How does an object become charged, and what rules does charge obey?

Objects gain or lose electrons by friction (rubbing), conduction (contact) or induction (a nearby charge rearranges charge without contact). Only electrons move in solids, so an object becomes negative by gaining electrons and positive by losing them. Charge is conserved (only transferred, never created or destroyed) and quantised in whole multiples of the elementary charge $e = 1.60 \times 10^{-19}$ C. The force between charges follows Coulomb's law, an inverse-square law on the Reference Tables.

What is the difference between electric field and potential difference?

The electric field is the force per unit charge at a point, $E = F_e/q$, in newtons per coulomb, a vector. Potential difference (voltage) is the work done per unit charge moved between two points, $V = W/q$, in volts, a scalar. Between parallel plates the field is uniform and equals $E = V/d$. The field describes the force on a charge; the potential difference describes the energy per charge that drives a current. All three equations are on the Reference Tables.

How do I analyze a circuit on the Regents?

Find the equivalent resistance using the series rule (resistances add) or the parallel rule (reciprocals add, then invert). Then use Ohm's law $I = V/R$ with the source voltage to find the total current. Apply the shared quantity: current is the same in series, voltage is the same across parallel branches. From there find the current through and voltage across each component, and use the power equations $P = VI = I^2R = V^2/R$ as needed.

When does a magnetic field exert a force on a charge?

A charge feels a magnetic force only when it moves across a magnetic field. The force $F_B = qvB$ (on the Reference Tables) is greatest when the velocity is perpendicular to the field and zero when it is parallel. The force is perpendicular to both the velocity and the field, so it changes direction but not speed. The same effect on the moving charges in a current-carrying wire gives the motor effect, $F = BIL$ (which is not on the tables, so recall it).

What causes electromagnetic induction?

An electromotive force is induced when the magnetic field through a conductor changes; a steady field induces nothing. Moving a magnet relative to a coil, rotating a coil in a field, or varying a current in a nearby coil all induce an EMF. The induced EMF is larger for a faster change, a stronger field, and more turns. Generators use induction to convert mechanical to electrical energy; transformers use it to change voltage. The tables give no induction equation, so this topic is qualitative.

New YorkPhysics

Regents Physics electricity and magnetism: a complete skills guide to charge, Coulomb's law, fields, current, Ohm's law, circuits, magnetism and induction

A deep-dive Regents Physics skills guide to the electricity and magnetism module: static electricity and Coulomb's law, electric fields and potential difference, current and Ohm's law, series and parallel circuits, magnetism and the motor effect, and electromagnetic induction. Includes worked examples and constructed-response technique.

Generated by Claude Opus 4.818 min readPS-PHYS 5.3Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

Why electricity and magnetism is a high-value Regents topic
Charge and Coulomb's law
Fields and potential difference
Current, Ohm's law and circuits
Magnetism and induction
Check your knowledge

Why electricity and magnetism is a high-value Regents topic

The electricity strand carries many marks, and most of them come from a handful of reliable calculations (Ohm's law, power, circuit analysis) plus conceptual questions on charging, fields, magnetism and induction. Because the Reference Tables print nearly all the equations, the lever is knowing which one to use and how to combine the circuit rules. This guide ties together the dot-point pages, each with its own practice: static electricity and Coulomb's law, electric fields and potential, current and Ohm's law, series and parallel circuits, magnetism and the motor effect and electromagnetic induction.

Charge and Coulomb's law

Objects charge by transferring electrons (friction, conduction, induction); charge is conserved and quantised in multiples of $e = 1.60 \times 10^{-19}$ C. Like charges repel, unlike attract. The force between point charges is Coulomb's law, $F_e = \dfrac{kq_1 q_2}{r^2}$ with $k = 8.99 \times 10^9$ N m squared per C squared, an inverse-square law (double the distance, quarter the force). It mirrors gravitation but is far stronger and can be attractive or repulsive.

Fields and potential difference

The electric field is force per unit charge, $E = \dfrac{F_e}{q}$ (N/C), pointing the way a positive charge is pushed; field lines run from positive to negative and are denser where the field is stronger. Between parallel plates the field is uniform, $E = \dfrac{V}{d}$ . Potential difference (voltage) is work per unit charge, $V = \dfrac{W}{q}$ (volts), and is what drives current around a circuit. The force on a charge in a field is $F_e = qE$ .

Current, Ohm's law and circuits

Current is charge flow rate, $I = \dfrac{q}{t}$ (amperes). Ohm's law is $R = \dfrac{V}{I}$ (used as $V = IR$ ). Power is $P = VI = I^2 R = \dfrac{V^2}{R}$ (watts) and energy is $W = Pt$ . In a series circuit the current is the same and resistances add ( $R_{total} = R_1 + R_2$ ); in a parallel circuit the voltage is the same across each branch and the resistances combine by $\dfrac{1}{R_{total}} = \dfrac{1}{R_1} + \dfrac{1}{R_2}$ , giving a total smaller than the smallest resistor. Reduce the circuit to an equivalent resistance, find the total current with Ohm's law, then work back to each component.

