Recitation 4.1 Electrostatics

EF 152 – Physics for Engineers
Summer, 2013
Recitation 4.1 Electrostatics
Task 1. Concept Questions
1. Two small spheres repel one another electrostatically. Which of the following statements is true?
a. At least one sphere must be charged.
b. Neither sphere needs to be charged.
c. Both spheres must be charged and the charges must have the same sign.
d. Both spheres must be charged and the charges must have opposite signs.
2. Two identical conducting spheres, one that has an initial charge +Q, the other initially uncharged, are brought
into contact. What is the new charge on each sphere?
a.) –Q
b.) −Q/2
c.) zero
d) +Q/2
e.) +Q
3. If you bring a positively charged insulator near two uncharged metallic spheres that
are in contact and then separate the spheres, the sphere on the right will have:
a.) no net charge
b.) a positive charge
c.) a negative charge
4. Two identical conducting spheres, one that has an initial charge +Q, the other initially uncharged, are brought
into contact. While the spheres are in contact, a positively charged rod is moved close to one sphere,
causing a redistribution of the charges on the two spheres so the charge on the sphere closest to the rod
has a charge of −Q. What is the charge on the other sphere?
a.) –2Q
b.) −Q
c.) zero
d) +Q
e.) +2Q
5. Charges A and B exert repulsive forces on each other. qA = 4qB.
Which statement is true?
a.) FA on B > FB on A
b.) FA on B < FB on A
c.) FA on B = FB on A
6. If a positive charge were placed at the origin (the crossing point of the vertical
and horizontal lines) of the figure, into which quadrant would it feel a net force?
a.) A
b.) B
c.) C
d) D
e.) None, it feels no net force.
Task 2. Van de Graaff Generator demonstrations
a) Hold the discharge sphere near the Van de Graff Generator.
b) Have a volunteer stand on an insulating object and place their hand on
the sphere of the Van de Graaff Generator.
c) Tape a paper clip to the surface of the Van de Graaff Generator. This
creates a point on the otherwise smooth sphere. Because this point
concentrates the electric filed here, the majority of leaking charge will
leave the sphere at this location.
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EF 152 – Physics for Engineers
Summer, 2013
Task 3. Practical Electricity
We will talk about three quantities: volts, amps, and ohms. An analogy with fluid flow is often used.
• Voltage is potential, and is like a water pump; it gives energy to the system.
• Current (charge per time), measured in amps, is like the flow rate of the fluid.
• Resistance, measured in ohms, is like a constriction in the pipe, or friction. It is something that takes
energy out of the system.
1. There are two types of currents:
AC: ____________________
2. What type of current is in your house?
3. What is the frequency of the current?
A. 60 Hz
B. 110 Hz
DC: ____________________
What is a Hz anyway? _____________
4. What is the voltage in a typical electrical circuit (the type you plug an appliance into)?
A. 10 V
B. 50 V
C. 120 V
D. 220 V
5. Use a multimeter to measure the voltage from a standard outlet in Estabrook 111.
What would be the setting of the multimeter? ____________ Measured voltage: ________
6. What are you measuring with the voltmeter? A. average voltage
What does rms mean again? __________________
B. peak voltage
C. rms voltage
Volts (V)
7. Fun with calculus:
• Define the voltage as .
• Determine the rms voltage by integrating the
square of the function for one period, dividing
by the period, and then taking the square root.
o Hint: sin x = 0.5(1-cos(2x))
8. If the rms voltage is 120 V, what is
the peak voltage? _______________
Time (sec)
A basic equation of electricity is P = VI, where P is power, V is voltage, and I is current (amps).
9. You plug a 1500 W hairdryer into the circuit. How many amps does the hairdryer draw?
A. 13 A
B. 110 A
C. 165000 A
10. To prevent excessive current (and a hazard), there are circuit breakers which trip if the amperage is
too high. A typical circuit breaker trips at how many amps?
A. 10 A
B. 20 A
C. 50 A
D. 100 A
11. To avoid excessive current, large appliances use a higher voltage circuit.
What is the typical voltage of a clothes dryer circuit?
A. 220 V
B. 440 V
C. 880 V
D. 1000 V
12. Where is 220 V a common voltage? ___________
13. Another basic equation of electricity is V = IR,
where V is the voltage, I is the current (amps),
and R is resistance (Ohms). Predict and measure
the resistance for the following items:
Air (open circuit)
Hair Dryer
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Resistance (Ω)
Resistance (Ω)
EF 152 – Physics for Engineers
Summer, 2013
Task 4. Kill a Watt
1. Plug the Kill a Watt meter into an outlet. Measure the voltage and the frequency.
Voltage __________
Frequency ____________
2. Connect a heater the Kill a Watt. Fill in the table below.
Switch Position
3. Determine the voltage regulation,
∗ 100, where VnL is the voltage with no load, VfL is the voltage
with full load. The voltage regulation is a measure of the ability of a system to maintain a constant voltage.
4. Try the Kill a Watt with different devices.
5. Extra information on power: The power in an AC circuit is obtained as
⋅ , where V is the
rms voltage, I is the rms current, and θ is the phase angle between the voltage and current. Inductance and
capacitance elements cause the current to either lag or lead the voltage. The term pf, which is equal to cosθ,
is called the power factor. The pf of a purely resistive device, such as a heater or a toaster, is almost 1. The
pf of a motor will be around 0.7 or 0.8, because of the existence of inductance in the windings of the motor.
This causes the current to lag the voltage.
Task 5: Switches
1. A simple circuit diagram with a battery, switch, and light is shown at the right.
2. At times we want two switches to control a single light, such as switches at
two entrances to a room. Would the circuit diagram at the right work for this?
3. Can you come up with a switch and wiring scheme that would work for two switches to control
one light? Hint: Use what is called a single-pole, double-throw switch (also called a three-way
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EF 152 – Physics for Engineers
Summer, 2013
4. The circuit diagram for three-way switches, or two switches
controlling a light, is shown at the right. Look at the model of
three-way switches made with knife switches. A wiring diagram
for a circuit is included at the bottom of the page.
5. It is also possible to have to have three switches control a light.
This requires the use of a double-pole, double-throw switch, or a
four-way switch. See if you can follow the logic of the circuit
diagram. Circuit diagrams are also included showing with the
four-way switch in the two different possible positions. A
simple animation of 4-way switches is at
html. Look at how the model of the 4-way switches is wired.
A wiring diagram for 4-way switches is also included. The
neat thing about 4-way switches is that you include as many
as you want in the circuit. By stringing together 4-way
switches, it is possible to have 100 switches control a single light.
Wiring diagram source:
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