of the current

EXAMPLE PROBLEM:

A nine volt battery supplies power to a cordless curling iron with a resistance of 18 ohms. How much current is flowing through the curling iron?

Sketch:

SOLUTION:

Know:V = 9 VR = 18 ohms

Solve for I:

I = V RI = 9 V = 0.5 A 18 ohms

ELECTRICAL POWER

The electrical power of an appliance is an indication of the quantity of work that it can do, the quantity of energy that it can transform in a certain period of time.

The unit of measure of electrical power is the Watt (W). An appliance with a power of one Watt does work of 1joule per second:

1 W = 1J 1 s

The mathematical equation of electrical power is

Pe = W Pe represents the electrical power (in W)

Δt W represents the work (in J) Δt represents the time required (in s)

Pe = VI Pe represents the electrical power (in W)

V represents the potential difference (in V)I represents the intensity of the current (in

A)

1 W = 1V * 1A

= 1J * 1C C s

= 1 J s

The relationship between Power and electrical energy

  It is possible to determine the quantity of

electrical energy consumed by an appliance by multiplying its power by time:

1 W * 1 s = 1 J * 1 s s  = 1 J

Electrical energy can be measured in joules but it can also be expressed in kilowatt hours (kWh)

1 kWh = 1000 W * 3600 s = 3 600 000 J

The kilowatt hour is used in the calculation of bills of consumption of electricity.

The following mathematical formula establishes the relationship between electrical energy and electrical power.

E= PeΔt E represents electrical energy consumed (in J or kWh)

Pe represents electrical power (in W or kW)

Δt represents the interval of time (in s or h)

For example, if we use a 1000 W microwave oven for 6 minutes, the quantity of energy consumed would be:

E= PeΔt E= PeΔt = 1000 W * 360s = 1 kW * 0.1 h = 360 000 J = 0.1 kWh   The microwave oven would have consumed

360 000 J after six minutes of use or 0.1 kWh.