Rhyl High School Physics Department

1. UNIFORM AND RADIAL FIELDS OF FORCE
CONTENT

• 1.1 Electrostatic and gravitational fields as examples of fields of force.
• 1.2 Field strength (or intensity).
• 1.3 The parallel plate capacitor and factors affecting its capacitance.
• 1.4 Electrical and gravitational inverse square laws.
• 1.5 Potential in force fields.
• 1.6 Relation between force and potential energy gradient.
• 1.7 Relation between intensity and potential gradient.
• 1.8 Vector addition of electric fields.
• 1.9 Potential energy of a system of charges.

1. define electric field strength, E, as the force per unit positive charge,
2. define gravitational field strength, g, as the force per unit mass,
3. show the similarities in the ways E and g are defined,
4. recall and use C = εA / D for a parallel plate capacitor,
5. describe and explain in molecular terms the effect of a dielectric on the capacitance of a capacitor,
6. define relative permittivity,
7. describe and explain an experiment to compare capacitances of two capacitors,
8. recall and use the Inverse square law for electric charges in the form
9. F = (k Q ₁ Q ₂ / r²) where k = 1/4πε ₀ and know the historical reasons for the choice of constants 4π and ε ₀ ,
10. recall, derive and use E = 1 / 4πε ₀ x Q / r² for the field strength of a point charge in free space or air,
11. recall and use the inverse square law for two masses in the form F = (k M ₁ M ₂ / r²) where k = G,
12. show the similarities and differences between electric and gravitation fields,
13. define potential at a point in terms of the work done in bringing unit positive charge, unit mass from infinity to that point and recall and use the equations
14. V = 1 / 4πε ₀ x Q / r x Vg = - GM / r
15. appreciate earth potential chosen as the arbitrary zero of potential,
16. recall that the field strength of the field at a point is equal to the negative of the potential gradient at that point i.e. E = - dV / dr and g = - dVg / dr
17. calculate the net potential and resultant field strength for a number of point charges.

• 2.1 Concept of magnetic fields.
• 2.2 Magnetic flux density.
• 2.3 Magnetic flux.
• 2.4 Magnetic fields due to currents.
• 2.5 Effect of a ferrous core: relative permeability.
• 2.6 Force on a current-carrying conductor.
• 2.7 Force on a moving charge.
• 2.8 Force between current-carrying conductors.
• 2.9 Definition of the ampere.
• 2.10 Measurement of magnetic flux density.
• 2.11 Deflection of beams of charged particles in electric and magnetic fields.

1. sketch the magnetic field of a permanent bar magnet,
2. appreciate that magnetic flux was, historically, the collective name for the magnetic field lines and that flux density was the normal flux per unit area,
3. recall that the magnetic flux density B represents the strength of a magnetic field,
4. recall and define magnetic flux as Φ = AB cos θ and flux linkage = N Φ,
5. sketch the magnetic fields due to a current in
    i) a long straight wire,
    ii) a long solenoid,
    iii) a flat circular coil,
6. use the equations
    B = μ ₀ I / 2πa,
    B = μ ₀ nI and
    B = μ ₀ NI / 2r which will be given when required,
7. for the flux densities due to a long straight wire, in a long solenoid and at the centre of a flat circular coil,
8. define relative permeability and calculate the flux density inside a solenoid which has a ferrous core,
9. predict the direction of the force on a current-carrying conductor in a magnetic field,
10. define magnetic flux density B by considering the force on a current-carrying conductor in a magnetic field, recall and use F = Bil sin θ,
11. define magnetic flux density B by considering the force on a charge moving in a magnetic field, recall and use F = BQv sin θ,
12. explain why current-carrying conductors exert a force on each other and predict the directions of the forces,
13. understand how the equation for the force between two currents in straight wires leads to the definition of the ampere,
14. recall the definition of the ampere,
15. describe how to investigate steady magnetic fields with a Hall probe and a current balance and changing magnetic fields with a search coil,
16. describe how ion beams i.e. charged particles are deflected in uniform electric and magnetic fields and derive the corresponding equations of motion.

• 3.1 Laws of electromagnetic induction.
• 3.2 Calculation of induced emf and charge.
• 3.3 Self induction.

1. recall Faraday's law and Lenz's law,
2. recall and use emf = - d / dt (N Φ) to derive an equation for the emf induced in
    i) a linear conductor moving at right angles to a uniform magnetic field,
    ii) a coil rotating at uniform angular velocity in a uniform magnetic field,
    iii) a search coil placed in an alternating magnetic field,
3. derive an expression for the charge which flows through a coil connected to a circuit of total
4. resistance R when the flux through it is removed i.e. Q = NAB / R,
5. explain the meaning of self inductance of a coil and recall and use the equation V = L dI / dt,
6. define the henry,
7. explain the origin of, and describe the uses of, eddy currents,

• 4.1 Peak and rms values of sinusoidal quantities.
• 4.2 The transformer.
• 4.3 Transmission of electrical energy.
• 4.4 Vector treatment of alternating voltages and currents.
• 4.5 Effect of resistance, inductance and capacitance in a series circuit. Phase lag and lead.
• 4.6 Resonance.
• 4.7 Rectification and smoothing.
• 4.8 Use of the cathode-ray oscilloscope.

1. understand and use the terms frequency, period, peak value and root-mean-square value when applied to alternating voltages and currents,
2. understand and explain that the rms value is related to the energy dissipated per cycle,
3. recall and use the relationship Vrms = V ₀ /√2 ,
4. explain the operation of a transformer,
5. recall and use Ns / Np = Vs / Vp for a transformer,
6. understand the advantages of transmission of electricity at high voltages and low currents,
7. understand how alternating voltages and currents may be represented by a rotating vector,
8. explain how a resistor behaves in an ac circuit,
9. explain how an inductor behaves in an ac circuit and derive an expression for its reactance,
10. explain how a capacitor behaves in an ac circuit and derive an expression for its reactance,
11. derive an expression for the mean power dissipated in a resistor, in an inductor and in a capacitor,
12. derive an expression for the impedance of a resistor, inductor and capacitor in series,
13. derive an expression for the resonant frequency of an R, L, C series circuit,
14. distinguish between half wave and full wave rectification,
15. describe and explain how a single diode can be used to produce half wave rectification,
16. describe and explain how an arrangement of four diodes (bridge rectifier) can be used to produce full wave rectification,
17. describe and explain the use of a single capacitor for smoothing, including the effect of changing the size of the load resistor,
18. describe the use of a cathode ray oscilloscope to measure
     i) ac and dc voltages,
     ii) frequencies,
     iii) phase difference using a double-beam CRO.