OBJECTIVES

  1. To understand how a potential difference (voltage) can cause an electric current flow through a conductor.
  2. To understand the relationship between voltage, current, and resistance in a DC circuit.
  3. To explain the physics behind the different current readings across the circuit.

THEORY

Direct Current (DC) is the constant flow of electric charge from high to low potential. A direct current circuit is a circuit that electric current flows through in one direction. A direct electric current flows only when the electric circuit is closed, but it stops completely when the circuit is open. A switch is a device for making or breaking an electric circuit. A direct current (DC) electrical circuit consists of a source of DC electricity with a conducting wire going from one of the source terminals to a set of electrical devices and then back to the other terminal, in a complete circuit. A DC circuit is necessary for DC electricity to exist. DC circuits may be in series, parallel or a combination. The electricity moving through a wire or other conductor consists of its voltage (V), current (I) and resistance (R). Voltage is potential energy, current is the amount of electrons flowing through the wire, and resistance is the friction force on the electron flow. The relationship between the voltage, current, and resistance is outlined by Ohm’s law. Ohm’s law states that electric current is directly proportional to voltage and inversely proportional to the resistance. This is represented by the equation [I=V/R]. This law means that as voltage increases, current increases whereas as resistance increases, current decreases.

MATERIALS

Bulb, DC power supply, resistor, alligator clips, current probe, Logger Pro, switch.

PRECAUTION

It was ensured that the connections were done as indicated by the circuit diagram. DC power source was used to supply a voltage of 3V throughout the experiment.

PROCEDURE

For this experiment, six different circuits were connected and in each case, the current flowing in and out of the circuit’s component(s) was recorded and discussed. In the first circuit, a bulb, two current probes, and a switch were connected in series to a 3V DC power source. Two alligator clips were used to form a switch. The switch was connected to the positive terminal of the power source and the two current probes were connected on either side of the bulb to ascertain the current entering and leaving the bulb when the switch is closed. In the second circuit, the number of bulbs was increased to two. The current entering both bulbs when the switch is closed was ascertained. The third and fourth circuits were the same as the first and second circuits. The difference in this case is that the third circuit consisted of a 10Ω resistor in the place of a bulb, while the fourth circuit consisted of a 10Ω and 51Ω resistors in the place of the two bulbs. In the fifth circuit, two switches, bulbs, and current probes were used respectively. The two bulbs were connected in parallel with a current probe placed before each. The parallel connection was done such that there was a loop between the switches. In this case, the behavior of the circuit was determined under the following conditions: when both switches were open and closed, when switch 1 was open and switch 2 closed vice versa. The current in each case was recorded. The sixth circuit was very similar to the fifth, the only difference being that one current probe was connected directly to the negative terminal of the power source, and before any switch. The behavior of the circuit was examined under the same conditions as the fifth circuit. The current on each probe was also recorded. All circuits were connected to a 3V DC power source.

RESULTS AND DISCUSSION

VoltageCP1CP2
3V0.1369A0.1369A

 

In this circuit, the two current probes, here and going forward referred to as CP1 and CP2, recorded currents of 0.1369A (same for the two) respectively. This is because the circuit is connected in series. In a series connection, current or electrons flow in one direction. This implies that the same amount of current flows in and out of each component of the circuit. The voltage will differ for the components however, the current will always be the same.

VoltageCP1CP2
3V0.0949A0.0950A

 

In this circuit, CP1 recorded a current of 0.0949A while CP2 recorded a current of 0.0950A. The currents are approximately the same, and the reason is same as the reason in the first circuit. The components are all connected in series, and as a result, the same amount of current flows through each component. In summary, the current readings from the probes imply that 0.0949A of current flows into bulb A while 0.0950A of current flows out of the bulb and into bulb B.

VoltageCP1CP2Resistor
3V0.2819A0.2822A10Ω

 

In this circuit a 10Ω resistor was used and CP1 recorded a current of 0.2819A while CP2 recorded a current of 0.2822A. Theoretically, using the equation I=V/R (where V is 3V), the current is expected to be 0.3A. The two current probes recorded nearly the same current values because the circuit is connected in series. Resistors connected in series have the current flowing in and out of them, however not the same voltage.

