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Faith… must be enforced by reason…When faith becomes blind, it dies.
Mahatma Ghandi
The ultimate ground of faith and knowledge is confidence in God.
Charles Hodge
Power:
Different electrical appliances consume energy at different rates. The rate at which energy is used is called its power. A hair dryer has a power rating of about 1200 W, whereas an alarm clock has a power rating of only 6 W.
Power is the rate at which electrical energy is used. It can be calculated as:
P = IV
Where,
P = power, measured in Watts (W)
I = current, measured in amps (A)
V = potential difference, measured in volts (V)
Practice Questions
- A light bulb has a power rating of 60 W. What is the current running into it if in is plugged into a 120 V outlet?
Answer
Given:
P = 60 W
V = 120 V
Required:
I = ?
Solution:
The current through the light bulb is 0.5 A.
By substituting Ohm’s Law into the power equation, two additional equations for power can be derived.
Second Equation for Power:
Sub V = IR into P = IV
This gives you P = I(IR)
Simplification yields the equation P = I2R.
Third Equation for Power:
Sub 
into P = IV
This gives you 
Simplification yields the equation 
.
Practice Questions
- A television has a power rating of 150 W and is plugged into a 120 V outlet. What is the resistance of the television?
Answer
Given:
P = 150 W
V = 120 V
Required:
R = ?
Solution:
Rearrange to get 
The television has a resistance of 96 ?
2. How much current will a 500 W vacuum cleaner draw if it has a resistance 30 ??
Answer
Given:
P = 500 W
R = 30 ?
Required:
I = ?
Solution:
Rearrange P = I2R to get 
The vacuum will draw a current of 4.1 A.
Producing electricity for a wide range of consumers is quite a challenge. In order to be efficient, power stations generate electricity at about 20 000 V. This is far too high a voltage for you to safely use in your home. The electrical outlets in Canadian homes is a much safer 120 V. But when electricity is transmitted over large distances at 120 , a tremendous amount of energy is lost. To minimize energy loss, many transmission lines carry voltages of up to 500 000 V. It is possible to increase the voltage form 20 000 V to 500 000 V for transmission and then reduce it over several stages to 120 V by the time it reaches an outlet in your home by using transformers. Transformers are able to step up or step down the voltage being transmitted.
The Transformer:
A transformer is made up of two coils of wire wrapped around a laminated iron core that transfers magnetic flux from one coil to the other. No electricity flows around the iron core.
The transformer shown below is a step-down transformer; this means that it will lower – or step-down – the voltage. You can tell that it is a step-down transformer because there are fewer turns on the secondary coil than on the primary. A transformer that increases the voltage is called a step-up transformer.
The primary coil is also considered the input coil. As electricity runs through the helix in the primary coil, it produces magnetic flux in the iron core which passes all the way around the core; the magnetic field is shown as a red arrow in the diagram above. This magnetic field in the right side of the helix is immediately “cut” but the wires in the helix of the secondary coil induce a voltage in it. This electricity is now transmitted through the wires connected to the secondary coil at the lower voltage.
You have seen how moving a magnet in and out of a coil of wire will induce a voltage in the wire according to Lenz’s Law. Transformers work using the same principle, except instead of moving the actual magnet, they just move the magnetic field. With AC electricity, each time the current changes direction, so does the magnetic field. And each time the magnetic field changes direction, voltage is induced in the secondary coil. This happens 60 times per second.
Transformer Equations:
The power going into the primary coil is equal to the power coming out of the secondary coil and if the voltage is increased, then the current must increase proportionally.
Pp = Ps
IpVp = IsVs
Practice Questions
- An electrical power plant generates electricity with a current of 50 A and a potential difference of 20 000 V. In order to minimize the power losses due to the high current heating up the line, a transformer steps up the potential difference to 500 000 V before it is transmitted. What is the current in the transmission lines?
Answer
Given:
Ip = 50 A
Vp = 20 000 V
Vs = 500 000 V
Required:
Is = ?
Solution:
IpVp = IsVs
The current in the transmission line is 2 A.
The change in the potential difference between the primary coil and the secondary coil is directly proportional to the change in the number of turns between the primary coil and the secondary coil.
Where Vp and Vs are the potential difference and Np and Ns are the number of turns of the wire in each coil.
Practice Questions
- Consider the transformer that was used at the top of this page.
- How do you know that this is a step-down transformer?
- How many turns are there on the primary coil?
- How many turns are there on the secondary coil?
- If the potential difference in the primary coil is 240 V, what is the potential difference in the secondary coil?
Answer
- This must be a step-down transformer because there are fewer turns on the secondary coil than on the primary coil.
- There are 8 turns on the primary coil.
- There are 4 turn on the secondary coil.
- Given:
Np = 8
Ns = 4
Vp = 240 V
Required:
Vs = ?
Solution:
