Required Practical: Investigating EMF & Internal Resistance

What is the independent variable of Investigating EMF & Internal Resistance?

resistance, R (Ω) 

What is the dependent variable of Investigating EMF & Internal Resistance?

voltage, V (V) & current, I (A)

What are the control variables of Investigating EMF & Internal Resistance?

• E.m.f of the cell

• Internal resistance of the cell

What are the steps for Investigating EMF & Internal Resistance

1. The cell and the resistor, labelled r, should be connected in series and considered to be a single cell

2. With the switch open, record the reading V on the voltmeter

3. Set the variable resistor to its maximum value, close the switch and record V and the reading I on the ammeter - make sure to open the switch between readings

4. Vary the resistance of the variable resistor up to a minimum of 8-10 readings and record values for V and I for each resistance. Ensure to take readings for the whole range of the variable resistor

What is the analysis of the expreriment?

• The relationship between e.m.f. and internal resistance is given by

E = I (R + r)

• Where:

  • E = electromotive force (V)

  • I = current (A)

  • R = resistance of the load in the circuit (Ω)

  • r = internal resistance of the cell (Ω)

 

• This can be simplified into the form:

E = IR + Ir = V + Ir

• Rearranging this equation for V:

V = –rI + E

• Comparing this to the equation of a straight line: y = mx + c

  • y = V (V)

  • x = I (A)

  • Gradient = –r (Ω)

  • Y-intercept = E (V)

1. Plot a graph of V against I and draw a line of best fit

2. Measure the gradient of the graph and compare it with the manufacturer’s value of the resistor

3. The y-intercept will be the e.m.f and the gradient will be the negative internal resistance:

What are the systemmatic errors when investigating EMF?

• Only close the switch for as long as it takes to take each pair of readings

  • This will prevent the internal resistance of the battery or cell from changing during the experiment

What are the random errors when investigating EMF?

• Only use fairly new cells otherwise the e.m.f. and internal resistance of run-down batteries can vary during the experiment

• Wait for the reading on the voltmeter and ammeter to stabilise (stop fluctuating) before recording the values

• Take multiple repeat readings (at least 3) for each voltage and current and calculate a mean to reduce random errors

What are the safety considerations when investigating EMF?

• This is a very safe experiment, however, electrical components can get hot when used for a long period

• Switch off the power supply right away if you smell burning

• Make sure there are no liquids close to the equipment, as this could damage the electrical equipment