# internal resistance and capacitor - lab instructions

```Practical work: Investigating internal resistance
Teacher instructions
Theoretical background:
Batteries and cells have an internal resistance (r) which is measured in ohms. When
electricity flows round a circuit the internal resistance of the cell itself resists the flow of
current and so thermal (heat) energy is dissipated in the cell itself. Electromotive force
on the source is given by: ε = IR + Ir.
Objective:
To record I-V characteristic of a real source of electric current and determine
internal resistance.
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Equipment:
Unstabilised DC Power supply 6 V
Variable resistor 5kΩ
Switch
Connecting wires (Jumper wires)
Voltmeter/ammeter
Circuit diagram:
Instructions:
1. Implement the circuit on the breadboard as shown in circuit diagram: voltmeter goes to B3-B4, Rp goes to C3C4, switch to B5-B6 and jumper wire to B1-C1.
2. Set voltmeter range to 20 V.
3. Turn on the supply and measure voltage. Don't press the switch. Write the voltage in the table. That is the
voltage when the current is zero and that is the voltage of unloaded source.
4. Disconnect the voltmeter/ammeter, set it to 20 mA and put it in B1-C1 instead of jumper wire.
5. Press the switch and hold it while setting Rp to 5 mA. That way the current in the circuit is set. In next step we
measure the voltage of the source when 5 mA current is flowing.
6. Return voltmeter to its position B3-B4.
7. Turn on the supply and hold the switch while reading the voltage. Write the voltage in the table.
8. Repeat the steps 4-8 for currents 10mA and 15 mA. Write the data in the table.
9. As the last step, put a jumper wire instead of Rp and measure the current and voltage in the same way as
described above. Set ammeter to 200 mA while measuring current. Write the data in the table.
10. Plot a graph to show I-V characteristic of the source.
11. Determine internal resistance of the source.
V/V
I/mA
0
5
10
15
20
Practical work: Investigating capacitors in RC circuit
Teacher instructions
Theoretical background:
When a capacitor is connected to a circuit with Direct Current (DC) source, two processes, which are called
&quot;charging&quot; and &quot;discharging&quot; the capacitor, will happen in specific conditions. Discharging formula for voltage:
−𝑡
𝑉 = 𝑉0 𝑒 𝜏
Objective: Investigate basic characteristics of a circuit with capacitor and resistor in series.
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Equipment:
DC Power supply 10 V
Capacitor 1000 μF
Resistor 20kΩ, 100Ω
Switch
Connecting wires (Jumper wires)
Voltmeter
Circuit diagram is shown below.
Instructions: For charging the capacitor.
1. Implement the circuit on the breadboard as shown in circuit diagram: voltmeter goes to D3-D4, capacitor
goes to C3-C4, resistor to A1-A2, jumper wires to: C2-D2, C5-D5, B6-C6, and switch to B1-C1.
2. Set voltmeter range to 20 V.
3. Voltage od C must be zero before doing this lab (if needed, discharge it through wire at D3-D4).
4. Turn on the supply. Press switch and chronometer at the same time and hold it for 10 s. Read the voltage at
10s.
5. Discharge the capacitor through a jumper wire at D3-D4.
6. Repeat the steps 4-5 for 20, 40, 60, 80, 100s. Capacitor must be discharged before each measurement.
7. Write data in the table.
8. Plot voltage versus time graph.
t/s
0
10
20
40
60
80
100
V/V
Instructions: For discharging the capacitor.
1. Implement the circuit on the breadboard as shown in circuit diagram: voltmeter goes to C3-C4, resistors
to A1-A2 and D3-D4, capacitor goes to B3-B4, jumper wires to: B2-C2, C5-D5, B6-C6, and switch to
C1-D1.
2. Set voltmeter range to 20 V.
3. Turn on the supply and wait for the capacitor to fully charge at 10V.
4. Disconnect the resistor 100Ω.
5. Press the switch and chronometer at the same time and read the voltage at 10s.
6. Return the resistor to its position A1-A2 and wait for it to charge again to 10 V.
7. Repeat the steps 3-6 for 20, 40, 60, 80, 100s. Capacitor must be at 10 V before each measurement.
8. Write the data in the table.
9. Plot voltage versus time graph.
t/s
V/V
0
10
20
40
60
80
100
Calculate the time constant of the circuit.
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