Download sc 90 electricity notes2012

Unit 4 - Electricity
1
The Electrical Nature of
Matter
• There are 2 types of electrical charges:
Static and Current.
• The study of static electric charge is called
electrostatics.
Electric Charge
• There are two kinds of electric charge: positive
and negative.
• A substance that is has no charge is called
neutral.
• When 2 neutral substances are rubbed together;
one substance becomes positively charged and
the other negatively charged.
Electric Charge
• Charged objects attract neutral objects including
liquids and gases.
• Objects with like charges repel each other.
• Objects with opposite charges attract each other.
• This constancy of behaviour is called the Law of
Electric Charges.
Electric Charge
• In some atoms, electrons transfer easily from one
type of substance to another.
• In most elements, the # of protons equal the # of
electrons thus the substance is neutral.
• When a substance loses electrons, there are more
positively charged protons than negatively
charged electrons thus the substance becomes
positively charged.
Electric Charge
• The opposite is also true.
• When a substance gains electrons, there are
more negatively charged electrons than
positively charged protons thus the substance
becomes negatively charged.
• Objects become neutral again when they
transfer these extra electrons to a positively
charged object.
Charging by Friction
• There are 3 ways that charges can be transferred
from neutral objects: friction, induction and
conduction.
• Charging by friction involves 2 substances
rubbing together and transferring electrons from
one object to another.
• Charging in this way has many of the same
effects as static electricity.
Charging by Friction
• Different substances hold onto their electrons
better than others.
• You can determine what kind of charge an object
will obtain when rubbed with a substance by
using a list
• This is called an electrostatic series.
Transferring charges by contact
• Transferring a charge by friction is hard to avoid.
• When charging by contact occurs, one object is
already charged, and the other may or not be
charged.
• The important factor is that there must be a
difference in charge between the 2 objects. The
charges transfer in order to try and make them
neutral again.
Transferring charges by contact
• This transfer of charges by contact is called
conduction.
• The shock that occurs during this transfer may be
painful due to the very rapid transfer of these
electric charges.
Insulators & Conductors
• An electrical insulator is a substance in which electrons
cannot move freely from atom to atom.
• Insulators can build up an electrical charge but will hold
onto that charge until they are removed by a substance
that exerts a stronger force on the electrons.
• Wood and glass are insulators that behave this way.
This is why dust particles are attracted to them and stick
to these surfaces.
Conductors
• No matter how hard you polish a metal surface,
it will never build up a static charge.
• This is because metals are conductors.
• A conductor is a substance in which electrons
can move freely from one atom to another.
• There are also many non-metallic conductors,
including graphite, solutions of salts, and all
living things.
Insulators
• An electrical insulator is a substance in which electrons
cannot move freely from atom to atom.
• Wood and glass are insulators that behave this way.
This is why dust particles are attracted to them and stick
to these surfaces. Teflon, some plastics, rubber,
porcelain, paper, varnish, and fiberglass
• Since electric charges cannot pass through them they
are used to protect us from electric shock.
• If electrical wires and appliances were not covered with
PVC or polyethylene (plastic)insulators, they would be
extremely dangerous.
1. Electricity – Static and Current
– can occur from rubbing your feet on the floor, or clothes
with a static cling
– a build up of electric charge is called electric forces
– an object acquires static electricity when it has a build up
of static charge
– electric discharge is the removal of electric charge from an
object by electron flow
• ex// shocking someone after rubbing your feet on a rug
14
Theory of Electricity
- there are two types of charges, positive
and negative
– an object with no charge is said to be neutral
– when two types of matter are brought close
together, one may lose electrons to another.
16
The 3 Laws of Electric Charges
1) Opposite charges attract each other
2) Like charges repel each other
3) Charged objects attract neutral objects
17
18
Rubbed by
silk
19
3.
4.
Does it do the same as with contact?
20
21
Question – What happens when a charged object is
brought near water?
a) When water falls from a tap it is neutral
b) When a charged object is brought near water, the
electrons rearrange themselves in order that there
is an attraction.
22
Induction
• This is the third way in which an object can
become charged.
• The other 2 types of charging involve the
transfer of charges by physical contact.
• Induction involves the movement of charges
within an object.
• When a neutral particle approaches a charged
object, the electrons within the neutral object
rearrange themselves.
The Leaf Electroscope
• An electroscope has a metal bulb on the top
and two metal leaves along the bottom of it
that all have the ability to conduct electricity
easily.
