Draw:
a) an atom that has 4 protons and a net charge of +2
b) an atom with 10 electrons and a net charge of -1
c) a neutral atom with 5 protons
SOLUTION:
a) + + + + - -
b) + + + + + + + + + - - - - - - - - - -
c) + + + + + - - - - -
The force between two charged bodies is proportional to their charges and inversely proportional to the square of the distance between them.
This means that the greater the charges, the greater the force, while the greater the distance, the lower the force.
It also means that in the expression to calculate the force the charges will appear in the numerator and the square of the distance in the denominator .
If the charges are of the same sign, the force will be of repulsion and if they are of opposite sign it will be of attraction .
Calculate the value of the force between two charges of 4 μC in vacuum, if the distance between them is 2 m.
Audio:
Known quantities: Q 1 = Q 2 = 4 μC = 4·10 -6 C
r = 2 m
k = 9·10 9
Unknown quantity: F
We write the Coulomb equation, substitute data and solve:
F = k · Q 1 · Q 2 / r 2 = 9 10 9 · 4 10 -6 · 4 10 -6 / 2 2 = 0.036 N
Classify these object as conductors or insulators:
Classify the following objects as conductors or insulators:
Conductors are those made of metal and insulators are those that are not.
Plastic pen: insulator
eraser: insulator
wooden ruler: insulator
tin can: conductor
gold necklace: conductor
towel: insulator
aluminum foil: conductor
paper sheet: insulator
key: conductor
coin: conductor
glass bottle: insulator
Indicate what type of material (conductor or insulator) would be adequate to build the following objects:
Choose the circuit element that converts electrical energy into:
light: ligt bulb
movement: engine
heat: resistance
sound: bell
a) Read these instructions to a class mate:
"Draw a circuit diagram containing a battery, a motor, an ammeter and a switch"
If there is any symbol that she/he can not remember, describe it for her/him.
Solution:
b) Now he/she has to invent a similar statement with the elements he/she choses for you to draw. And you can receive clues as well if there is a symbol that you don´t remember.
Repeat the exercise one more time each of you, changing the elements.
Read the paragraph below and complete the missing words.
The energy supplied by the generator per unit charge is called electromotive force. It is measured in volts .
The energy per unit charge between two points in a circuit is called potential difference, voltage or tension, and is measured in volts. The instrument used to measure it is the voltmeter.
Current intensity is the amount of electric charge that passes through a section of material per unit time. It is measured in Amperes and its measuring instrument is the ammeter.
Resistance measures the opposition that a material presents to the flow of electric current. It is measured in Ohms.
Now try to do the exercise in reverse. Define electromotive force, voltage, intensity and resistance orally, and check, by looking at the solution, that you are able to construct a definition that contains the key words. Record the definitions with your mobile phone. Recording what you want to learn and listen to it while you are walking, cleaning, resting,... is a good trick for memorisation!
Do the problems in your notebook and write the result.
1. R = V / I = 12 / 20 = 0,6 A
2. V = I · R = 2 · 4 = 8 V
3. I = V / R = 4,5 / 3 = 1,5 A
a) Calculate the equivalent resistance of a series circuit with two light bulbs of resistances 1 and 1.25 Ω .
b) Calculate the current that passes through them when they are connected to a 4.5 V battery .
R = R1 + R2 = 1 + 1,25 = 2,25 A
I = V/R = 4,5 /2,25 = 2 A
Draw a circuit diagram with a battery, a push button and a light bulb in parallel with a bell. Add a voltmeter to measure the potential difference between the ends of the bulb.
SOLUTION:
a) Calculate the equivalent resistance of a parallel circuit with two 6 Ω resistors .
b) Calculate the intensity that would pass through the circuit battery if it supplies a voltage of 9 V.
c) Calculate the intensity that would pass through each of the resistors .
1/R = 1/R1 + 1/R2 = 1/6 + 1/6 = 2/6
R = 6/2 = 3 Ohm
I = V/R = 9/3 = 3 A
I1 = I2 = V1/R1 = V/R1 = 9/6 = 1,5 A
Talk to your class mate and discuss why the lamps of this circuit bright with different intensity. Afterwards, try to prove your point mathematically.
SOLUTION:
It is a parallel circuit. The higher the resistance in a branch, the less current will circulate through it:
I 1 = V/R 1 = 9/1000 = 0.009 A
I 2 = V/R 2 = 9 /200 = 0.045 A
I 3 = V/R 3 = 9/10 = 0.9 A
Click "retry" to start
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