Electricity and Circuits
Conceptual Physics
What is electricity?
What is needed for electricity to work?
A circuit is...
Quantity
Variable
Unit and Unit Symbol
Voltage
(Potential Difference)
V
Joule per Coulomb
Volt (V)
Current
I
Coulombs per second
Amperes/Amps (A)
Resistance
R
Ohms (Ω)
Power
P
Watts (W)
1
Voltage (Potential Difference)
The change in electrical potential energy per unit charge. Measured in Volts (V). The power
source provides the energy to the charges by separating them (+, -)
 Charges have electrical potential energy when they are separated from opposite charges.
Chemical reactions in the battery separate the charges to the negative and positive terminals of
the battery.
Electric Current is the flow of charge; the amount of charge that passes a point per unit time.
Current is measured in Amps (A).

A closed pathway of conducting material is needed for electricity to work.
The circuit provides a pathway for the energized charges for flow. The conductors in the circuit
contain many loosely bound electrons, so the energy of the charges can move quickly from the
negative terminal of the power source, through the conductors to the positive terminal of the
power source.
Resistance, R, is the resistance to the flow of charge. The resistance in the circuit determines the
amount of current that moves through the circuit. Resistance is minimal in most conducting
metals. Why?
Bulb filaments and resistors and any electrical appliance transform electrical potential energy into
other forms of energy. The charges "work" to get through the bulb, resistor or clock, so the
electrical potential energy (PEE) is transformed into other forms of energy.
Examples:
Power is..
the rate at which work is performed
energy per unit time
measured in joules per second or Watts
batteries are power sources
loads are power consumers
2
Ohm’s Law
In any circuit, there is a mathematical relationship between the voltage, current and resistance.
This relationship is stated in Ohm’s Law. Ohms Law states that the ratio of voltage to current is
equal to the resistance of the circuit.
Mathematically, this relationship
between V,I and R can be represented
with the following equation:
Graphically, the relationship looks like this:
(Do some practice problems!)
Circuit Diagrams, Circuit Schematics:
Symbols are used to represent the connections in a circuit. Which symbol is used to represent
each part of a circuit? What is each element used for in a circuit? (fill it in below)
conducting wire
battery/power source
bulb
resistor
voltmeter
ammeter
switch
Draw a circuit diagram that
includes a battery and a resistor.
3
Series and Parallel Circuits
Many circuits can have more than one element in them. Resistors (or bulbs or electrical
appliances) in circuits can be connected in series or in parallel.
Two basic ways to build circuits...
Series Circuit - elements are connected so
that there is only one pathway for the current
to go from the negative terminal to the positive
terminal of the power source.
Parallel Circuit - elements are connected
so that there is more than one pathway for
the current to go from the negative terminal
to the positive terminal of the power source.
4
Name:
Electricity and Circuits 1
Conceptual Physics
Finish the following statements, answer the questions or match the phrase with the word in the space provided. For calculations,
identify the given quantities, write the equation used, substitute the values into the equation and write the answer with the units.
1. Electricity is…
2. What is needed for electricity? (three items)
3. Charge flows when there is a
between the ends of a conductor.
4. How do charges gain electrical energy? What happens to the electrical energy of the charges as they
move through the circuit?
5. Complete the chart using the following terms, variables and units: resistance, power, electron, voltage,
current, amps (amperes),Watts, Ohms, Volts, Coulombs, I, R, V, A, V, Ω, W
Definition
Term and Variable
Unit and Unit Symbol
Particles in a conductor that carry charge and
move through a closed circuit when energized.
Flow of electric charge in a closed circuit.
A measure of the change in electrical energy of
charges as they move through a power source
or an electrical appliance. Also called potential
difference.
Impedes the flow of current. As the electrical
energy of the charges is transformed into other
forms of energy, the flow of charged is reduced.
The rate at which electrical energy is
transformed into other forms of energy.