Electroscope
24
Charging by Induction
• When a positive charge is brought near the
bulb, all electrons from the leaves are brought
to the bulb. This causes both leaves to
become positive and they repel each other.
• When a negative charge is brought near the
bulb, all electrons from the leaves are pushed
to the leaves. This causes both leaves to
become negative and they repel each other.
25
26
Charging by Conduction
• Same idea as induction, but the bulb is
actually contacted by the charged object, so
electrons are transferred. This means that
when the charged object is removed, the
leaves will remain open.
Grounding (-)
Grounding (+)
27
2. Current Electricity
–
–
–
–
–
when dealing with static electricity, once the charges
are passed, they do not move. The term used to
describe this is “at rest”
usually electricity is moving.
Conduction of electricity means that electricity is
flowing through an object
Current electricity is when the charged particles move
• Radio, toaster, blow dryer
energy sources are used to produce energy in the form
of electric current
• batteries, generators, solar panels
28
3. Detecting Current – Measuring Current
– the ammeter is generally used to measure current
(the flow of electrons)
– the greater the number of electrons flowing past
the detector in a second, the greater the current.
– The unity used to measure current is called the
“ampere” (amp) and the symbol used is “A”
29
4. Voltage of Cells and Batteries
– a cell is a device that converts chemical
energy into electrical energy
– two or more cells joined together are called
a battery
– a volt V is a unit used to measure electric
potential ( how much charge; quantity is
delivered by a cell.)
– The higher the voltage, the more electrons
leave the cell
– If two 1.5V batteries are used in a piece of
equipment, 3V are being used
– A voltmeter is used to measure the number
of volts released by a battery
30
5. Types of Cells and Batteries
– two main features
a) contain two different metals(electrodes)
b) separated by a solution that conducts
electricity (electrolyte)
31
Negative electrode
Positive electrode
electrolyte
32
How Do Batteries Work?
– Electrons begin at the positive electrode.
– The electrons then move across the electrolyte to
the negative electrode.
– The electrons gather at the negative terminal and
then are pushed out of there.
– The electrons provide energy to the device being
used and then are pulled back in by the positive
terminal.
– The electrons are then returned to the positive
electrode.
– For every electron that leaves the negative
terminal, one is returned to the positive
terminal. This process repeats itself.
33
When do batteries Die?
• Batteries die when the paste of chemicals dry
out, or when the zinc layer becomes used up.
34
Types of Batteries
• Rechargeable Batteries
– generally use Ni and Cd (Nickel and
Cadmium)
– electrolyte is very caustic (corrosive)
• Alkaline Batteries
– use Zn and MnO2
– useful for toys (last a long time)
• Car Battery
– use Pb and PbO
– provide current to starter, then a
generator (alternator) passes current
back to the battery to recharge it.
35
6. Resistance
– current can be affected by the type and amount of
wire that is used
– a resistor offers resistance to the flow of charges
– the greater the resistance, the greater the amount
of energy is given up
ex// light bulb, toaster, etc.
– there is tungsten in the light bulb that does not
conduct very well
– therefore energy is given up to the bulb and heat
and light are produced
– if copper was used, light nor heat would be
produced because the electricity would be easily
conducted
36
 Three things effect resistance
1. Length of the Wire
2. Diameter of the Wire
3. Type of Wire
 The longer the wire, the more resistance.
 The thinner the wire, the more resistance.
 Copper would be a better conductor than a pickle
for example.
37
A Light Bulb is an Example of a Resistor
38
7. Circuits and Switches
– if you connect one wire from a battery terminal
to a light bulb, nothing happens until you
connect a wire from the other end
– there must be a complete pathway for the
electrons to flow
– this is called an “ELECTRIC CIRCUIT”
– if electrons flow through, the circuit is called
closed
– if the electrons can not flow through, the circuit
is called open
– a switch makes a closed circuit open by
disconnecting one of the wires (this enables us to
turn things on and off)
39
8. Series and Parallel Circuits
– some Christmas lights all go out if one bulb is
removed
– this is known as a series circuit
– the removal of one bulb interrupts the flow of
electricity in the circuit
– most lights do not work like this however
– these types of circuits are known as parallel
circuits
– electrons can flow in two or more alternative
paths
– if one bulb is removed, the path remains closed.
40
Symbols for Circuit diagrams
cells
Lamp
Ammeter
Voltmeter
Switch
Resistor
Motor
Series Circuit
• Has only one pathway for
electrons to flow
Electrons flow out through the negative end of the battery,
through the device and back into the positive end of the
battery.