VoltageCP1CP2Resistor
3V0.0469A0.0471A10Ω and 51Ω

 

In this circuit, two resistors of 10Ω and 51Ω were connected in series. Theoretically, with the total resistance in the circuit being 61Ω, the current in this circuit is expected to be 0.049A (using I=V/R where V=3V). Experimentally, CP1 recorded a current of 0.0469A and CP2 recorded a current of 0.0471A.Compared to the circuit in (3), the current here is less because the resistance is more. This proves Ohm’s law which states that current and resistance are inversely proportional. Also, approximately the same amount of current is flowing in and out of the resistors because they are connected in series.

VoltageCP1CP2
S1 and S2 are open3V0A0A
S1 and S2 are closed3V0.1569A0.1226A
S1 open, S2 closed3V0A0A
S1 closed, S2  open3V0.1310A0A

 

When both switches were open, there was no current flowing through the circuit hence the two current probes recorded no current. When both switches were closed, there was flow of current and CP1 recorded a current of 0.1569A while CP2 recorded a current of 0.1226A. This implies that the current flowing into bulbs D and E were not equal. This is because in a parallel connection, the voltage is the same for all the components, however the current is split. Bulb D experienced a higher current because the resistance to the flow of current in bulb D is less than the resistance to the flow of current in bulb E. When S1 (switch 1) was open and S2 (switch 2) was closed, there was no flow of current in the entire circuit. This is because S1 is directly connected to the power source and keeping it open means the circuit is incomplete. When S1 was closed and S2 was open, there was flow of current into bulb D whereas no current flowed into bulb E, hence CP1 recorded a current of 0.131A while CP2 recorded 0A. This is because the circuit containing S1 and bulb D is independent of S2. This is in line with the theory that the components of a parallel circuit are independent of one another. Hence, when one component is not functioning, the other components won’t be affected unlike in a series connection.

 

VoltageCP1CP2
S1 and S2 are open3V0A0A
S1 and S2 are closed3V0.1408A0.2338A
S1 open, S2 closed3V0A0A
S1 closed, S2  open3V0.1220A0.1549A

 

There was no flow of current when both switches were open because the circuit was incomplete, however when both switches were closed, CP2 recorded 0.2338A while CP1 recorded 0.1408A. The current recorded by CP2 is the total current flowing in the circuit while CP1 indicates the current flowing into bulb D. This current is less because in a parallel connection, the current flowing in the circuit is split between the components of the circuit. When S1 was open and S2 was closed, there was no flow of current in the circuit. This is because S1 is connected before the loop, hence if S1 is open, the circuit is incomplete and current won’t flow. When S1 is closed and S2 is open, there is a flow of current in the circuit containing CP2, CP1, S1, and bulb D but not to bulb E. This is because the circuit containing bulb D is a complete circuit that is independent of S2. It can be better put that in a parallel circuit, the components are independent of one another unlike in a series connection where all components are dependent on one another. CP1 recorded a current of 0.1220A while CP2 recorded a current of 0.1549A.

CONCLUSION

Direct current is a type of current that flows only in one direction. The flow of direct current in a circuit is only possible when the circuit is closed, however, if the circuit is open, flow of current stops. A simple circuit requires a power source that supplies a certain amount of voltage as well as wires to connect the components. A switch is used to open or close the circuit. In a DC circuit, supply of voltage creates a potential difference which causes current to flow. The more the voltage supplied, the more the current. This relationship is outlined by Ohm’s law. Also, in a circuit, there is resistance to the flow of current, and this resistance can come from the wires and components in the circuit, or through a resistor. Since resistance impedes the flow of current, it implies that resistance is inversely proportional to the flow of current. Using a current probe, the current flowing in and out of different components of a circuit can be determined. These circuits can either be connected in series or in parallel. In a series connection, the current flowing in and out of the components is equal, however the voltage is different. In parallel circuit, the voltage is the same for every component of the circuit, however the current is not the same. Also the components are independent of one another, i.e. when the switch of one component is open, the other components will still be functioning. For example, in circuit 5 when S1 was closed and S2 was open, current flowed to bulb D not bulb E. Also, when resistors are connected in a circuit, current decreases, and if the resistor is increased, current further decreases. This can be seen from circuits 3 and 4.

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