emfs

What is the definition of electromotive force (emf)?

the energy supplied per unit charge by a power source. It is the work done to move a unit charge completely around the circuit, including through the source itself

emf formula

E = W/Q

W = energy supplied (J)

unit of an emf

volt

1V = 1JC^-1

how is energy transferred in a battery

1. chemical to electrical energy - (chemical reactions release energy)

2. charge movement - (the battery moves charges through the circuit)

3. energy dissipation - ( electrical energy is converted to heat, light, ect)

equation linking emf, terminal p.d., and internal resistance

E = V + Ir or E = I(R+r)

V= IR

E = emf (V)

V = terminal p.d (V)

I = current (A)

r = internal resistance (ohm)

difference between emf and terminal pd

emf - total voltage supplied by the source

terminal pd - voltage available after accounting for internal resistance

how to measure an emf

• open circuit method - measure V across terminals with no current flowing

• variable resistor method - vary resistance, measure terminal pd and current, and plot a graph

how does internal resistance affect a circuit

• higher internal resistance - greater energy loss inside the battery

• lower internal resistance - more energy available to the external circuit

what is the emf of a battery in series

the sum of individual emfs

E_total = E₁ + E₂

emfs of battery in parallel (assuming all baterries have the same emf)

total emf = the emf of one cell, but the total internal resistance decreases