43
Parallel Circuit
• There are 2 or more ways pathways for
electrons to flow.
44
46
Series Circuit
electrons flow out of the short end
and into the positive end of the cell !!!
Parallel Circuit
47
48
49
50
51
(three cells) for each question.
52
53
9. Electricity in the Home
– electrical devices can be grouped into 4
main categories
1) Light producing devices (lamps,
flashlights)
2) Heat producing devices (stoves, hair
dryers)
3) Mechanical producing devices (vacuum,
drill, saw)
4) Audio visual devices (TV, VCR, radios)
54
10. Measurement and Cost of Electricity
– appliances use up energy
– in order to calculate the cost of energy, we
need to determine the amount of energy
used per second
55
SaskPower charges 9 cents for each kWh of
electricity. Your TV consumes 200W and you
watch the TV 150h in a month. How much will
it cost to watch the TV for the month?
Step 1  Convert the watts to kilowatts
200 W = 0.200 kW
Step 2  Multiply kW by the number of hours
used
(0.2 kW)(150h) = 30 kWh
Step 3  Multiply the kWh by the cost of
electricity
(30kWh)($0.09) = $2.70
56
Questions
If SaskPower charges 11 cents per kWh,
calculate the following:
a) Using a toaster for 10h at 1000W
b) Using a desk lamp for 20h at 60W
c) Using a clock for 720h at 4W
d) Using a stove for 10h at 12000W
e) Using a dryer for 30h at 4600W
57
11. A Safe Supply of Electricity
• A meter is used to measure the amount of electricity
used in our homes.
• There are 3 wires found at this meter, 1 is said to be
neutral and the other 2 are considered hot wires.
• There are fuses and breakers found in the house for
safety reasons.
• The neutral wire is attached to the ground
• 120V to 240V is the maximum amount of voltage
provided for major appliances.
58
12. Fuses
• Consist of a thin metal strip that melts if too
much current is passed through it
• This opens up the circuit and stops the flow of
electrons
• Fuses of 5A, 10A, 15A, 20A and 30A are used
• If a fuse melts, it needs to be replaced before
the appliance can be used again
59
13. Breakers
• Consist of a bi-metallic strip that bends if too much
current passes through.
• This causes the breaker to trip which opens up the
circuit and stops the flow of electrons.
• Breakers do not need to be replaced, because when
they cool down, they return to their original position.
61
14. Calculations
A. Electric Energy
The total amount of electric energy
used depends on the total power
used by all of the electric appliances
and the total time they are used.
Energy = Power  Time
(E = P  t)
Kilowatt-hours = Kilowatts  Hours
62
B. Cost of Electricity
How much a person pays for electricity depends on two
factors.
a) the price of electricity.
b) the amount of energy consumed.
The formula used for this is:
Total Cost = Price × Energy
T.C. = Price × E
64
Examples:
1) If a blow dryer uses 350W and it is used for
19h in a month, how much will it cost if
SaskPower charges 9 cents per kWh?
65
2) If it costs a household $2.25 to operate their
computer that uses 625W, how long did they
use their computer for at 9 cents per kWh?
66
3) How many watts does a machine use if it
costs $1.10 to use the machine for 2h and it
costs 9.5 cents per kWh?
68
4) A grocery store is open from 9AM to 9PM
seven days a week. If the store has twelve
120W light bulbs, how much would lighting
cost in the month of June if SaskPower is
charging 10 cents per kWh?
69
Ohm’s Law
Ohm’s Law states that the current in a wire (I) in
amps(A) is equal to the voltage (V) in Volts(V)
divided by the resistance (R) in ohms(Ω).
Current = Voltage
Resistance
I=V
R
70
D. Electric Power
Electric power is a measure of the rate at
which electricity does work or provides
energy. Watts (W) are the unit that power is
measured in. Voltage must be in volts(V) and
current in amps(A)
Power = Voltage  Current
(P = V  I)
71
Electricity Problem Examples:
1) The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and
the power.
72
Electricity Formulas
I=V
R
V
I
R
P=VI
E = PX t
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours, time in hours
(hrs)
73
Electricity Problem Examples:
1) The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and
the power.
V = 120V
I = 20A
R= ?
P=?
74
Electricity Formulas
I=V
R
V
I
R
P=VI
E = PX t
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours, time in hours
(hrs)
75
Electricity Problem Examples:
1) The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and
the power.
