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# It is Now Believed that Protons and Neutrons (Which Constitute Nuclei of Ordinary Matter) Are Themselves Built Out of More Elementary Units Called Quarks - CBSE (Science) Class 12 - Physics

ConceptBasic Properties of Electric Charge

#### Question

It is now believed that protons and neutrons (which constitute nuclei of ordinary matter) are themselves built out of more elementary units called quarks. A proton and a neutron consist of three quarks each. Two types of quarks, the so called ‘up’ quark (denoted by u) of charge (+2/3) e, and the ‘down’ quark (denoted by d) of charge (−1/3) e, together with electrons build up ordinary matter. (Quarks of other types have also been found which give rise to different unusual varieties of matter.) Suggest a possible quark composition of a proton and neutron.

#### Solution

A proton has three quarks. Let there be n up quarks in a proton, each having a charge of +2/3e.

Charge due to up quarks=(2/3e)n

Number of down quarks in a proton = 3 − n

Each down quark has a charge of -1/3e.

Charge due to (3 − n) down quarks =(-1/3e)(3-n)

Total charge on a proton = + e

therefore e=(2/3e)n+(-1/3e)(3-n)

e=((2n e)/3)-e+("ne")/3

2e= n e

n=2

Number of up quarks in a proton, n = 2

Number of down quarks in a proton = 3 − n = 3 − 2 = 1

Therefore, a proton can be represented as ‘uud’.

A neutron also has three quarks. Let there be n up quarks in a neutron, each having a charge of +3/2 e.

Charge on a neutron due to n up quarks =(+3/2e)n

Number of down quarks is 3 − n,each having a charge of (-1/3)e.

Charge on a neutron due to (3-n)down quarks = (-1/3e)(3-n)

Total charge on a neutron = 0

0=(2/3e)n+(-1/3e)(3-n)

0=2/3 en -e +(n e)/3

e=n e`

n=1

Number of up quarks in a neutron, n = 1

Number of down quarks in a neutron = 3 − n = 2

Therefore, a neutron can be represented as ‘udd’.

Is there an error in this question or solution?

#### APPEARS IN

NCERT Solution for Physics Textbook for Class 12 (2018 to Current)
Chapter 1: Electric Charge and Fields
Q: 31 | Page no. 49
Solution It is Now Believed that Protons and Neutrons (Which Constitute Nuclei of Ordinary Matter) Are Themselves Built Out of More Elementary Units Called Quarks Concept: Basic Properties of Electric Charge.
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