Electricity – In-class Practice (This is not homework!)

 

1.      A pith ball with charge –2.0 nC is located 30.0 cm from a metal sphere with charge +5.0 mC.  Find the electric force on the pith ball.  Find the electric force on the metal sphere.

2.      Assuming other forces are negligible, find the acceleration of each object due to electric force.
                  m1 = 1.50 g                                          m2 = 3.00 g
                  q1 = -2.40 mC                                     q2 = -975 nC






3.      Three charges are placed in a line, in order from left to right:  q1 = -3.0 mC, q2 = 5.0 mC, q3 = -4.0 mC.  The distance between q1 and q2 is 30.0 cm, the distance between q2 and q3 is 20.0 cm.  Find the net electric force acting on q1.

4.      A pith ball with mass 0.10 grams is suspended from a thread as shown.  Use the given information to determine the angle, q.



5.      Two pith balls, each mass 0.10 grams, are touched by a charged rod and separate as shown.  Determine the charge on each pith ball based on the values of q and d.

6.      In a certain dry cell, 0.500 J of work is done in transferring 0.330 C of charge from the positive to negative terminals.  (a) Find the voltage of the cell.  (b) How much energy may be obtained by allowing 2.00 mC of charge to go from the negative to positive terminals.

7.      A car’s battery maintains an electric potential of 14.0 V.  (a) If 200 C of charge is transferred through the battery to start the car, how much energy is used?  (b) How much charge must be “pulled” from the battery in order to “obtain” 100 J of energy?

8.      A Van de Graaff generator produces 100 kV of potential as it transfers 2 mC of charge from one sphere to another.  (a) How much work is needed to transfer one more electron once the two spheres are at this potential?  (b) How much energy is released when a spark jumps between the two spheres?

9.      As everyone knows, the energizer bunny runs off a 1.50 V dry cell.  Suppose he has a mass of 800 g.  At least how much charge pass through the cell in order for the bunny to march up a hill with elevation 10.0 m?  Can he really keep going and going and going and going?

10.  An electron volt (eV) is an amount of energy used by scientists to describe subatomic particles.  It is equal to the change in energy of an electron moving through a potential difference of 1 Volt.  Find its value in Joules.

11.  An electron is accelerated from rest to a speed of 50 Mm/s between two oppositely charged parallel plates.  Find the electric potential between the plates.

12.  What is the maximum amount of charge that can pass through a 20 A fuse in 1.0 minute?

13.  In how much time does 1.0 C of charge pass through a bulb that has a current of 0.83 A?

14.  A certain 1.5 V “AA” rechargeable cell is rated 500 mA-h.  It takes 3.0 hours for the charger to recharge the cell.  (a) How much charge can be “stored” in the cell according to its rating?  (b) How much current must the charger deliver to the battery as it is charging?  (c) How much energy is stored when the cell is fully charged?

15.  A certain type of rechargeable “C” cell is rated at 1200 mA-h.  Two of these are placed in a flashlight in which the bulb draws 0.25 A current.  (a) How much charge can each cell deliver before “dying”?  (b) How long will the flashlight operate?  (c) How much total energy is stored in the combination of two cells?  (d) Based on the previous two answers what is the power of the flashlight?

16.  A light bulb requires 0.50 A current and 120 V potential.  Determine its Wattage.

17.  A certain electric motor draws 2.0 A current and runs on 240 V.  Determine its power.

18.  Determine the current in a 75 W bulb connected to 120 V.

19.  A certain appliance is rated at 3.0 kW and runs on 240 V.  Determine its current requirement.

20.  A 100 Ω resistor is connected to a 1.5 V battery.  Determine the current that will flow.

21.  A certain galvanometer reads full scale when the current through it is 425 μA and the voltage across its terminals is 35 mV.  Determine the resistance of the galvanometer.

22.  If a current of 140 mA passes through a 4.0 Ω resistor, by how much will the electric potential drop?

23.  A light bulb is connected to a 6.00 V battery.  A current of 175 mA flows through the bulb.  (a) Find the resistance of the bulb.  (b) Find the power of the battery.  (c) What total amount of energy is given off by the bulb in 2.00 minutes?

24.  A cigarette lighter in a car draws 4.0 A from the car’s 14.0 V battery.  The lighter takes 25 seconds to heat up and “pop out”.  (a) Determine the resistance of the lighter.  (b) Determine the amount of heat generated by the lighter in the 25 seconds.

25.  A home stereo speaker is rated 8.0 Ω and maximum power 100 W.  Determine the maximum current that can pass through the speaker before it “blows”.

26.  Two resistors, R1 = 450 Ω and R2 = 225 Ω, are connected in series with a 12.0 V battery.  (a) Determine the equivalent resistance of the two resistors.  (b) Determine the current through the battery.  (c) Determine the voltage and power for each resistor.

27.  A certain 6.0 V battery has an internal resistance of 2.0 Ω.  Suppose it is connected to a light bulb with resistance 40.0 Ω.  (a) Determine the total resistance.  (b) Determine the power output of the bulb.  (c) Determine the power “wasted” due to the internal resistance.

28.  When resistors are connected in series it is sometimes referred to as a “voltage divider” circuit because the voltage applied to the resistors will be “divided” in proportional parts.  Show that this is the case if resistors of 6.0 Ω, 9.0 Ω, and 15.0 Ω are connected in series with a 10.0 volt power source.  (i.e. solve for the voltage across each resistor)

29.  A certain flashlight consists of three “D” cells (1.5 V ea) stacked end to end powering a single bulb.  Suppose the flashlight has power of 4.0 W, determine the current and resistance of the bulb.

30.  A particular string of Christmas lights consists of 50 identical bulbs connected in series across the 120 volts found at a wall outlet.  Each bulb has power of 0.41 W.  Determine the current and resistance for each bulb.

31.  Two resistors, R1 = 450 Ω and R2 = 225 Ω, are connected in parallel with a 12.0 V battery.  (a) Determine the equivalent resistance of the two resistors.  (b) Determine the current through the battery.  (c) Determine the current and power for each resistor.

32.  Pick any number of resistors.  Assign any value of resistance to each and calculate the equivalent resistance if connected in a parallel arrangement.  The equivalent resistance will always be less than that of any of the individual resistances – why?

33.  A PA amplifier drives a single speaker of 8.0 Ω.  Now suppose two more speakers of the same type are connected in parallel to the amplifier – what happens to the total power output of the amplifier?  To answer this, let’s just assume the amplifier supplies a constant voltage of 20 V.  Use this to calculate the power with first one and then three speakers.  Note: what happens with an actual PA system is more complicated than this!

34.  A current of 100 mA flows through a 330 Ω resistor in a certain circuit.  A voltmeter of resistance 10.0 k Ω is connected across this resistor.  Assuming the total current “arriving” at the resistor does not change, what will be the change in the voltage across the resistor.  (Hint: some of the current originally flowing through the resistor will flow through the meter.)  This calculation shows how a voltmeter cannot work perfectly because its presence in the circuit will actually change the voltage that is being measured.  Would it be better for the voltmeter to have a higher resistance or a lower resistance?