cahier physique - physics workbook

INTRODUCTION: PRODUCING ELECTRIC CURRENTS

In class, get a 1.5V battery, 1 bulb and only 1 wire. Your challenge is to light the bulb. Draw here the right connection(s)
(source: Physics of everyday phenomena, Mac-Graw-Hill)
You just have built a simple electric circuit. An electric circuit is analogous to a water circuit.

1) PLEASE READ

A) The battery, like a water wheel or water fall, give the charges POTENTIAL ENERGY. A water wheel gives water GRAVITATIONAL potential energy but a battery gives charges ELECTRIC potential energy. Each coulomb of charge (1C), coming out of the positive pole has an energy of 1.5 J. (see figure). The battery has a potential (or voltage ) of 1.5V.
B) the charges flow through the wire carrying their potential energy like water flowing through pipes. THey flow trough the side of the bulb, through the filament and through the bottom of the bulb back to the battery. While passing through the filament the POTENTIAL ENERGY is converted to LIGHT ENERGY and THERMAL ENERGY.
In an incandescent lamp only 10% is used as light. The rest is lost to heat. Fluorescent bulb is much more efficient.

C) The charges have lost all their POTENTIAL energy they go through the battery again get another boost of 1.5 J per charge. The battery is analogous to a water pump. THe cycle starts again until there is no more energy in the battery.

D) A battery of potential 12V (voltage) gives each charge of 1C a potential energy of 12J. WHile flowing in the circuit, the charges lose their energy because some device will transform the potential energy into another form of energy. All these devices have a resistance noted R. THe energy lost per second (Electric power) is proportional to R. In the same way, the water has a potential energy because it is raised above ground level, the initial energy is spent has it go through the pipes and encounter resistance.
source: http://www.windows.ucar.edu/tour/link=/spaceweather/how_circuit_works.html

There is some limitation to this model:

A) To ease your understanding we will pretend that positive charges , each carrying 1C of charge, are flowing from + to -. EAch charge , carrying each 1C, has an energy of 12J (If the battery voltage is 12V). Actually, only electrons are flowing and they flow from - to +. You need a large group of electrons of make 1C (1 electron = 1.6 E -19C).

B) If you cut a pipe, the water will spill. If you cut a circuit, the charges will stop flowing. An open switch don't hold charges the same way a faucet holds water. But it is a nice model, and I will use it:

The battery gives each charge of 1C potential energy. The potential energy is converted to another form of energy. The conversion is never 100% efficient. Some energy will converted to thermal energy. (A bulb gets hot, the wires get hot, the battery gets hot ...). Once the charge have to more energy, they flow through the battery (the pump here) and get a new boost.
(source: Physics of everyday phenomena, Mac-Graw-Hill)

The bulb, the battery and the wire resist the flow of charges and take some of their energy. We say that they have resistance.

This is the way you should draw a simple circuit:

2)
Electricity provides the mean to transfer large quantities of energy great distance with little lost. Electric energy can be converted into other forms of energy:
A motor transforms electric energy to ____________ and thermal energy.
A bulb transforms .................................. to ------------------ and ----------------/
A speaker transforms _______________ to ___________ and ________________.
A radiator transforms ________________ to ______________.
A generator converts (tricky) __________ to ______________.and ____________. A generator produce electricity by spinning a magnet inside a coil (circuit). To make the wheel spins you can use a crank if it is a small generator.

3) If the potential of a battery is 9V, then each coulomb of charge carries ___________ J.

4) REmember, in a given circuit, the voltage or potential is the energy (PE energy) carried by a charge of 1C. (per coulomb of charge). So, when a charge of 1C has an electric of 1J, it has a potential of 1V. When a charge of 2C has an electric energy of 2J, its potential is ________ V. (potential (V)= energy(J)/charge (C) )

5) If a conductor connected to the terminal of a battery has a potential of 12 volts, then each coulomb of charge on the conductor has an energy of ______________.

6) If a charge of 1C has an energy of 5000J, its voltage is __________ V.

7) If a charge of 0.0001C has an energy of 0.5J , its voltage is _____________V.

8) Voltage is PE (potential energy ) per ___________

THe electric current and Electric Power
remember ? Power (watt) = Energy (J) supplied per second (for every second)

1) read:The electric current is noted I. The current is the number of charges of 1C that flow in a circuit in 1s.
Or I is the rate of flow of electric charge. The unit is the ampere (A).
I = 1 A, means that 1 coulomb of charge flows every second in a circuit. 1C/s = 1A. The ampere is named after a FRench scientists Andre Marie Ampere (1775-1836). An ammeter is a device that measures current.

2) Suppose you build a circuit made of a generator (V= 120V) and a motor. Suppose the current through a motor is 3A that is 3C/s. The potential of the generator is 120V.
THis means that each coulomb supplies the motor with ______J.