V = 120V
R = V/ I
I = 20A
R= ?
P=?
76
Electricity Problem Examples:
1) The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and
the power.
V = 120V
R = V/ I = V ÷ I =
I = 20A
R= ?
P=?
77
Electricity Problem Examples:
1) The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and
the power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R= ?
P=?
78
Electricity Problem Examples:
1) The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and
the power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R=6
R= ?
P=?
79
Electricity Formulas
I=V
R
V
I
R
P=VI
E = PX t
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours, time in hours
(hrs)
80
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
81
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
82
Electricity Formulas
I=V
R
V
I
R
P = VI
E=Pt
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours, time in hours
(hrs)
83
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
P = VI =
84
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
P = VI = V X I =
85
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
P = VI = V X I = 120V x 20A=
86
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
P = VI = V X I = 120V x 20A=
P = 2400
87
Electricity Formulas
I=V
R
V
I
R
P = VI
E=Pt
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours, time in hours
(hrs)
88
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
P = VI = V X I = 120V x 20A=
P = 2400W
89
Electricity Problem Examples:
1)The voltage in a computer is 120V and the
current is 20A. Calculate the resistance and the
power.
V = 120V
R = V/ I = V ÷ I = 120V ÷ 20A=
I = 20A
R = 6 Ω (ohms)
R= ?
P=?
P = VI = V X I = 120V x 20A=
P = 2400W
The resistance is 6 Ω and the power is 2400W.
90
2) The current in a MP3 player is 10 000mA and
the resistance is 2Ω. With this information,
calculate the power being used.
91
2) The current in a MP3 player is 10 000mA and
the resistance is 2Ω. With this information,
calculate the power being used.
I = 10 000 mA
R=2Ω
P= ?
92
Electricity Formulas
I=V
R
V
I
R
P = VI
E=Pt
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours, time in hours
(hrs)
93
2) The current in a MP3 player is 10 000mA and
the resistance is 2Ω. With this information,
calculate the power being used.
I = 10 000 mA = 10A
R=2Ω
P= ?
94
2) The current in a MP3 player is 10 000mA and
the resistance is 2Ω. With this information,
calculate the power being used.
I = 10 000 mA = 10A
R=2Ω
P= ?
P=VI
95
2) The current in a MP3 player is 10 000mA and
the resistance is 2Ω. With this information,
calculate the power being used.
I = 10 000 mA = 10A
R=2Ω
P= ?
P = V I = 10A x 2 Ω =
96
2) The current in a MP3 player is 10 000mA and
the resistance is 2Ω. With this information,
calculate the power being used.
I = 10 000 mA = 10A
R=2Ω
P= ?
P = V I = 10A x 2 Ω =
P = 20W
97
Electricity Formulas
I=V
R
V
I
R
P = VI
E=Pt
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours(kWh),
time in hours (h)
98
2) The current in a MP3 player is 10 000mA and
the resistance is 2Ω. With this information,
calculate the power being used.
I = 10 000 mA = 10A
R=2Ω
P= ?
P = V I = 10A x 2 Ω =
P = 20W
The power is 20W.
99
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
100
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
t = 3h
V = 110V
I=?
101
Electricity Formulas
I=V
R
V
I
R
P = VI
E=Pt
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours(kWh),
time in hours (h)
102
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 25kWh
P = E/t = E ÷ t =
t = 3h
V = 110V
I=?
103
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
P = E/t = E ÷ t = 15 kWh ÷ 3 h=
t = 3h
V = 110V
I=?
104
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
P = E/t = E ÷ t = 15 kWh ÷ 3 h=
t = 3h
P = 5kW
V = 110V
I=?
105
Electricity Formulas
I=V
R
V
I
R
P = VI
E=Pt
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours(kWh),
time in hours (h)
106
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
P = E/t = E ÷ t = 15 kWh ÷ 3 h=
t = 3h
P = 5kW= 5000W
V = 110V
I=?
107
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
P = E/t = E ÷ t = 15 kWh ÷ 3 h=
t = 3h
P = 5kW= 5000W
V = 110V
I=?
I = P/V =
108
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
P = E/t = E ÷ t = 15 kWh ÷ 3 h=
t = 3h
P = 5kW
V = 110V
I=?
I = P/V = 5000W ÷ 110V =
109
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
P = E/t = E ÷ t = 15 kWh ÷ 3 h=
t = 3h
P = 5kW= 5000W
V = 110V
I=?
