Unit 4 – Electricity & Magnetism
Electricity & Its Production
In this chapter we will explore;
• The quantities of electrical energy,
power, electric current, potential
difference, and resistance.
• Understand the movement of charge
through a circuit.
• Describe properties of electric potential
difference, current, and resistance for
series, parallel, and mixed circuits.
• Analyze and solve problems in series,
parallel, and mixed circuits.
11.1 Electrical Energy & Power Plants
In physics, electricity refers to electrical
energy and the movement of charge.
The generation of electricity at power
plants convert mechanical energy, thermal
energy, and radiant energy into electrical
energy.
For example, in a hydroelectric power
plant, kinetic energy from falling water is
converted into electrical energy.
Electrical devices in your home convert
electrical energy into other forms of useful
energy to perform particular tasks.
11.1 Electrical Power & Energy
Recall in Section 5.5 that power (P) is the rate at which energy is transformed.
Electrical power (P) is the rate at which electrical energy is produced, or consumed,
in a given time, expressed in Watts (W).
Electrical energy is measured in units of kilowatt hours (kWh) because the joule (J)
is too small to be a convenient measurement. 1 kWh = 3.6 x 106 J. A typical home in
Ontario uses 1000 kWh of electrical energy each month.
When describing the electrical energy generated at a power plant, it is measured in
megawatt hours (MWh). 1 MWh = 3.6 x 109 J; 1 MWh = 1000 kWh.
11.1 Efficiency of Power Plant Technologies
p.506
11.1 Homework
Practice # 1, 2 p.505
Practice # 1, 2 p.506
Questions # 2-5 p.507
11.3 Electric Potential Difference
Electrical energy is transferred from a power
plant to your home through conducting wires
called transmission lines.
The wires are made of metal (aluminum),
where electrons are free to move. This provides
the means for the transfer of electrical energy.
Negatively charged electrons repel one
another. The closer electrons are pushed
together, the greater the increase in their
potential energy.
Energy must be transferred to the electrons to
overcome this repulsion; the movement in
electrons requires a change in energy.
11.3 Electric Potential Difference
Since electrons repel, the amount of energy needed to move a group of electrons
closer together is the electric potential. The more work that is done to move them
together, the higher the electric potential.
A group of electrons is a quantity of charge (Q), measured in coulombs (C).
Instead of measuring electric potential at one point in the circuit, it is more useful
to measure the electric potential difference (V) between two points in a circuit,
because energy is transformed in a circuit. Charge at the battery of a circuit has a
different amount of potential than charge at a light bulb.
SI units: ⤍ Joules per Coulomb of Charge (J/C)
⤍ Volt (V); 1 V = 1 J/C
SP #1 p.511
11.3 Measuring Electric Potential Difference
A voltmeter is an electrical device used to
measure the electric potential difference in a
circuit.
Voltmeters only work accurately if a separate path
is created in the circuit, which allows a small
amount of electrons to go through the meter; In
other words, the voltmeter must be connected in
parallel.
Circuits contain sources of electrical energy, such
as a battery or a power supply, which cause an
increase in electric potential (voltage gain).
Loads, such as lamps, motors, and resistors cause
a decrease in electric potential (voltage drop).
11.3 Homework
Practice # 1 p.511
Questions # 1-6, 9 p.513
11.5 Electric Current
The movement of electrons, or electric current, is required for an electric device to
operate. Direct current (DC) is the flow of electrons in only one direction through a
circuit.
Electric current (I), is the rate of charge flow, measured in amperes (A).
One coulomb of charge passing a point in a circuit every second is equivalent to
one ampere.
SI units: ⤍ Coulombs per second (J/s)
⤍ Ampere (A); 1 A = 1 C/s
SP #1 p.516
11.3 Measuring Electric Current
An ammeter is an electrical device used to
measure the amount of electric current in a
circuit.
Ammeters must be connected in series so all
electrons flowing through the circuit also flow
through the meter.
If the ammeter were connected in parallel, there
would be more than one path for the current to
flow through; not all of the current will flow
through the ammeter, providing an inaccurate
reading.
11.5 Homework
Practice # 1, 2 p.517
Questions # 2-6 p.518