in volts, what potential difference will you measure across the terminals of a dry cell? in what way will it change when you use it in a torch light for a few days? give a reason for the change you observe
Answers
Answer:
Ohm's Law
Journey of a Typical Electron
Resistance
Ohm's Law
Power Revisited
There are certain formulas in Physics that are so powerful and so pervasive that they reach the state of popular knowledge. A student of Physics has written such formulas down so many times that they have memorized it without trying to. Certainly to the professionals in the field, such formulas are so central that they become engraved in their minds. In the field of Modern Physics, there is E = m • c2. In the field of Newtonian Mechanics, there is Fnet = m • a. In the field of Wave Mechanics, there is v = f • λ. And in the field of current electricity, there is ΔV = I • R.
The predominant equation which pervades the study of electric circuits is the equation
ΔV = I • R
In words, the electric potential difference between two points on a circuit (ΔV) is equivalent to the product of the current between those two points (I) and the total resistance of all electrical devices present between those two points (R). Through the rest of this unit of The Physics Classroom, this equation will become the most common equation which we see. Often referred to as the Ohm's law equation, this equation is a powerful predictor of the relationship between potential difference, current and resistance.
Ohm's Law as a Predictor of Current
The Ohm's law equation can be rearranged and expressed as
As an equation, this serves as an algebraic recipe for calculating the current if the electric potential difference and the resistance are known. Yet while this equation serves as a powerful recipe for problem solving, it is much more than that. This equation indicates the two variables that would affect the amount of current in a circuit. The current in a circuit is directly proportional to the electric potential difference impressed across its ends and inversely proportional to the total resistance offered by the external circuit. The greater the battery voltage (i.e., electric potential difference), the greater the current. And the greater the resistance, the less the current. Charge flows at the greatest rates when the battery voltage is increased and the resistance is decreased. In fact, a twofold increase in the battery voltage would lead to a twofold increase in the current (if all other factors are kept equal). And an increase in the resistance of the load by a factor of two would cause the current to decrease by a factor of two to one-half its original value.
The table below illustrates this relationship both qualitatively and quantitatively for several circuits with varying battery voltages and resistances.
Circuit
Diagram
Battery
Voltage
(ΔV)
Total
Resistance
()
Current
(Amps)
1.
1.5 V
3
0.50 Amp
2.
3.0 V
3 Ω
1 Amp
3.
4.5 V
3
1.5 Amp
4.
1.5 V
6
0.25 Amp
5.
3.0 V
6
0.5 Amp
6.
4.5 V
6 Ω
0.75 Amp
7.
4.5 V
9 Ω
0.50 Amp
Rows 1, 2 and 3 illustrate that the doubling and the tripling of the battery voltage leads to a doubling and a tripling of the current in the circuit. Comparing rows 1 and 4 or rows 2 and 5 illustrates that the doubling of the total resistance serves to halve the current in the circuit.
Because the current in a circuit is affected by the resistance, resistors are often used in the circuits of electrical appliances to affect the amount of current that is present in its various components. By increasing or decreasing the amount of resistance in a particular branch of the circuit, a manufacturer can increase or decrease the amount of current in that branch. Kitchen appliances such as electric mixers and light dimmer switches operate by altering the current at the load by increasing or decreasing the resistance of the circuit. Pushing the various buttons on an electric mixer can change the mode from mixing to beating by reducing the resistance and allowing more current to be present in the mixer. Similarly, turning a dial on a dimmer switch can increase the resistance of its built-in resistor and thus reduce the current.