Course

A2 Physics: Electric Fields, Capacitance, Magnetism and A.C. (9702)

Name: A2 Physics: Electric Fields, Capacitance, Magnetism and A.C. (9702)
Price: 1999 BDT

9702 topics 18 to 21. A field is what one charge leaves behind for another to find. Strength adds as arrows and potential adds as numbers, and most of the marks live in that difference.

56 lessons5h 47mEnglish

What you'll learn

Add field contributions as vectors and potential contributions as numbers, and say why the rule differs
Use Coulomb law and the inverse-square field, and read a radial field off its potential gradient
Compute the energy stored in a capacitor, and why doubling the p.d. quadruples it
Handle exponential discharge with the time constant, including the log-graph method
Apply the motor rule and the generator effect without confusing the two

Requirements

AS topic 9, electricity: charge, current, potential difference and the meaning of one volt
AS topic 10, d.c. circuits: series and parallel, and what stays the same in each
AS topic 1: vectors, and adding two of them to a resultant
O Level graphs: reading a gradient and an area, since capacitance IS a gradient and energy IS an area
Exponentials and natural logarithms, used by every discharge question

Course content

56 lessons · 5h 47m

18.1-18.5 · Electric fields

A charge changes the space around it. Field lines and field strength, the uniform field between parallel plates, Coulomb inverse-square law, potential and potential energy, and where the parallel with gravity stops.

0/13

A field is a force waiting at every point

Drop a test charge between two charges and the force appears before you touch anything.

Preview6m 2.

Electric field strength is force per unit charge

Double the test charge, double the force, and E = F/Q does not move at all.

Preview6m

Two charges, one resultant field

Each charge sends its own field to the point. Add them as vectors, never as numbers.

The uniform field between parallel plates

A storm cloud and the ground are two plates 2 km apart. Find the field between them.

Why the deflected path is a parabola

An electron enters parallel to the plates. Predict the shape of the path before you run it.

Coulomb law: the force between two point charges

Move the charges twice as far apart and the force falls to a quarter, not a half.

The radial field of a point charge

One charge, field lines straight out, and a graph that falls fast then keeps falling.

Electric potential is one number per point

Walk a test charge around a ring and the potential does not change at all.

Work done is W = QΔV, whatever the path

Take the long way from A to B and the work is the same. Only the two ends count.

10.

Flux is the field counted through an area

Turn a loop edge-on to the field and the count of lines through it falls to zero.

11.

Where the gravity parallel stops

Both laws are inverse square. Only one of the two fields can push as well as pull.

12.

One exam question, end to end

Ten marks, four parts, no options: field strength, acceleration, deflection, then why weight is ignored.

13.

Electric fields: quick revision

Reconnect force, field and potential, then repair charged-particle direction and sign traps.

19.1-19.3 · Capacitance

What a capacitor holds, C = Q/V and the farad, the parallel and series rules, the energy stored as the area under a charge against voltage graph, exponential discharge through a resistor, the time constant RC and how to read a logarithmic plot.

0/12

A capacitor stores charge, and stops

A camera flash charges slowly and empties in milliseconds. Predict what the current does.

Preview6m 2.

Capacitance is the gradient of Q against V

Charge is proportional to potential difference, and C = Q/V is that constant.

Preview6m

Capacitors in parallel add

Two 100 microfarad capacitors side by side. Predict the combined value before you take the move.

Capacitors in series share the charge

Same two capacitors, now end to end. The rule is the opposite of the resistor rule.

Energy stored is the area under Q against V

The first coulomb is cheap and the last is expensive, so the answer carries a half.

Double the p.d., quadruple the energy

W = half C V squared is not proportional. Predict the energy at twice the voltage.

Discharge is exponential, not steady

Predict the shape of the fall: a straight slope, or something that never quite ends.

The time constant is RC

Double the resistance and predict the new time constant before you change it.

After one time constant, 37% is left

Every equal interval removes the same fraction, wherever on the curve you start.

