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Electric charge and fields exercises · Physics Playground

ExercisesPhysics

Electric charge and fields

28 problems with worked solutions, from Physics and College Physics.

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Learn it first: Electric Charge & Force

Physics18.1 Electrical Charges, Conservation of Charge, and Transfer of Charge

ExampleWhat is the missing charge?Figure 18.7 shows two spheres that initially have +4 C and +8 C of charge. After an interaction (which could simply be that they touch each other), the blue sphere has +10 C of charge, and the red sphere has an…
ExampleCharging Ink DropletsElectrically neutral ink droplets in an ink-jet printer pass through an electron beam created by an electron gun, as shown in Figure 18.14. Some electrons are captured by the ink droplet, so that it becomes charged.…With a figure

Physics18.2 Coulomb's law

ExampleUsing Coulomb’s law to find the force between charged objectsSuppose Coulomb measures a force of 20× 10^(−6) N between the two charged spheres when they are separated by 5.0 cm. By turning the dial at the top of the torsion balance, he approaches the spheres so that they are…
ExampleUsing Coulomb’s law to find the distance between charged objectsAn engineer measures the force between two ink drops by measuring their acceleration and their diameter. She finds that each member of a pair of ink drops exerts a repulsive force of F=5.5mN on its partner. If each…

Physics18.3 Electric Field

ExampleWhat is the charge?Look at the drawing of the electric field in Figure 18.20. What is the relative strength and sign of the three charges? Figure 18.20 Map of electric field due to three charged particles.With a figure
ExampleElectric field from doorknobA doorknob, which can be taken to be a spherical metal conductor, acquires a static electricity charge of q=−1.5nC. What is the electric field 1.0 cm in front of the doorknob? The diameter of the doorknob is 5.0 cm.

Physics18.4 Electric Potential

ExampleX-ray Tube (part 1)Dentists use X-rays to image their patients’ teeth and bones. The X-ray tubes that generate X-rays contain an electron source separated by about 10 cm from a metallic target. The electrons are accelerated from the…With a figure
ExampleElectric Potential Energy of Doorknob and Dust SpeckConsider again the doorknob from the example in the previous section. The doorknob is treated as a spherical conductor with a uniform static charge q1 =−1.5nC on its surface. What is the electric potential energy…With a figure

Physics18.5 Capacitors and Dielectrics

ExampleCapacitance and Charge Stored in a Parallel Plate Capacitor (part 1)(a) What is the capacitance of a parallel-plate capacitor with metal plates, each of area 1.00 m2, separated by 0.0010 m? (b) What charge is stored in this capacitor if a voltage of 3.00 × 103 V is applied to it?
ExampleWhat battery is needed to charge a capacitor?Your friend provides you with a 10μF capacitor. To store 120μC on this capacitor, what voltage battery should you buy?
ExampleCapacitor for Camera Flash (part 1)A typical flash for a point-and-shoot camera uses a capacitor of about 200μF. (a) If the potential difference between the capacitor plates is 100 V—that is, 100 V is placed “across the capacitor,” how much energy is…

College Physics18.2 Conductors and Insulators

Quick checkCan you explain the attraction of water to the charged rod in the figure below? Figure 18.16With a figure

College Physics18.3 Coulomb’s Law

Example 18.1How Strong is the Coulomb Force Relative to the Gravitational Force?Compare the electrostatic force between an electron and proton separated by 0.530×10^(−10)m with the gravitational force between them. This distance is their average separation in a hydrogen atom.

College Physics18.4 Electric Field: Concept of a Field Revisited

Example 18.2Calculating the Electric Field of a Point ChargeCalculate the strength and direction of the electric field E due to a point charge of 2.00 nC (nano-Coulombs) at a distance of 5.00 mm from the charge.
Example 18.3Calculating the Force Exerted on a Point Charge by an Electric FieldWhat force does the electric field found in the previous example exert on a point charge of –0.250μC?

College Physics18.5 Electric Field Lines: Multiple Charges

Example 18.4Adding Electric FieldsFind the magnitude and direction of the total electric field due to the two point charges, q1 and q2, at the origin of the coordinate system as shown in Figure 18.24. Figure 18.24 The electric fields E1 and E2 at the…With a figure

College Physics18.8 Applications of Electrostatics

Example 18.5Acceleration of a Charged Drop of GasolineIf steps are not taken to ground a gasoline pump, static electricity can be placed on gasoline when filling your car’s tank. Suppose a tiny drop of gasoline has a mass of 4.00×10^(–15)kg and is given a positive…

College Physics19.1 Electric Potential Energy: Potential Difference

Example 19.1Calculating EnergySuppose you have a 12.0 V motorcycle battery that can move 5000 C of charge, and a 12.0 V car battery that can move 60,000 C of charge. How much energy does each deliver? (Assume that the numerical value of each…
Example 19.2How Many Electrons Move through a Headlight Each Second?When a 12.0 V car battery runs a single 30.0 W headlight, how many electrons pass through it each second?
Example 19.3Electrical Potential Energy Converted to Kinetic EnergyCalculate the final speed of a free electron accelerated from rest through a potential difference of 100 V. (Assume that this numerical value is accurate to three significant figures.)

College Physics19.2 Electric Potential in a Uniform Electric Field

Example 19.4What Is the Highest Voltage Possible between Two Plates?Dry air will support a maximum electric field strength of about 3.0×10^6 V/m. Above that value, the field creates enough ionization in the air to make the air a conductor. This allows a discharge or spark that…
Example 19.5Field and Force inside an Electron Gun(a) An electron gun has parallel plates separated by 4.00 cm and gives electrons 25.0 keV of energy. What is the electric field strength between the plates? (b) What force would this field exert on a piece of plastic…

College Physics19.3 Electrical Potential Due to a Point Charge

Example 19.6What Voltage Is Produced by a Small Charge on a Metal Sphere?Charges in static electricity are typically in the nanocoulomb nC to microcoulomb µC range. What is the voltage 5.00 cm away from the center of a 1-cm diameter metal sphere that has a −3.00 nC static charge?
Example 19.7What Is the Excess Charge on a Van de Graaff GeneratorA demonstration Van de Graaff generator has a 25.0 cm diameter metal sphere that produces a voltage of 100 kV near its surface. (See Figure 19.7.) What excess charge resides on the sphere? (Assume that each numerical…With a figure

College Physics19.5 Capacitors and Dielectrics

Example 19.8Capacitance and Charge Stored in a Parallel Plate Capacitor(a) What is the capacitance of a parallel plate capacitor with metal plates, each of area 1.00m^2, separated by 1.00 mm? (b) What charge is stored in this capacitor if a voltage of 3.00 × 10^3 V is applied to it?

College Physics19.6 Capacitors in Series and Parallel

Example 19.9What Is the Series Capacitance?Find the total capacitance for three capacitors connected in series, given their individual capacitances are 1.000, 5.000, and 8.000 µF.
Example 19.10A Mixture of Series and Parallel CapacitanceFind the total capacitance of the combination of capacitors shown in Figure 19.22. Assume the capacitances in Figure 19.22 are known to three decimal places ( C1=1.000 µF, C2=5.000 µF, and C3=8.000 µF), and round…

College Physics19.7 Energy Stored in Capacitors

Example 19.11Capacitance in a Heart DefibrillatorA heart defibrillator delivers 4.00 × 10^2 J of energy by discharging a capacitor initially at 1.00 × 10^4 V. What is its capacitance?