Per second, 3 coulombs flow in the motor. So the total amount of energy delivered by the generator in 1 second is therefore: ________ J/s = __________ watts.
(the energy delivered per second is the power delivered).
conclusion: the power rating of an electric device is found by multiplying the voltage V by the current I:

P = V x I

INDEX CARD
Voltage = Energy (J)/Charge (C)
Power (W)= Energy(J)/time(S) = voltage (V) x I (A)
I= current (A) = charge (C)/time (S)

3) A motor delivers 0.5A of current to an electric motor connected across its terminals .
voltage = 12V

A) What is the power rating of the motor (P = I x V)
B) How much energy does the motor use is 5 minutes ?

4) Answer these questions:
A) The units of electric potential difference are:
amperes ? volts? ohms? watts?
B) It is common to call electric potential difference (like 1.5V)
voltage? amperage? wattage?
C) The flow of electric charge is called electric
voltage ? current? power ?
and is measured in: ohms ? amperes? watts?

5) A current of 1 ampere is a flow of charge at the rate of ____ coulomb per second.
B) When a charge of 15C flows through any area in a circuit each second, the current is ______A.
c) When a lamp is plugged into a 120V socket, each coulomb of charge that flows in the current is raised to a potential energy of __________ J.

6) The current through a toaster connected to a 120V source is 8A. What is the power rating of the toaster ?

7) What current flows through a 75W light bulb connected to a 120V outlet?

8) The current through the starter motor of a car connected to a 120V battery is 210A. WHat electric energy is delivered to the starter in 10s? in 10 minutes? in 10 hours?

Ohm's law
lab electricity 1: introduction circuits, Ohm's law

1) Read and complete. Any conductor (bulb, battery, motor, generator, AC , the connecting wires) offers resistance to an electric current. The German Scientists Georg Simon Ohm found that the ratio between the potential difference V between the ends of the conductor and the current I is constant for many materials
This ratio is the resistance R of the material . So R = ____ / ______ or I = ____ / _____
So the current varies directly with the applied _____ and inversely with __________________.
The current I is in amperes (A) , the potential V is in volts (V) and the resistance is in ohms (Ω ).
The relationship says: for a potential of 1V, and a resistance of 1 Ω the current is 1A.

Some devices are called resistors. They are designed to have a designed resistance.

2) What is the current through a 30 Ω resistance that has a potential difference of 120 V?

3) AN automobile headlight with a resistance of 30Ω is placed across a 12V battery. What is the current through the circuit ?

4) A voltage of 75V is placed across a 15 Ω resistor what is the current through the resistor ?

5) A lamp draws a current of 0.5 A when it is connected to a 120V source.
A) what is the resistance of the lamp?
B) What is the power rating of the lamp ?
C) compute the expression RI². conclusion?

6) A transistor uses 2 10^-4 A of current when it is operated by a 3V battery. What is the resistance of the radio circuit?

7) Match each item in Column 1 with the most appropriate item in column2.
1. energy used a. represents one coulomb of charge flowing per second
2. KWh b. joule per second
3. watt c. unit of charge
4. kilowatt d. measured in volts
5. coulomb e. flow of electrical charges
6. current f. unit of measure in electric meters
7. ampere g. 1000 watts
8. potential difference h. power x time

8) A resistor was held at constant temperature in an operating electric circuit. A student measured the current thought the resistor and the potential difference across it. The measurements are shown in the data table below.

Current (A)	Potential Difference (V)
0.01	2.3
0.02	5.2
0.03	7.4
0.04	9.9
0.05	12.7

A) Using the information in the data table, construct a graph. (y = potential, x= current)
- Mark an appropriate scale on the axis labeled current (A).
- Plot the data points - give a title and label the axis - Draw the best fit line
B) Using your graph, find the slope of the best fit line. Use a ruler. Don't connect the points
C) What physical quantity does the slope represent? What is the Unit ?

9) THe graph below represents the relationship between the potential difference across a metal conductor and the current through the conductor at constant temperature.
What is (approximately) the resistance of the conductor ?

Diagramming Electric Circuits

1)read and observe. Here are some electric circuit symbols:

source: http://www.pschweigerphysics.com/dccircuits.html

An Ammeter is used to measure the current flowing in a circuit.
The Voltmeter is used to measure the voltage of a device.

Here is an example. Suppose we have a battery of 30V connected to a resistance R of15 Ω in a simple circuit. In our circuit we also include an ammeter to read the current and a voltmeter to read the potential difference across the resistance R.
Note that the voltmeter one terminal connected to one side of the element (resistor here) and the other one to the other side. This connection is called a parallel connection. The potential difference across the element is equal to the potential difference across the voltmeter.
Note that the ammeter measures the current. All the charges have to pass through it. It behaves like the main wire. Such a connection is called a series connection

2) Draw a schematic to show a circuit that includes a 90V battery, an ammeter, and a resistance of 45 Ω. What is the ammeter's reading ?

3) Draw a circuit diagram to include a 60V battery, an ammeter, and a resistance of 12.5 Ω. Indicate the ammeter reading.

4) Draw a circuit diagram to include a 16 Ω resistor, a battery, and an ammeter that reads 1.75A. Indicate the voltage of the battery. Include also a voltmeter to read the voltage of the battery and another one to read the voltage of the resistor.