What is an electric charge?
An electric charge results from the movement of electrons and is responsible for electric forces.
Name three methods of charging a body.
Friction, induction, and contact.
What is charging by friction?
Transfer of electrons between two different materials when rubbed together.
What is charging by induction?
Charging an object without direct contact by using a nearby charged object and grounding.
State Coulomb’s Law.
\( F = k \frac{Q_1 Q_2}{r^2} \)
What is the value of Coulomb's constant?
\( k = 9 \times 10^9 \, \text{Nm}^2/\text{C}^2 \)
What is electric field strength?
The force experienced per unit charge: \( E = \frac{F}{Q} \)
Give the formula for electric field strength in terms of a source charge.
\( E = \frac{Q}{4 \pi \varepsilon_0 r^2} \)
What is the unit of electric field strength?
Newton per coulomb (N/C)
Define electric potential.
Work done in bringing a unit charge from infinity to a point: \( V = \frac{Q}{4 \pi \varepsilon_0 r} \)
What is the unit of electric potential?
Volt (V)
State the fundamental law of electrostatics.
Like charges repel; unlike charges attract.
What is a capacitor?
A device used to store electric charge and energy.
What is capacitance?
The ratio of charge stored to potential difference: \( C = \frac{Q}{V} \)
Unit of capacitance?
Farad (F)
Give the formula for energy stored in a capacitor.
\( W = \frac{1}{2} QV = \frac{1}{2} CV^2 = \frac{1}{2} \frac{Q^2}{C} \)
State three factors affecting the capacitance of a parallel plate capacitor.
Plate area, distance between plates, and dielectric constant.
Formula for capacitance of a parallel plate capacitor.
\( C = \frac{\varepsilon A}{d} \)
What is the dielectric?
Insulating material placed between plates of a capacitor to increase capacitance.
Capacitance in series formula?
\( \frac{1}{C_{\text{eq}}} = \frac{1}{C_1} + \frac{1}{C_2} + \cdots \)
Capacitance in parallel formula?
\( C_{\text{eq}} = C_1 + C_2 + \cdots \)
What happens to the total capacitance when more capacitors are added in parallel?
Total capacitance increases.
What happens to the total capacitance when capacitors are added in series?
Total capacitance decreases.
What is an equipotential surface?
A surface on which all points have the same electric potential.
What is the relationship between electric field and potential gradient?
\( E = -\frac{dV}{dr} \)
Describe the electric field around a point charge.
Radial field; lines point away for positive charge and toward for negative charge.
What is electric field intensity at a point?
Force experienced by a unit positive charge at that point.
Electric potential energy formula?
\( U = \frac{Q_1 Q_2}{4 \pi \varepsilon_0 r} \)
What is the effect of a dielectric on a capacitor?
Increases capacitance by reducing the electric field strength.
Why does a capacitor block DC current but allow AC?
DC builds up charge and reaches full potential; AC constantly changes direction and allows current to alternate.
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