Sample Mid-Term Exam Answers

MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question.

1) The reason that the white-hot sparks that strike your skin from a 4th-of-July-type sparkler don't harm you is because
A) the energy per molecule is high, but the total energy transferred is small.
B) the energy per molecule is very low.
C) they have a low temperature.

The answer is A. The amount of material in the spark is very small, so the total heat content to be transferred to your skin is small.


2) Heat energy is measured in units of
A) joules.
B) calories.
C) both of these

We use both units in our calculations. One calorie is 4.2 joules. The answer is C.


3) A substance can absorb heat energy by the process of
A) radiation.
B) convection.
C) conduction.
D) all of these

We discussed all three processes. The answer is D.


4) A good absorber of radiation is a
A) good reflector.
B) poor emitter of radiation.
C) good emitter of radiation.
D) none of these

Good absorbers are also good emitters, but are poor reflectors. The answer is C.


5) Hydrogen and oxygen molecules in a sample gas have the same temperature. This means the hydrogen molecules, on average, have the same
A) kinetic energy, but more speed.
B) speed, but less kinetic energy.
C) speed and the same kinetic energy.
D) speed, but more kinetic energy.
E) kinetic energy, but less speed.

Remember that temperature measures average kinetic energy of the molecules. Since hydrogen molecules are much lighter than oxygen molecules, they would move faster to have the same kinetic energy. The answer is A.


6) Which force binds atoms together to form molecules?
A) nuclear
B) centripetal
C) gravitational
D) electrical
E) none of these

We talked about how all chemistry is due to the electrical force. The answer is D.


7) Every proton in the universe is surrounded by its own
A) electric field.
B) gravitational field.
C) both of these
D) none of these

Since the proton has mass and is charged, it is surrounded by both a gravitational field and an electric field. The answer is C.


8) The electric field around an isolated electron has a certain strength 1 cm from the electron. The electric field strength 2 cm from the electron is
A) four times as much.
B) twice as much.
C) the same.
D) half as much.
E) none of these

Remember that the electric force is an inverse square law force. So if you double the distance, you cut the field strength by the square. So the force and therefore the field is one-fourth the size. the answer is E.


9) Several paper clips dangle from the north pole of a magnet. The induced pole in the bottom of the lowermost paper clip is a
A) north pole.
B) south pole.
C) north or south poleÑno difference really.

Each paper clip has its magnetic field oriented in the same way as the original magnet. Since the north pole is down in the original magnet, the north pole is down on each paper clip. the answer is A.


10) Which force field can increase a moving electron's speed?
A) electric field
B) magnetic field
C) both of these
D) none of these

Electric fields apply forces to electrons and can increase the electron's speed. A magnetic field acts at right angles to the direction of the electron's motion so can only change the direction of the electron's motion, but not its speed. The answer is A.



Heat = (Specific Heat) x mass x (temperature change)
Heat of fusion of water is 80 calories per gram
Heat of vaporization of water is 540 calories per gram
Specific Heat of water is 1 cal/gram degree

1. A pot of water containing 0.5 kg of liquid is heated on a stove. If the amount of heat applied to the water is 3000 calories, what will be the temperature change of the water?

Use the formula above. C = 1, m = 500 grams, Q = 3000 calories.
3000 - 1 x 500 x Temp change
Temp change = 3000/500 = 6 degrees Celsius

2. A glass of water containing 0.5 kg of liquid has a temperature of 20 degrees Celsius. An ice cube of mass 10 grams is dropped into the water. When the ice cube melts, what will be the temperature of the water?

Calculate the amount of heat needed to melt the ice. Then use the heat equation to compute the temperature change. 10 grams of ice x heat of fusion of water = 10 x 80 = 800 calories. Now in the heat equation c = 1, m = 500, so 800 = 1 x 500 x temp change.
Temp change = 800/500 = 1.6 degrees Celsius. Since the water had to give up energy to melt the ice, the water will cool down. So the final water temperature is 20 - 1.6 = 18.4 degrees Celsius.

3. A 15 watt light bulb is connected to 1.5 volt circuit. What current flows in the circuit? If a second identical light bulb is added to the circuit in series, what current will flow?

Power = volts x amps. 15 = 1.5 x amps. Current = 15/1.5 = 10 amps. If you add a second identical bulb, you will double the resistance, so half the current will flow or 5 amps.

4. The transformer at a power plant converts the voltage from the plant (100 volts) to a much higher value (100,000 volts) for long distance transmission. If the plant supplies 500 megawatts of power, how much current flows in the long distance line?

The current being generated at the plant can be calculated from watts = volts x amps. So, 500,000,000 watts = 100 volts x current.
Current = 500,000,000/100 = 500,000 amps.
The transformer increases the voltage by a factor of 1000, so to conserve energy, the current decreases by a factor of 1000. The long distance line current is 500 amps.