C3 - Emf and Internal Resistance Flashcards
Where do charges move, and how is the movement described?
Charges move ‘downhill’ from the cathode to the anode as they transfer electrical energy to the resistance in the circuit. At the anode, they move ‘uphill’ across the cell, gaining electrical energy.
What is emf, and what is it the sum of?
Emf is the total energy supplied to each coulomb of charge.
Emf=sum of p.d.s across all resistances. (Kirchoff’s second law)
How is voltage lost in the cell?
Emf makes current flow. Chemicals provide resistance to current in the cell, causing energy to be lost to this resistance as lost volts. Therefore, the battery has internal resistance.
What’s the formula for emf?
E = V + Vr, or E = I(R+r)
What is terminal P.D.?
This is the voltage across the battery terminals - energy transferred per coulomb of charge in the external circuit.
How can electromotive force and the internal resistance be found on a V-I graph?
V= (-r)I +E
The gradient of the graph, change in voltage over current, is the internal resistance.
The electromotive force is y-intercept.
What has too low an internal resistance to be measured?
Lead-acid car batteries. Therefore, these cannot be used in an experiment.
How can emf and internal resistance be found in an experiment?
Connect two dissimilar metals to a conduction fluid, fruit or vegetable, to form a cell. Use a variable resistor to change the resistance over a range of values and measure current and voltage for this. Draw a V-I graph and find the gradient for r and y-intercept for E.
What happens when load resistance falls?
Current increases, causing more volts to be lost in the battery. This lowers terminal p.d.