6. In a closed circuit, (electrons/energy) move at the speed of light. (pick one!)
7. True or False: The power company supplies all the electrons for us to use in our electrical appliances.
Electrons must move from the power company, through the power lines and into our houses before we
have electricity.
5
Ohmwork Problems
1. Find the resistance of an electric light bulb
if there is a current of 0.8 A and a potential
difference of 120 V. Ans: 150 
2. What is the potential difference across a
toaster of 13.7  resistance when the current
through it is 8.75 A? Ans: 120 V
3. What is the current through an electric heater
with a resistance of 38  when the potential
difference is 240 V? Ans: 6.3 A
4. A light bulb uses 1.2 A when connected
across a 120 V source. What power is
dissipated by the bulb? Ans: 144 W
5.
How much current flows through a 75 W bulb
connected to a 120 V outlet? Ans: 0.625 A
6. A 6 V battery delivers 0.5 A of current to an
electric motor. What power is dissipated by
the motor? How much electrical energy does
it use in 5 min? Ans: 3 W; 900 J
6
Name:
Electricity and Circuits 2: Series & Parallel Circuits
Series Circuits
Conceptual Physics
For each problem, draw the circuit diagram using appropriate symbols and meters.
1. A 10 , a 15 , and a 5  resistor are connected in series across a 90 V battery. Find the equivalent
resistance (Req) of the circuit. What is the current (I) through each resistor? Ans: 30 , 3 A
V (V)
I (A)
R (Ω)
P (W)
R1
R2
R3
Total
2. A 20  and a 30  resistor are connected in series
across a 110 V potential difference. What is the total
resistance of the circuit? What is the current through
each resistor? What is the voltage drop across each
resistor? Ans: 50 ; 2.2 A; 44 V and 66 V
V (V)
I (A)
R (Ω)
P (W)
R1
R2
Total
3. A 3 , a 5 , and a 4  resistor are connected in
series across a 12 V battery. What is the total
resistance of the circuit? What is the current through
each resistor? What is the voltage drop across each
resistor? Ans: 12 ; 1 A; 3 V, 5 V, and 4 V
V (V)
I (A)
R (Ω)
P (W)
R1
R2
R3
Total
4. A 60 , a 90 , and an unknown resistor are connected
in series across a 130 V battery. An ammeter in the circuit
reads 0.67 A. What is the total resistance of the circuit? What
is the resistance of the unknown resistor? What is the power dissipated
in each resistor? Ans: 195 ; 45  ; 26.7 W, 40 W, and 20 W
V (V)
I (A)
R (Ω)
P (W)
R1
R2
R3
Total
7
Parallel Circuits
For each problem, draw the circuit diagram using appropriate symbols and meters.
1. Three 15  resistors are connected in parallel across a 3 V battery. Find the total resistance of the
circuit. What is the total current? What is the current through each resistor? Ans: 5 ; 0.6 A; 0.2 A in each
V (V)
I (A)
R (Ω)
P (W)
R1
R2
R3
Total
2. A 120 , a 60 , and a 40  resistor are connected in parallel and placed across a 120 V potential
difference. What is the total resistance of the circuit? What is the total current? What is the current
through each resistor? Ans: 20  ; 6 A; 1 A. 2 A, and 3 A
V (V)
I (A)
R (Ω)
P (W)
R1
R2
R3
Total
3. A 40 , a 8 , and a 10  resistor are connected in parallel across a 120 V battery. What is the total
resistance of the circuit? What is the circuit current? What is the current through each resistor? What is
the power dissipated? Ans: 4  ; 30 A; 3 A, 15 A, 12 A; 360 W, 1800 W, 1440 W
V (V)
I (A)
R (Ω)
P (W)
R1
R2
R3
Total
4. A 12 , a 15 , and an unknown resistance are connected in parallel. The total current of the
combination is 12 A. The current across the 15  branch is 4 A. What is the total voltage? What is the
total resistance? What is the unknown resistance? Ans: 60 V; 5 ; 20 
V (V)
I (A)
R (Ω)
P (W)
R1
R2
R3
Total
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