AQA A-Level Physics: Mechanics and Electricity Revision Guide

Mechanics and Electricity are two of the most heavily examined topics on AQA A-Level Physics Paper 1. Together they account for a substantial share of the marks, and the skills they develop -- resolving vectors, analysing forces, applying conservation laws, and building circuit models -- underpin almost every other area of the course. If you are confident in these two topics, you have a strong foundation for the rest of A-Level Physics.

This guide works through the key content for both topics as set out in the AQA specification, explains the concepts you need to understand, and highlights the common pitfalls that cost marks in exams.

Where These Topics Fit in the Specification

Mechanics sits within Section 4 (Mechanics and Materials) of the AQA specification, while Electricity is Section 5. Both are assessed on Paper 1, which is worth 85 marks, lasts 2 hours, and accounts for 34% of the A-Level. The calculation techniques and physical reasoning you develop here will reappear across Papers 2 and 3.

Mechanics

Scalars and Vectors

The distinction between scalars and vectors is fundamental. A scalar quantity has magnitude only -- examples include speed, mass, energy, temperature, and distance. A vector quantity has both magnitude and direction -- examples include velocity, acceleration, force, displacement, and momentum.

You must be comfortable resolving vectors into perpendicular components. For a vector of magnitude F acting at angle theta to the horizontal, the horizontal component is F cos(theta) and the vertical component is F sin(theta). To find the resultant of two perpendicular components, use Pythagoras' theorem for the magnitude and trigonometry for the direction.

Common error: Confusing sine and cosine when resolving. Always draw a clear diagram and label the angle. If theta is measured from the horizontal, the horizontal component uses cosine and the vertical uses sine. If the angle is from the vertical, the assignments swap.

Moments

The moment of a force about a point is defined as force multiplied by the perpendicular distance from the line of action of the force to the point:

Moment = F x d

The principle of moments states that for an object in rotational equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that same point. This principle is used to solve problems involving beams, levers, and other rigid bodies.

A couple is a pair of equal and opposite forces whose lines of action do not coincide, producing a turning effect but no net translational force. When solving moments problems, choose your pivot point carefully -- picking where an unknown force acts eliminates that force from the equation.

SUVAT Equations and Kinematics

The SUVAT equations describe motion under constant acceleration. The five quantities are s (displacement), u (initial velocity), v (final velocity), a (acceleration), and t (time). The four equations are:

1. v = u + at
2. s = ut + 0.5at squared
3. v squared = u squared + 2as
4. s = 0.5(u + v)t

Each connects four of the five variables. Identify three known quantities and the one you want, then choose the equation containing those four. These equations only apply when acceleration is constant -- if it varies, use graphical analysis or calculus.

Newton's Laws of Motion

Newton's first law: An object remains at rest or moves at constant velocity unless acted upon by a resultant force.

Newton's second law: The resultant force equals the rate of change of momentum. For constant mass, F = ma -- the most frequently used equation in Mechanics.

Newton's third law: When two objects interact, the forces they exert on each other are equal in magnitude, opposite in direction, and act on different objects. Third-law pairs never cancel because they act on different bodies.

For multi-body problems (connected particles, inclined planes), draw a free-body diagram for each object, resolve forces, and set up simultaneous equations.

Projectile Motion

Projectile motion is the motion of an object under the influence of gravity alone (ignoring air resistance). The key principle is that horizontal and vertical motions are independent:

• Horizontally, there is no acceleration (ignoring air resistance), so the horizontal velocity remains constant.
• Vertically, the object accelerates downward at g = 9.81 m/s squared.

To solve projectile problems, resolve the initial velocity into horizontal and vertical components, then apply the SUVAT equations to each direction separately. The time of flight links the two directions because the object is in the air for the same duration in both.

For an object launched horizontally, the initial vertical velocity is zero and you use the vertical displacement to find the time of flight. For an object launched at an angle, resolve the initial velocity into components -- the time to reach maximum height comes from the vertical component (v = 0 at the top), and the total flight time for a symmetric trajectory is double this.

Work, Energy, and Power

Work done is the force multiplied by the displacement in the direction of the force: W = Fs cos(theta). When the force is parallel to the displacement, W = Fs. When perpendicular, no work is done.

