Voltmeters are tools used to measure the potential difference between two points in a circuit. The voltmeter is connected in parallel with the element to be measured, meaning an alternate current path around the element to be measured and through the voltmeter is created. You have connected a voltmeter correctly if you can remove the voltmeter from the circuit without breaking the circuit. In the diagram at right, a voltmeter is connected to correctly measure the potential difference across the lamp. Voltmeters have very high resistance so as to minimize the current flow through the voltmeter and the voltmeter's impact on the circuit. AmmetersAmmeters are tools used to measure the current in a circuit. The ammeter is connected in series with the circuit, so that the current to be measured flows directly through the ammeter. The circuit must be broken to correctly insert an ammeter. Ammeters have very low resistance to minimize the potential drop through the ammeter and the ammeter's impact on the circuit, so inserting an ammeter into a circuit in parallel can result in extremely high currents and may destroy the ammeter. In the diagram at right, an ammeter is connected correctly to measure the current flowing through the circuit.
An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter of resistance 500 Ω are connected in parallel to a 220 V source. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?
Given, Equivalent resistance, Rp of the three appliances connected in parallel is given by, 1Rp = 1R1+1R2+1R3 = 1100+150+1500= 16500 Rp = 50016Ω = 31.25 Ω Resistance of electric iron will be equivalent resistance of the three appliances connected in parallel = 31.25 Ω Applied voltage, V = 220 V Current through the appliance, I = VR = 220 V31.25Ω =7.04 A.
Answer :
To measure the potential difference between two points, a voltmeter should be connected in parallel across these points. Voltmeter is always connected in parallel to the element of any electrical circuit across which potential difference is to be measured. |