Magnetism and induction

A magnetic field runs north to south outside a magnet, and a current produces a magnetic field (electromagnets). A charge moving across a field feels a force $F_B = qvB$ (maximum when perpendicular, zero when parallel; perpendicular to both $v$ and $B$ ). The same effect on a current-carrying wire is the motor effect, $F = BIL$ (recall, not on the tables), which turns motors. The converse is electromagnetic induction: a changing magnetic field through a conductor induces an EMF (a steady field does not). The induced EMF grows with the speed of the change, the field strength and the number of turns. Generators (motion to electricity) and transformers (changing voltage) both rely on induction.

A combined circuit calculation

A $12$ V battery is connected to a $2.0$ ohm resistor in series with a parallel pair of $6.0$ ohm and $3.0$ ohm resistors. Find the total current from the battery.

step 1 Combine the parallel pair

$\dfrac{1}{R_p} = \dfrac{1}{6.0} + \dfrac{1}{3.0} = \dfrac{1}{6.0} + \dfrac{2}{6.0} = \dfrac{3}{6.0}$ , so $R_p = 2.0$ ohms.

step 2 Add the series resistor

The parallel combination ( $2.0$ ohms) is in series with the $2.0$ ohm resistor: $R_{total} = 2.0 + 2.0 = 4.0$ ohms.

step 3 Apply Ohm's law for the total current

$I = \dfrac{V}{R_{total}} = \dfrac{12}{4.0} = 3.0$ A.

step 4 State the answer

The total current from the battery is $3.0$ A. Reducing the parallel part first, then adding the series resistor, gives the equivalent resistance the battery "sees".

Electricity and magnetism rests on a handful of Reference-Table equations and the circuit rules. Charge is conserved and quantised; Coulomb's law $F_e = kq_1q_2/r^2$ is inverse-square. The electric field is $E = F_e/q$ (and $V/d$ between plates); potential difference is $V = W/q$ . Current is $I = q/t$ ; Ohm's law is $R = V/I$ ; power is $P = VI = I^2R = V^2/R$ ; energy is $W = Pt$ . Series circuits share current and add resistances; parallel circuits share voltage and combine resistances by reciprocals (total smaller than the smallest). A current makes a magnetic field; a charge moving across a field feels $F_B = qvB$ (maximum perpendicular, zero parallel); the motor effect $F = BIL$ is recall. Electromagnetic induction needs a changing field and drives generators and transformers.

Check your knowledge

A mix of recall, calculation and reasoning questions covering the module. Attempt them under timed conditions, then check against the solutions.

State how an object becomes positively charged in terms of electrons. (1 mark)
State what happens to the Coulomb force if the distance between two charges doubles. (1 mark)
A $4.0 \times 10^{-6}$ C charge feels a force of $0.024$ N in a field. Calculate the field strength. (2 marks)
A $20.$ V source drives a current through a $5.0$ ohm resistor. Calculate the current. (2 marks)
Calculate the power dissipated by a $6.0$ ohm resistor carrying $2.0$ A. (2 marks)
Two $6.0$ ohm resistors are in series. Calculate the total resistance. (1 mark)
Two $6.0$ ohm resistors are in parallel. Calculate the total resistance. (2 marks)
State the condition for a moving charge to feel a magnetic force. (1 mark)
A $2.0$ C charge moves at $5.0$ m/s perpendicular to a $0.30$ T field. Calculate the magnetic force. (2 marks)
State the condition required for electromagnetic induction to occur. (1 mark)

Solutions

Q1: It loses electrons (only electrons move in solids; losing them leaves a net positive charge).
Q2: It becomes one quarter as large (inverse-square law).
Q3: $E = \dfrac{F_e}{q} = \dfrac{0.024}{4.0 \times 10^{-6}} = 6.0 \times 10^3$ N/C.
Q4: $I = \dfrac{V}{R} = \dfrac{20.}{5.0} = 4.0$ A.
Q5: $P = I^2 R = (2.0)^2(6.0) = (4.0)(6.0) = 24$ W.
Q6: $R_{total} = 6.0 + 6.0 = 12$ ohms.
Q7: $\dfrac{1}{R_{total}} = \dfrac{1}{6.0} + \dfrac{1}{6.0} = \dfrac{2}{6.0}$ , so $R_{total} = 3.0$ ohms.
Q8: The charge must move across (with a component perpendicular to) the magnetic field.
Q9: $F_B = qvB = (2.0)(5.0)(0.30) = 3.0$ N.
Q10: The magnetic field through the conductor must be changing.

Sources & how we know this

Reference Tables for Physical Setting/Physics — NYSED (2006)
Physical Setting/Physics Regents examinations — NYSED (2019)