The key energy equations are kinetic energy Ek = 0.5mv squared and gravitational potential energy Ep = mgh. The work-energy theorem states that the net work done on an object equals the change in its kinetic energy -- a powerful problem-solving tool that lets you track energy transfers instead of analysing forces.

Power is the rate of doing work: P = W / t. For an object moving at constant velocity v under a driving force F, P = Fv. This is particularly useful for vehicle problems where the driving force equals the total resistive force.

Conservation of Energy

The principle of conservation of energy states that energy cannot be created or destroyed -- only transferred between stores. In Mechanics, this is most often applied as conservation of mechanical energy: loss in Ep = gain in Ek (and vice versa) when no dissipative forces act. When friction is present, you must account for the energy dissipated (friction force multiplied by distance), since mechanical energy is no longer conserved even though total energy is.

Conservation of Momentum

The momentum of an object is defined as the product of its mass and velocity:

p = mv

Momentum is a vector quantity -- direction matters.

The principle of conservation of linear momentum states that total momentum remains constant provided no external resultant force acts. This applies to collisions and explosions:

• Elastic collision -- both momentum and kinetic energy conserved.
• Inelastic collision -- momentum conserved, kinetic energy is not.
• Perfectly inelastic collision -- objects stick together; maximum kinetic energy lost.
• Explosion -- total momentum remains zero, so fragments have equal and opposite momenta.

Exam technique: Always define a positive direction at the start. Sign errors are the most common source of lost marks in momentum questions.

Impulse

Impulse is the change in momentum: Impulse = F x t = mv -- mu. The area under a force-time graph equals the impulse. This connects to car safety -- crumple zones, seat belts, and airbags increase the collision time, reducing the peak force on the occupant.

Electricity

Charge and Current

Electric current is the rate of flow of charge:

I = Delta Q / Delta t

The charge carriers in a metallic conductor are free (delocalised) electrons. By convention, current flows from positive to negative (opposite to electron flow). One coulomb is the charge transferred when a current of one ampere flows for one second.

Potential Difference

The potential difference (p.d.) between two points is the energy transferred per unit charge as charge moves between those points:

V = W / Q

where W is the energy transferred and Q is the charge. One volt equals one joule per coulomb. Potential difference is measured in parallel with the component using a voltmeter, which must have a very high resistance so that it draws negligible current.

Resistance

Resistance is the ratio of potential difference to current: R = V / I. For resistors in series:

R(total) = R1 + R2 + R3 + ...

For resistors in parallel, the reciprocal of the total resistance is the sum of the reciprocals:

1/R(total) = 1/R1 + 1/R2 + 1/R3 + ...

Exam tip: A common error in parallel resistance calculations is forgetting to take the reciprocal at the final step. If you calculate 1/R(total) = 0.5, the total resistance is 2 ohms, not 0.5 ohms.

Resistivity

Resistivity is a material property that quantifies how strongly it opposes current flow. The resistance of a uniform conductor depends on resistivity, length, and cross-sectional area:

R = rho x L / A

where rho is the resistivity (measured in ohm-metres), L is the length, and A is the cross-sectional area.

Metals have low resistivity (around 10 to the power of -8 ohm-metres), insulators have very high resistivity, and semiconductors fall in between. The resistivity of a metal increases with temperature because increased lattice vibrations cause more frequent collisions with charge carriers.

Practical connection: The required practical on measuring resistivity involves measuring resistance at different wire lengths, plotting R against L, and using the gradient (rho/A) to find the resistivity. You need the wire diameter (measured with a micrometer) to calculate A.

I-V Characteristics

The I-V characteristic of a component is a graph of current against potential difference (or vice versa). The shape of the graph reveals how the component behaves:

Ohmic conductor (e.g. metallic wire at constant temperature): A straight line through the origin -- current is directly proportional to p.d.

Filament lamp: A curve that becomes less steep at higher voltages. The filament heats up, increasing resistance, so current does not rise proportionally. The graph is symmetrical about the origin.

Thermistor (NTC): Resistance decreases as temperature increases, because more charge carriers gain enough energy to conduct. Used as temperature sensors.

Diode: Conducts above the threshold voltage (approximately 0.6 V for silicon) but has very high resistance in reverse. The I-V graph shows negligible current below the threshold, then a sharp increase.

