Electricity and CircuitsConceptual Physics

What is electricity?

What is needed for electricity to work?

A circuit is...

QuantityVariableUnit and Unit Symbol

Voltage VVolt (V)

(Potential Difference) Joule per Coulomb

CurrentIAmperes/Amps (A)

Coulombs per second

ResistanceROhms (Ω)

PowerPWatts (W)

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.Resistanceis 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

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 Graphically, the relationship lookslike this:

between V,I and R can be represented

with the following equation:

(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.

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.

Name:Electricity and Circuits 1Conceptual 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.

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

Name:Electricity and Circuits 2: Series & Parallel CircuitsConceptual Physics

Series Circuits

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 thecircuit

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

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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