A-level Physics OCR A module 6 Particles and Medical Physics

• An electrical component that stores charge on 2 separate metallic plates

• An insulator called a dielectric is placed between the plates to prevent the charge from travelling across the gap.

The charge stored per unit potential difference across the two plates. C=Q/V measured in Farads. 1F=1CV^-1

• The ratio of the capacitances of a capacitor with and without the dielectric in place.

• ϵ_{r =} Q/Q₀

• Greater the relative permittivity, the greater the capacitance of the capacitor.

The energy stored by the capacitor.

Time it takes for the charge in a capacitor to fall to 37% of its initial value

• Start with Q = Q₀e^-t/CR

• When t = RC the formula becomes Q = Q₀e^-1

• e^-1 = 0.37

T_½ = (RC)ln(2)

1. Electrons move from negative to positive around the circuit

2. The electrons are deposited on plate A, making it negatively charged

3. Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge

4. Electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a potential difference across the plates

5. When the p.d across the plates = source p.d the capacitor is fully charged and current stops flowing.

1. Electrons move in opposite direction than when the capacitor was charging up

2. Charge on plate A decreases as it loses electrons, and plate B gains electrons, neutralising them

3. P.d decreases exponentially across the plates

• Flash photography

• Nuclear fusion

• Backup power supplies

• Capacitance as affects the amount of charge that can be stored by the capacitors per unit p.d

• Resistance of the circuit as affects how quickly the current flows in the circuit.

A region of space in which charged particles are subject to an electrostatic force.

Radial field

As a point charge at the centre of the sphere

The path a positive test charge would take when placed in an electric field.

Positive to negative

By how close together the field lines are

Force per unit charge on a positive test charge placed in the field

Electric Field Strength (E) = Force (F) / Charge (q)

The force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

The ability of a material to transmit an Electric Field

• Both follow inverse square law for the force

• Point masses and point charges both produce a radial field.

• Field strength defined by force per unit charge/mass

• Gravitational fields are always attractive, Electric Fields can be attractive of repulsive.

W=Fx

The work done per unit charge in moving a positive test charge from infinity to that point in the Electric Field.

Projectile motion

A region of space in which moving charged particles are subject to a magnetic force.

place iron filings on a piece of paper and put magnet on paper and filings will align to field.

how close together field lines are

Force per unit length on a current carrying conductor placed in a Magnetic Field perpendicular to the field lines.

T Tesla

Tesla (T)

1T = 1Nm^-1A^-1

Point thumb in direction of conventional current and field goes around in direction of fingers.

When a current-carrying conductor is placed within a Magnetic Field it experiences a force perpendicular to the flow of current and the field lines which pushes it out of the field.

First finger - Field lines

Second finger - current

Thumb - motion

1. Place horseshoe magnet on a digital balance and zero it

2. Connect rigid piece of straight wire to DC supply, variable resistor and ammeter

3. Align the wire so the force on it acts upwards

4. Measure the length of the wire in the field

5. Record extra mass on the balance and use this to calculate the force

6. Plot a graph of current against mass - gradient gives BL/g

F=BIL

I=Q/t and L=vt

F=BQvt/t

F=BQv

Force is always perpendicular to the velocity of the partice.

mv²/r = BQv

isolate particles of a specific velocity

A velocity selector works by applying perpendicular electric and magnetic fields to allow only charged particles with a specific velocity to pass through unaffected.

Product of magnetic flux density and the area perpendicular to the field lines

Weber Wb

1Wb = 1Tm²

The product of magnetic flux and the number of turns in the coil

Induced emf is always in a direction so as to oppose the change that caused it.

The induced emf in a circuit is proportional to the rate of change of flux linkage throughout the circuit.

1. Increase the speed of rotation

2. Increase the magnetic flux density of the field

3. Increase the cross-sectional area of the coil

4. Increase the number of turns in the coil

Change peak value of an alternating PD to a different value. Step up increase it, Step down decrease it.

Two coils, primary and secondary wrapped around two sides of a laminated iron ring. For step up there’s more on the secondary coil.

alternating current is run through the primary coil which induces an alternating magnetic field in iron core. this in turn induces an alternating emf in the secondary coil

Step-up increase voltage before the electricity travels long distances to reduce energy lost as heat due to resistance in the wires as electricity passes through them.

A few alpha particles bounce back. This wouldn’t happen if the positive charge in the atom was distributed evenly throughout (as in the Plum Pudding Model), which suggests they must be hitting a dense positive charge. The fact it only happens to a very small number of alpha particles shows the nucleus must be small.

Approximately 100,000 times.

A particle that makes up the nucleus: a protons or a neutron.

Isotopes are atoms of an element (with the same number of protons) with a different number of neutrons

The force that holds the nucleus together. It must overcome the electrostatic force of repulsion between protons

Repulsive up to 0.5fm. Attractive up to 3fm.

Similarity: Mass. Difference: Charge (eg. for protons/anti-protons).

A type of particle which is affected by the strong nuclear force.

Quarks

● Baryon (three quarks)

● Mesons (two quarks)

Protons and neutrons.

● Strong nuclear

● Weak nuclear

● Electrostatic

● Gravity

Fundamental particles which are not subject to the strong nuclear force.

● Electron

● Muon

● Neutrino

● And their corresponding antiparticles

● Up (u)

● Down (d)

● Strange (s)

● Proton (uud)

● Neutron (udd)

Up = +⅔e

Down = -⅓e

Strange = -⅓e

When a neutron turns into a proton, the atom releases an electron and an anti electron neutrino.

A down quark turns into an up quark.

Charge, mass, baryon and lepton numbers.

Radioactive decay is spontaneous and random - you can’t predict when an individual nucleus will decay

● Too many or too few neutrons

● Too heavy overall (too many nucleons)

● Too much energy

● Alpha

● Beta (plus and minus)

● Gamma

● Alpha

● Beta

● Gamma

A particle which contains two protons and two neutrons, the same as a helium nucleus.

50cm - 1m

● Alpha - paper

● Beta - ~5mm thick aluminium

● Gamma - thick lead sheet

It decreases.

The number of radioactive decays per second (measured in Becquerels, Bq).

A = activity

λ = decay constant

N = number of radioactive nuclei

The average time taken for the activity of a sample (or the number of radioactive nuclei) to halve.

Carbon-14 (in radiocarbon dating).

E = mc²

● Annihilation.

● When a particle and its antiparticle meet, they will annihilate each other and releases two gamma rays.

● Two rays are released in order to conserve momentum.

● The mass of the particles will transform into the energy equivalent.

When a gamma ray has enough energy to produce a particle and its antiparticle.

The difference between the total mass of all the nucleons separately compared to the mass of the nucleus.

Because energy is released as the nucleons bind together into a nucleus.

The energy required to separate a nucleus into its constituent parts.

Where a unstable nucleus splits into 2 smaller nuclei. Often occurs with the larger nuclei.

When two small nuclei fuse together to create a larger nuclei. The new nucleus has a larger binding energy per nucleon than the old nuclei therefore energy is released in the process.

Fusion releases a lot more energy per reaction. This is because the change in binding energy is very drastic.

There is a large repulsion between the two positively charged nuclei, therefore a lot of energy is required to overcome the repulsion and fuse them together. It is hard to get a material that can withstand the heat and be cost effective.

Rods of uranium-235 absorb neutrons and become unstable and then split into two daughter nuclei. It also releases 2 or 3 more neutrons. These then go on to be reabsorbed by another uranium-235.