Electric Circuits

Electric Circuit

• Is a closed path for the flow of electrons.

• Consists of:

1. Source of electricity

2. Wires to conduct the flow of electrons (electric current)

3. Objects (resistors or loads) along the path that require electricity to operate (ex. Lamps)

Current

• Current: the movement of negative charge (flow of electrons). The amount of charge that ‘flows” past a certain point in a conducting wire every second.

• If you were to describe a current of water you might state the amount of litters of water that flows past a point in a pipe in one minute. (watch)

•Current/Water analogy

Watch

Current

• One coulomb of charge is equal to the amount of charge in 6.25x1018 electrons (6.2 billion billion electrons).

• The symbol for current is I

• Current is measured in amperes A

• The symbol for charge is Q

• Charge is measured in coulombs C

• Time (t) is usually measured in seconds s

Current

• The mathematical relationship among these variables is:

Amount of current=amount of charge moving past a point time

t

QI

Current

• A current of 2.0 A means that 2.0 C of charge is moving past a point in the circuit every second.

I=Q t

2.0 A= 2.0 C1 s

Calculations

• Example 1

If 350 Coulombs (C) of charge pass a point in a conductor in 20 seconds, what is the electric current (I) through that point?

Formula: I=Q/t

I= ?(unknown)

Q= 350C

t=20 s

I=Q / tI=350C / 20sI=17.5 AThere are 17.5 Amperes of current passing though that point.

Calculations• Example 2

If 200 C of charge pass a point in 3 minutes, what is the electric current through that point?

• What is your first step?

• Covert minutes to seconds

• 3 minutes x 60 s = 180s 1 min

I= ? I=Q/t Q= 200 C I= 200/180s t= 180 s I= 1.1A

There are 1.1 amps of current passing though

that point.

Calculations

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• Read p.324-325; 328-329• Questions p.329 (1-5)

Review

Electricity on the Move

Source of Electricity:

• You can think of a battery as an object that has energy to make electrons move around a circuit.

Energy

Electrical Potential Energy• The purpose of a battery in an electric

circuit is to provide energy to move the negative charge (electrons) through the conductors in the circuit.

• All Energy is measured in Joules (J)

Batteries

• Chemical reactions occur in a battery that result in buildup of electrons in the negative terminal

• Electrons in the battery move to the other terminal making it positive.• These “energized” electrons now have the ability to do work on

something.

Potential Energy

•This electrical energy stored in a battery is called Potential Energy. (This is the driving force responsible for the moving charges in a circuit.)

•In order to actually do work the battery must be connected to an object (load or resistor) and the circuit must be complete. (See page 330, figure 10.3 a & b)

Potential Energy

The units for potential energy per unit of charge are:

J (Joules)C (Coulomb) or Volt (V)

Another name is given to J/C. It is called a Volt (V)

Formula

• Voltage can be looked at as how much energy is carried per unit charge

• Voltage (potential energy) = Energy / charge

Q

EV

Potential difference = energy/charge

Potential Difference

• When describing energy in a circuit we speak of potential difference.

• Potential Difference: the difference in potential energy per coulomb charge at one point in a circuit compared to another. This is measured in Volts (V).

• For example: If one coulomb of charge at one point in a circuit has one more joule, of potential energy, than at another point in the circuit the potential difference is one volt.

1C+ 1J vs. 1C + 2J = Potential Difference of 1 V

• Using a voltmeter, you can measure voltage by looking at the difference in energy between two points on the circuit

Example

• In a battery, 45J of chemical energy are converted into electrical energy by separating positive and negative charges. This energy places 15C of charge at the negative terminal, leaving a deficit at the positive terminal. What is the potential difference between the two terminals of the battery?

Given• E= 45 Joules• Q= 15 Coulombs• V=?

V =

V = = 3.0 V

A battery that uses 45J of chemical energy to separate 15C of charge generates a potential difference of 3V.

Q

E

C

J

15

45

Example 2

• Within a battery, 180 J of chemical energy are converted into electrical potential energy. This amount of energy produces 30 C of negative charge (electrons) at the negative terminal, and a deficit of electrons at the positive terminal. What is the potential difference between the negative and positive terminal of the battery?

