 ## Definitions for Module 6 – OCR A Physics

### Units

Tesla

The S.I. unit for magnetic flux density;
Equivalent to 1 newton per ampere per metre

Weber

The S.I. unit for magnetic flux

Coulomb per volt

### Quantities/ Definitions

Electric field strength

Force per unit positive charge

Magnetic flux density

Is the force per unit current per unit length; $B = \frac{F}{IL}$

Magnetic flux

Magnetic flux density multiplied by the area (perpendicular to field direction)

Magnetic flux multiplied by the number of turns

Capacitance

Charge per unit potential difference

Time constant of a circuit

Time taken for current/charge/voltage of a discharging capacitor to fall to 37% of its original value (equal to the product of capacitance and resistance

Proton number

Number of protons in the nucleus of an atom (a.k.a. atomic number)

Nucleon number

Number of neutrons and protons in the nucleus of an atom (a.k.a. mass number)

Activity

The rate of decay of nuclei in a radioactive sample

Decay constant

Probability of a decay of a nucleus per unit time

Half life

The mean time taken for half the number of active nuclei in a radioactive sample to decay

Binding energy

Minimum energy to separate protons and neutrons of a nucleus

Binding energy per nucleon

The total binding divided by the number of nucleons in a nucleus

Intensity

power per unit cross-sectional area

### Concepts

The induced e.m.f. is proportional to the rate of change of magnetic flux

Lenz’s law

The induced current or e.m.f. is in a direction so as to produce effects which oppose the change producing it

Isotope

A version of an element with a different number of neutrons (but the same number of protons and electrons)

Piezoelectric effect

The application of a potential difference across a crystal causes a vibration

Compton effect

When an X-ray photon interacts with electron; the scattered photon has a longer wavelength and the electron is ejected

Doppler effect

The change in the frequency and wavelength of a wave caused by the relative movement between source and detector

• random – cannot predict when or which nucleus will decay next
• spontaneous – decay cannot be induced

### Nature and Range of Fundamental Forces Acting on Nucleons:

• electrostatic force – repulsive between protons, no effect on neutrons; long-ranged
• gravitational force – attractive; long-ranged
• strong nuclear force – attractive; short-ranged

### Principals of Positron Emission Tomography (PET):

• electrostatic force – repulsive between protons, no effect on neutrons; long-ranged
• gravitational force – attractive; long-ranged
• strong nuclear force – attractive; short-ranged

### Principals of a CAT Scan:

• electrostatic force – repulsive between protons, no effect on neutrons; long-ranged
• gravitational force – attractive; long-ranged
• strong nuclear force – attractive; short-ranged

### Principals of Ultrasound Scanning:

• electrostatic force – repulsive between protons, no effect on neutrons; long-ranged
• gravitational force – attractive; long-ranged
• strong nuclear force – attractive; short-ranged