10.

A log graph makes the decay a straight line

Plot ln V against t and the gradient hands you the time constant.

11.

One exam question, end to end

Nine marks, four parts, no options: type the answer, then read the mark scheme you were being marked against.

12.

Capacitance: quick revision

Reconnect charge, voltage, energy and exponential discharge through one reliable calculation route.

20.1-20.5 · Magnetic fields

Fields made by magnets and by currents, the force on a wire and on a moving charge, circular paths, the Hall voltage, flux linkage, and the two laws of induction that make a dynamo work.

0/14

A compass needle maps the field

Predict where the needle points, then drag the compass right around the magnet.

Preview6m 2.

A current makes a magnetic field

No magnet needed. Find the shape of the field around a straight current-carrying wire.

Preview6m

Two currents, and the neutral point between them

Midway between two like currents the field is exactly zero. Reverse one and the same point becomes a maximum.

The force on a current-carrying conductor

Turn the wire along the field lines and the force disappears. F = BIL sin θ says why.

Fleming left-hand rule

Three quantities, three perpendicular directions, and one hand that keeps them in order.

From F = BIL to F = BQv

A current is drifting charge, so the push on the wire is the push on its carriers.

Why the path closes into a circle

A force always at right angles turns the particle without ever changing its speed.

The Hall effect and the Hall voltage

Carriers pile up on one edge until the electric force balances the magnetic one.

Magnetic flux and flux linkage

Turn a coil edge-on to the field and the flux through it falls to zero.

10.

Faraday law: the e.m.f. is a gradient

A bicycle dynamo is dark while you stand still. Only a changing flux linkage induces.

11.

Lenz law and the minus sign

Push a magnet into a coil and the coil pushes back. Pull it out and the coil pulls back. Anything else would be free energy.

12.

Inside a simple generator

A coil turning at a steady rate gives an e.m.f. that peaks when the flux linkage is zero.

13.

One exam question, end to end

Ten marks, four parts, no options: a force, two induced e.m.f.s, then why an e.m.f. needs no current.

14.

Magnetic fields: quick revision

Reconnect magnetic force and induction, then repair the generator and Lenz-law traps.

21.1-21.2 · Alternating currents

The mains reverses fifty times a second, so its mean is zero and useless. Square it first, and the number you get is the one printed on the socket. Then rectify, smooth, and see why the grid runs on alternating supply at all.

0/13

A supply that reverses fifty times a second

One cycle of the mains, and why the socket says 230 V when the peak is 325 V.

Preview5m 2.

The mean voltage is exactly zero

A 325 V supply and a 5 V supply have the same mean. So the mean is the wrong number.

Preview5m

Square it, and the halves stop cancelling

Power depends on v², and v² is never negative. Predict the mean of the squared curve.

Where the square root of two comes from

Square, take the mean, take the root. The name is the recipe read backwards.

R.m.s. is defined by equal heating

The steady voltage that would deliver the same mean power. Not an average of the wave.

Preview7m

Mean power is half the peak power

The power pulses twice per cycle. Predict how its mean compares with its peak.

One diode throws half the cycle away

The output is direct current, but half of every cycle delivers nothing.

A bridge flips the negative half instead

Twice the mean output of half-wave, and a trap about which mean the question wants.

A capacitor holds the voltage up between peaks

Raise the capacitance and watch the ripple shrink. It is the RC decay again.

10.

Why the grid is alternating at all

Transformers need a changing current, and I²R punishes current far more than voltage.

11.

Reactance and resonance: the frequency a circuit prefers

A capacitor and an inductor oppose each other, and at one frequency they cancel exactly. The current peaks there.

12.

One exam question, end to end

Eight marks, three parts, no options: r.m.s. voltage, mean power, then what a bridge rectifier changes.

13.

Alternating currents: quick revision

Reconnect peak values, r.m.s. heating, rectification, smoothing and high-voltage transmission.

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