I = P/V = 5000W ÷ 110V =
I = 45.5
110
Electricity Formulas
I=V
R
V
I
R
P = VI
E=Pt
Voltage in Volts(V), Current in Amps (A),
Resistance in Ohms (Ω),
Power in Watts (W) or Kilowatts(kW)
Energy in Kilowatt-Hours(kWh),
time in hours (h)
111
3) In a stereo, the energy being used is 15kWh.
If it is used for 3h and the voltage of the
stereo is 110V, find the current.
E = 15kWh
P = E/t = E ÷ t = 15 kWh ÷ 3 h=
t = 3h
P = 5000W
V = 110V
I=?
I = P/V = 5000W ÷ 110V =
I = 45.5 A
The current is 45.5 A.
112
7.
113
V = 30V
V = 9V
P = 192 000 W
R = 0. 323 Ω
P = 111.1W
I = 2.29A
7.
P= 24 576W
61¢
114
Ohm’s Law Electricity Problems
# Voltage
Current
Resistance
1
12V
2
2A
5.5A
Power
time
2000h
20Ω
3000h
3
8V
1.5A
9000h
4
100V
0.5 A
2500h
5
24V
0.25A
200h
6
2A
60Ω
50h
7
50V
0.5A
150h
8
75V
0.3A
900h
9
120V
10
120V
500mA
2h
11
220V
0.75A
5h
12
30Ω
2000mA
Energy
4.5Ω
500h
20h
115
12 W
108kWh
116
4) If the voltage in a machine is 30V and the
resistance is 2Ω, calculate the power being
used.
117
NAME
Electricity and Magnetism Station Lab
Purpose: to study electricity and magnetism topics
Procedure: Go from station to station & answer questions
Data:
Station Drawing
1.
Leave 3-4 lines
Answer to Question
2.
3.
15.
118
15. Magnetism
Lodestone was the first naturally magnetic substance found.
It attracts iron.
Pieces of lodestone, suspended so they could turn, were
the first compasses which helped ancient sailors.
Magnetic substances include iron, nickel, cobalt and
some of their alloys.
• How do you remove the magnetism from a permanent
magnet?"
- You can heat it past its Curie Point.
- Stroking one magnet with another in a random
fashion will sometimes work.
- Hammering it will usually work.
-Putting it into or near an electromagnet being
powered by AC.
119
15. Magnetism
Magnetism – is the force of attraction or repulsion of a
magnetic material due to the arrangement of its
atom domains – particularly its electrons.
Poles – the two ends where the magnetic effects are
the strongest.
• One pole is labeled North and the other is labeled
South.
• Magnetic forces apply the same forces as electric
forces. Opposite charges attract and like charges
repel.
120
Magnetic Field – the region in which the
magnetic forces can act.
• A magnetic field, represented by lines of force
extending from one pole of a magnet to the
other, is an area over which the magnetic
force is exerted.
121
• The earth exerts magnetic
forces and is surrounded by a
magnetic field that is the
strongest near the north and
the south magnetic poles.
This is due the high iron (and
nickel) content of the earth’s
core.
122
• A compass needle does not point exactly to the
earth’s geographic poles. It points to the magnetic
poles. The difference in the location of the earth’s
magnetic and geographic poles is called magnetic
declination.
• The deflection of charged particles in a magnetic
field is responsible for several phenomena; the
protection of the earth from solar wind; radio
astronomy; and future applications of nuclear
reactions to produce energy.
• The region in which the magnetic field of earth is
exerted is known as the magnetosphere.
123
124
16. Electromagnetism
• A magnetic field is created around a wire that is
conducting electric current.
• The relationship between electricity and magnetism
is called electromagnetism.
• A coiled wire, known as a selenoid, acts as a magnet
when current flows through it. A selenoid with a core
of iron acts as a strong magnet called an
electromagnet.
• A magnetic field exerts a force on a wire conducting
current.
125
• An electric motor converts electric
energy into mechanical energy that is
used to do work.
• During electromagnetic induction, an
electric current is induced in a wire
exposed to a changing magnetic
field.
• One of the most important uses of
electromagnetic induction is in the
operation of a generator, which
converts mechanical energy into
electric energy.
• A transformer is a device that
increases the voltage of alternating
current. A step-up transformer
increases voltage. A step-down
transformer decreases voltage.
126
EXAM TIME!
127