Careful Measurement of Emf and or Internal Resistance of a Cell
Careful measurement of emf and or internal resistance of a cell
Aim
To measure voltage and current in the circuit and from that figure work out emf and internal resistance of the cell, to identify errors involved and deal with them in the most effective way and maintain a safe working environment at all times.
Background
EMF (electromotive force) is a measure of work done per unit of charge and is measured in volts. Internal resistance is resistance in ohms of the cell. I will be using a 1.5V battery in the experiment. I will measure the voltage and current using multimeters.
Calculation method
I intend to rearrange the equation ‘ε=Ir+IR’ to form ‘V= -Ir +ε’ and then draw the y=mx+c graph equation to find EMF and internal resistanc`e. The y axis intercept measures EMF and the gradient gives you internal resistance.
Projected main sources of error
The multimeter has some inaccuracies: Resistance is accurate to 0.1 ohms (+/-0.8%), Current is accurate to 0.03 A (+/- 2%) and voltage is accurate to 1 mV (+/- 0.5%). These on their own are minor problems but may have a cumulative affect. The resistance of all the wires joined together is 0.6 ohms. But the biggest uncertainty after early testing is the fluctuations on the ammeter, the ammeter very rarely settles on a value which leads to an inaccuracy of 0.02 volts (+/- 0.01 volts) and 0.02 amps (+/- 0.01 amps). This uncertainty could be very high especially with lower values
First experiment
I started with a simple experiment to get figures from which improvements could be achieved.
This is the circuit diagram of my first experiment
It was soon clear that the variable resistor didn’t have the necessary resistance to induce the current necessary for the experiment. I replaced the original variable resistor with a rheostat as the resistance could be changed in a more linear way rather than with the exponential increases of the variable resistor.
There are few safety hazards in my experiment but to be sure I made sure
Aim
To measure voltage and current in the circuit and from that figure work out emf and internal resistance of the cell, to identify errors involved and deal with them in the most effective way and maintain a safe working environment at all times.
Background
EMF (electromotive force) is a measure of work done per unit of charge and is measured in volts. Internal resistance is resistance in ohms of the cell. I will be using a 1.5V battery in the experiment. I will measure the voltage and current using multimeters.
Calculation method
I intend to rearrange the equation ‘ε=Ir+IR’ to form ‘V= -Ir +ε’ and then draw the y=mx+c graph equation to find EMF and internal resistanc`e. The y axis intercept measures EMF and the gradient gives you internal resistance.
Projected main sources of error
The multimeter has some inaccuracies: Resistance is accurate to 0.1 ohms (+/-0.8%), Current is accurate to 0.03 A (+/- 2%) and voltage is accurate to 1 mV (+/- 0.5%). These on their own are minor problems but may have a cumulative affect. The resistance of all the wires joined together is 0.6 ohms. But the biggest uncertainty after early testing is the fluctuations on the ammeter, the ammeter very rarely settles on a value which leads to an inaccuracy of 0.02 volts (+/- 0.01 volts) and 0.02 amps (+/- 0.01 amps). This uncertainty could be very high especially with lower values
First experiment
I started with a simple experiment to get figures from which improvements could be achieved.
This is the circuit diagram of my first experiment
It was soon clear that the variable resistor didn’t have the necessary resistance to induce the current necessary for the experiment. I replaced the original variable resistor with a rheostat as the resistance could be changed in a more linear way rather than with the exponential increases of the variable resistor.
There are few safety hazards in my experiment but to be sure I made sure