Given• E= 180 Joules• Q= 30 Coulombs• V=?

V =

V = = 6.0 V

A battery that uses 180J of chemical energy to separate 30C of charge generates a potential difference of 6.0V.

Q

E

C

J

30

180

Review

• Read pages 330 to 336

• Answer questions 1 to 5 on page 336.

Resistance

• When electrons move through a conductor the atoms resist the flow of electrons.

• Resistance is the property of a substance which indicates how much that substance will interfere with the flow of electrons

• Example: The resistance of Tungsten (filament in light bulbs) is 400 times greater than copper wire.

Resistance

• There is a mathematical relationship between resistance potential difference and current.

I

VR

resistance = potential difference / current

•The unit for resistance is V/A or Ohm (Ω)

Ohm’s Law

• The scientist Ohm did experiments that found resistance was always the same no matter how much voltage (potential difference) was placed on them.

RIV Potential difference = current x resistance

•An ohmic resistor has constant resistance. Many electrical appliances are NOT ohmic

Example

• What is the resistance of a heating coil if a current of 10.0 A goes through it when connected to a wall outlet providing a potential difference or voltage of 120 V?

I

VR

0.120.120.10

120

A

V

A

V

I

VR

V = 120V

I = 10.0 A

Example 2

• What is the current running through a light bulb having a resistance of 5.0 Ω (V/A) when the potential difference across the bulb is 12.0 V?

I

VR

AV

R

VI 4.2

0.5

0.12

V = 12.0 V

R = 5.0 Ω (V/A)

Power

• Power is defined as energy per unit time.• Electrical Power is defined as the amount of

electrical energy that is converted (into light, sound, heat or motion) every second.

• The mathematical equation for power (P) is:

t

EP

Power = energy / time

Power

• The unit for power, Joules/second, is called a watt W.

• Example: when sixty joules of electrical energy is converted into light and heat by a light bulb every second.

• The bulb has a power of 60 watts (W)• It’s not convenient to talk about power in electric

circuit this way so another equation has been developed. (page 344)

Power

• The new mathematical equation for power (P) is:

VIP Power = current x potential difference (voltage)

•Power gives us information about how many joules of electrical energy are being converted from electrical energy into another form of energy every second.

Example

• A current of 0.83 A passes through a light bulb which is connected to a 120 V wall outlet. What is the power of the light bulb??

IVP WVAIVP 10012083.0

V = 120V

I = 0.83 A

Power Rating

• It’s useful to know how much energy an electrical device would use in a certain amount of time.

• To calculate this we use a Power Rating

• Many electrical devices, like a light bulb have this stamped on them.

This stamp tells us that the light bulb has a power rating of 60 W, 14 W, or 12.5 W when connected to a 120 V wall

outlet

Power Rating

• If we want to calculate how much energy an electrical device uses, we would multiply the power (watts) by time (seconds).

PtE Energy (joules) = power (watts) x time (seconds)

Energy Efficiency

Percent efficiency of electrical device = (useful energy output/total electrical energy input) x 100%

•The useful energy output of a lamp is the amount of energy that a lamp actually converts to light.

•There is no electrical device that converts all of the electrical energy going into it , into the energy the electrical device is producing.

•Not all electrical energy going into a light bulb is converted to light.

•The efficiency of an electrical device can be calculated but using the following equation:

Example

• An electric kettle has power rating of 1000 W. it takes this kettle 3.5 min to heat up 600ml of water from 22.0 °C to 100.0 °C. This required 196 000 J of energy to heat the water. What is the efficiency of the kettle?

• The energy used by the kettle is

JE

sWE

PtE

210000

2101000

PtE P = 1000 W

t = 3.5 min = 210s

Percent efficiency = (196 000 J/210 000 J) x 100%

Percent efficiency = 93.3 %

• Read p.337 – 339; 342-346; 358• Questions p.342 (1-6)

»p.348 (1-6)»p.350 (15, 16; 21-22; 24-30)

Review