Physics 4 Marks Questions

Microwave oven: The spectrum of electromagnetic radiation contains a part known as microwaves. These waves have frequency and energy smaller than visible light and wavelength larger than it. What is the principle of a microwave oven and how does it work? Our objective is to cook food or warm it up. All food items such as fruit, vegetables, meat, cereals, etc., contain water as a constituent. Now, what does it mean when we say that a certain object has become warmer? When the temperature of a body rises, the energy of the random motion of atoms and molecules increases and the molecules travel or vibrate or rotate with higher energies. The frequency of rotation of water molecules is about 2.45 gigahertz (GHz). If water receives microwaves of this frequency, its molecules absorb this radiation, which is equivalent to heating up water. These molecules share this energy with neighbouring food molecules, heating up the food. One should use porcelain vessels and non-metal containers in a microwave oven because of the danger of getting a shock from accumulated electric charges. Metals may also melt from heating. The porcelain container remains unaffected and cool, because its large molecules vibrate and rotate with much smaller frequencies, and thus cannot absorb microwaves. Hence, they do not get eaten up. Thus, the basic principle of a microwave oven is to generate microwave radiation of appropriate frequency in the working space of the oven where we keep food. This way energy is not wasted in heating up the vessel. In the conventional heating method, the vessel on the burner gets heated first and then the food inside gets heated because of transfer of energy from the vessel. In the microwave oven, on the other hand, energy is directly delivered to water molecules which is shared by the entire food.
(i) (b) less than visible light
Explanation: Microwaves have frequency and energy smaller than visible light and wavelength larger than it.

(ii) (c) the energy of the random motion of atoms and molecules increases
Explanation: When the energy of the random motion of atoms and molecules of a substance increases and the molecules travel or vibrate or rotate with higher energies, the substance becomes hot.

(iii) (d) 2.45 GHz
Explanation: The frequency of rotation of water molecules is about 2.45 gigahertz.

OR

(c) Energy is directly delivered to water molecules which is shared by the entire food
Explanation: In the conventional heating method, the vessel on the burner gets heated first and then the food inside gets heated because of transfer of energy from the vessel. In the microwave oven, on the other hand, energy is directly delivered to water molecules which is shared by the entire food.

(iv) (c) Because of the danger of getting a shock from accumulated electric charges
Explanation: One should use porcelain vessels and non-metal containers in a microwave oven because of the danger of getting a shock from accumulated electric charges. Metals may also melt from heating. The porcelain container remains unaffected and cool, because its large molecules vibrate and rotate with much smaller frequencies and thus cannot absorb microwaves. Hence, they do not get heated up

The triboelectric series is a list that ranks materials according to their tendency to gain or lose electrons. The process of electron transfer as a result of two objects coming into contact with one another and then separating is called triboelectric charging. During such an interaction, one of the two objects will always gain electrons (becoming negatively charged) and the other object will lose electrons (becoming positively charged). The relative position of the two objects on the triboelectric series will define which object gains electrons and which object loses electrons.
In triboelectric series, materials are ranked from high to low in terms of the tendency for the material to lose electron. If an object high up on this list (Glass, for example) is rubbed with an object low down on the list (Teflon, for example), the glass will lose electrons to the teflon. The glass will, in this case, become positively charged and the teflon will become negatively charged. Materials in the middle of the list (steel and wood, for example) are items those do not have a strong tendency to give up or accept electrons.



(i) (c) high
Explanation: In triboelectric series, materials are ranked from high to low in terms of the tendency for the material to lose electron i.e., they are ranked high to low tendency of getting positively charged.

(ii) (a) steel, wood
Explanation: Materials in the middle of the list (steel and wood, for example) are items those do not have a strong tendency to give up or accept electrons.

(iii) (c) Hair will be positively charged, Amber will be negatively charged.
Explanation: Since, human hair is placed at the upper portion of the list, it will leave electron and will be positively charged. Since, amber is placed at the lower portion of the list, it will accept the electron and will be negatively charged.

OR

(a) By contact
Explanation: The process of electron transfer as a result of two objects corning into contact with one another and then separating is called triboelectric charging.

(iv) (d) positively, negatively
Explanation: During triboelectric charging, one of the two objects always gains electrons and become negatively charged. The other object loses electrons and become positively charged.