A field predicts a force.

Electric force between point charges depends on their charge product and inverse square separation. Like charges repel; unlike charges attract. Electric field direction is the force direction on a positive test charge.

Field contributions add as vectors. A negative charge experiences force opposite to the field. At a midpoint between identical positive charges the fields cancel, even though each individual field is nonzero.

INTERACTIVE MODEL

Electric force at a distance

Charge product / μC²2

READ THE GRAPH

Explore the relationship

Positive charge product means repulsion; negative means attraction. Point-charge model.

Signed force / N

Tap or focus a plotted point to read its values.

View plotted data as a table

Explore the relationship
Series	Separation / m	Signed force / N
Signed force / N	0.1	1.798
Signed force / N	0.115	1.35955
Signed force / N	0.13	1.06391
Signed force / N	0.145	0.855172
Signed force / N	0.16	0.702344
Signed force / N	0.175	0.587102
Signed force / N	0.19	0.498061
Signed force / N	0.205	0.427841
Signed force / N	0.22	0.371488
Signed force / N	0.235	0.325577
Signed force / N	0.25	0.28768
Signed force / N	0.265	0.256034
Signed force / N	0.28	0.229337
Signed force / N	0.295	0.206607
Signed force / N	0.31	0.187097
Signed force / N	0.325	0.170225
Signed force / N	0.34	0.155536
Signed force / N	0.355	0.14267
Signed force / N	0.37	0.131337
Signed force / N	0.385	0.121302
Signed force / N	0.4	0.112375
Signed force / N	0.415	0.104398
Signed force / N	0.43	0.0972418
Signed force / N	0.445	0.0907966
Signed force / N	0.46	0.0849716
Signed force / N	0.475	0.0796898
Signed force / N	0.49	0.0748855
Signed force / N	0.505	0.0705029
Signed force / N	0.52	0.0664941
Signed force / N	0.535	0.0628177
Signed force / N	0.55	0.059438
Signed force / N	0.565	0.0563239
Signed force / N	0.58	0.0534483
Signed force / N	0.595	0.0507874
Signed force / N	0.61	0.0483203
Signed force / N	0.625	0.0460288
Signed force / N	0.64	0.0438965
Signed force / N	0.655	0.041909
Signed force / N	0.67	0.0400535
Signed force / N	0.685	0.0383185
Signed force / N	0.7	0.0366939
Signed force / N	0.715	0.0351704
Signed force / N	0.73	0.0337399
Signed force / N	0.745	0.0323949
Signed force / N	0.76	0.0311288
Signed force / N	0.775	0.0299355
Signed force / N	0.79	0.0288095
Signed force / N	0.805	0.0277458
Signed force / N	0.82	0.02674
Signed force / N	0.835	0.0257879
Signed force / N	0.85	0.0248858
Signed force / N	0.865	0.0240302
Signed force / N	0.88	0.023218
Signed force / N	0.895	0.0224462
Signed force / N	0.91	0.0217124
Signed force / N	0.925	0.0210139
Signed force / N	0.94	0.0203486
Signed force / N	0.955	0.0197144
Signed force / N	0.97	0.0191094
Signed force / N	0.985	0.0185318
Signed force / N	1	0.01798

Probe along the horizontal axis0.1Signed force / N: 1.798

Explore: Reverse the sign of one charge through the charge-product control. What changes?

F=\frac{k|Qq|}{r^2}\qquad E=\frac Fq\qquad E_{\rm point}=\frac{k|Q|}{r^2}

WORKED EXAMPLE

A +2.0 μC charge sits in a uniform field of 300 N $C^{-1}$ to the right.

F = qE = 2.0 × $10^{-6}$ × 300.
Force = 6.0 × $10^{-4}$ N to the right.

Assumed knowledge

Vectors and charge units.

Learning checkpoints & source

YOUR LEARNING CHECKPOINT

Apply Coulomb’s law and electric field strength.
Determine directions and superpose fields.

QCAA Physics · Unit 3 · Electromagnetism ↗

READY TO TRY IT?

A field predicts a force.

Electric force at a distance

Explore the relationship

A +2.0 μC charge sits in a uniform field of 300 N C−1C^{-1}C−1 to the right.

Put the idea to work.

A +2.0 μC charge sits in a uniform field of 300 N $C^{-1}$ to the right.