EMF, Grade 10 Physics: Electric Circuits EMF When you measure the potential difference across (or between) the terminals of a battery you are measuring the ”electromotive force” (emf) of the battery. This is how much potential energy the battery has to make charges move through the circuit. This driving potential energy is equal to the total potential energy drops in the circuit. This means that the voltage across the battery is equal to the sum of the voltages in the circuit. We can use this information to solve problems in which the voltages across elements in a circuit add up to the emf. EMF = V$_{total}$ Worked Example: Voltages I Question: What is the voltage across the resistor in the circuit shown? Answer Step 1 : Check what you have and the units We have a circuit with a battery and one resistor. We know the voltage across the battery. We want to find that voltage across the resistor. V$_{battery}$ = 2V Step 2 : Applicable principles We know that the voltage across the battery must be equal to the total voltage across all other circuit components. V$_{battery}$ = V$_{total}$ There is only one other circuit component, the resistor. Vtotal = V1 This means that the voltage across the battery is the same as the voltage across the resistor. V$_{battery}$ = V$_{total}$ = V1 V1 = 2V Worked Example: Voltages II Question: What is the voltage across the unknown resistor in the circuit shown? Answer Step 1 : Check what you have and the units We have a circuit with a battery and two resistors. We know the voltage across the battery and one of the resistors. We want to find that voltage across the resistor. V$_{battery}$ = 2V V$_{resistor}$ = 1V Step 2 : Applicable principles We know that the voltage across the battery must be equal to the total voltage across all other circuit components. V$_{battery}$ = V$_{total}$ The total voltage in the circuit is the sum of the voltages across the individual resistors V$_{total}$ = V1 + V$_{resistor}$ Using the relationship between the voltage across the battery and total voltage across the resistors V$_{battery}$ = V$_{total}$ V$_{battery}$ = V1 + V$_{resistor}$ 2V = V1 + 1V V1 = 1V Worked Example : Voltages III Question: What is the voltage across the unknown resistor in the circuit shown? Answer Step 1 : Check what you have and the units We have a circuit with a battery and three resistors. We know the voltage across the battery and two of the resistors. We want to find that voltage across the unknown resistor. V$_{battery}$ = 7V V$_{known}$ = 1V + 4V Step 2 : Applicable principles We know that the voltage across the battery must be equal to the total voltage across all other circuit components. V$_{battery}$ = V$_{total}$ The total voltage in the circuit is the sum of the voltages across the individual resistors V$_{total}$ = V1 + V$_{known}$ Using the relationship between the voltage across the battery and total voltage across the resistors V$_{battery}$ = V$_{total}$ V$_{battery}$ = V1 + V$_{known}$ 7V = V1 + 5V V1 = 2V Worked Example: Voltages IV Question: What is the voltage across the parallel resistor combination in the circuit shown? Hint: the rest of the circuit is the same as the previous problem. Answer Step 1 : Quick Answer The circuit is the same as the previous example and we know that the voltage difference between two points in a circuit does not depend on what is between them so the answer is the same as above V$_{parallel}$ = 2V. Step 2 : Check what you have and the units - long answer We have a circuit with a battery and three resistors. We know the voltage across the battery and two of the resistors. We want to find that voltage across the parallel resistors, Vparallel. V$_{battery}$ = 7V V$_{known}$ = 1V + 4V Step 3 : Applicable principles We know that the voltage across the battery must be equal to the total voltage across all other circuit components. V$_{battery}$ = V$_{total}$ The total voltage in the circuit is the sum of the voltages across the individual resistors V$_{total}$ = V$_{parallel}$ + V$_{known}$ Using the relationship between the voltage across the battery and total voltage across the resistors V$_{battery}$ = V$_{total}$ V$_{battery}$ = V$_{parallel}$ + V$_{known}$ 7V = V1 + 5V V$_{parallel}$ = 2V Xem thêm: Physics 10.IX Electric Circuits High School Students Studying the Sciences Physics