LDR (light-dependent resistor): Resistance decreases as light intensity increases because more photons liberate charge carriers. Used as light sensors.

EMF and Internal Resistance

The electromotive force (EMF) of a source is the energy transferred per unit charge when driving charge around a complete circuit. Every real source has internal resistance (r), so some energy is dissipated inside the source as heat. The terminal p.d. (V) is therefore less than the EMF:

EMF = V + Ir

which can be rearranged as:

V = EMF -- Ir

As the current increases, the terminal p.d. decreases. When no current flows (open circuit), V equals the EMF. The term Ir is the "lost volts" -- energy per coulomb dissipated inside the source.

Practical connection: The required practical on determining EMF and internal resistance involves measuring V for different values of I and plotting V against I. The y-intercept gives the EMF and the gradient gives --r. You vary the current using a variable resistor and should avoid drawing large currents for extended periods to prevent the cell heating up.

Kirchhoff's Laws

Kirchhoff's first law (junction rule): The total current entering a junction equals the total current leaving it -- a consequence of conservation of charge.

Kirchhoff's second law (loop rule): The sum of the EMFs around any closed loop equals the sum of the p.d.s around that loop -- a consequence of conservation of energy.

These laws are essential for analysing circuits that cannot be reduced to simple series or parallel combinations.

Potential Dividers

A potential divider is a circuit that uses two (or more) resistors in series to produce an output voltage that is a fraction of the input voltage. For two resistors R1 and R2 connected in series across a supply of voltage V(in):

V(out) = V(in) x R2 / (R1 + R2)

where V(out) is the potential difference across R2.

Potential dividers are used to provide a variable output voltage, to create sensor circuits (using thermistors or LDRs as one of the resistors), and to supply a specific voltage to a component.

Sensor circuits: If R2 is a thermistor, then as temperature rises, the thermistor's resistance falls and V(out) decreases. If R2 is an LDR, increased light intensity reduces its resistance and V(out) falls.

Key point: A potential divider only works as expected if negligible current is drawn from the output. If a load is connected in parallel with R2, the effective resistance of the lower arm changes and the output voltage is no longer given by the simple formula. This is a common exam question.

Linking Mechanics and Electricity

Although Mechanics and Electricity might seem like separate topics, the principles that connect them run deep. Conservation of energy appears in both -- in Mechanics as the interplay between kinetic and potential energy, and in Electricity as Kirchhoff's second law. The concept of "work done" links force and displacement in Mechanics to EMF and charge in Electricity. Developing fluency in both areas builds a general problem-solving toolkit that serves you well across the entire A-Level.

Exam Strategy for Mechanics and Electricity Questions

Mechanics

Draw diagrams. For every force problem, draw a free-body diagram showing all forces with their magnitudes and directions. Resolve into components if forces are not aligned with the axes.

Show your working. Write out the equation, substitute values with units, and carry units through. Method marks are awarded for correct substitution even if you make an arithmetic error.

Check units and signs. A negative velocity or acceleration is physically meaningful -- it tells you the direction. Keep your sign convention consistent throughout.

Electricity

Redraw the circuit. If the circuit diagram looks complicated, redraw it in a simpler form to identify series and parallel sections clearly.

Apply Kirchhoff's laws systematically. Label all currents with assumed directions and write equations using both laws. A negative result simply means the current flows opposite to your assumed direction.

Watch for internal resistance. When a question mentions internal resistance, the terminal p.d. is not equal to the EMF. Use EMF = V + Ir throughout.

Potential divider reasoning. Describe what happens to the sensor's resistance as conditions change, how this alters the resistance ratio, and the effect on V(out).

Prepare with LearningBro

These topics reward consistent practice. The following LearningBro courses are designed to build your confidence and fluency:

• AQA A-Level Physics: Mechanics in Depth -- focused, in-depth coverage of every Mechanics topic on the AQA specification, from vectors and SUVAT to energy and momentum
• AQA A-Level Physics: Electricity in Depth -- detailed coverage of charge, current, resistance, EMF, internal resistance, and potential dividers, with practice questions at every level
• AQA A-Level Physics: Mechanics and Electricity -- combined coverage of both topics, ideal for building connections between Mechanics and Electricity and preparing for Paper 1