CURRENT ELECTRICITY
Let us look at the basic terms used in current electricity. They are
- Electric current (I): This can be defined as the rate at which charge is transported.
I =Q/T, where I is electric current in ampere, Q is electric charge in coulomb, T is time in second. Also Q = ne, where n is the number of charges.
- Ammeter: this is a device used to measure current flowing in a circuit. It has a low resistance. It must be connected in series with the circuit elements. An ideal ammeter must have a negligible resistance.
- Resistance: this is the measure of opposition to the flow of current through a material. A body with lower resistance will allow greater current to flow through it. Its unit is ohms (Ω). According to ohms law, it states that the electric current flowing through a metallic conductor is directly proportional to the potential difference provided that other factors remain constant (V =IR).
NB: Conductors that obey ohms law are called ohmic conductors while conductors that do not obey ohms law are called NON- ohmic conductors. Examples are transistors, radio valves, semiconductor ( silicon and germanium), rectifiers etc
- Resistor(R): this is a device which is specially designed to provide a known amount of resistance in a circuit.
- Internal resistance(r): this is the resistance offered by the electrolyte to the flow of electric current. The internal resistance is responsible for the reduction in the efficiency of the cell. An ideal cell is a cell with negligible internal resistance.
- Potential difference: this is the workdone in moving unit charge from one end of the circuit to the other. Its unit is volts(V)
- Voltmeter: this is a device used to measure the potential difference or voltage in a circuit. It has a high resistance.It must be connected in parallel with the circuit element.
Question: Why is ammeter connected in series?
Answer: Since the same amount current flow through circuit element connected in series, ammeter is connected in series so that it will be able to measure the current flowing through them.
Question: Why is voltmeter connected in parallel?
Answer: a voltmeter is connected in parallel so as to measure the potential difference between two points
The efficiency of most cells is less than 100% as a result of lost volt arising from the internal resistance. An ideal cell is a cell with 100% efficiency.
- Electromotive force (EMF): this is the workdone in moving unit charge round the entire circuit. The EMF of a cell is not exactly the same as the terminal voltage. The difference in value is called the lost volt resulting from the internal resistance of the cell.
EMF = Terminal Voltage + Lost Volt
Terminal voltage, V = IR
Lost volt = Ir
EMF = V + Ir, V = EMF - Ir
EMF = IR + Ir
EMF = I(R +r) ,
Electromotive force is also the voltage across the cell in an open circuit i.e. when the cell is not delivering any current.
- Efficiency of a cell: this can be defined as the ratio of the output power to the input power. To know more about electric cell, click on it
- Potentiometer: this is a device used to measure the internal resistance of a cell accurately. It can also be used to compare the EMF of two cells.
- Metre Bridge: this device is used for measuring the resistance of a given resistor. It consist of a standard resistor, a standard cell, a jockey and a galvanometer.
- Wheatstone bridge: this consists of three standard resistors and one unknown resistor. The galvanometer is also used to established balance point.
At balance point, potential difference
across AB is zero.
Examples
- A cell of internal resistance 1Ω supplies current to an external resistance 3Ω. The efficiency of the cell is
Solution
- A cell can supply current of 0.4A and 0.2A through a 4Ω and 10Ω resistor respectively. The internal resistance of the cell is ?
Solution
- Three cells each of EMF 1.5V and an internal resistance of 1Ω are connected in parallel across a load resistance of 2.67Ω. calculate the current in the load.
- Find the value of R
- From the diagram below, the value of X is
Solution
- If 16milli ampere of current flows through a conductor in one second. The number of electrons transported per seconds is (I electronic charge = 1.6 ×10^-19C)
Solution
Questions
- Six dry cells each of emf 2V and internal resistance of 1Ω are connected in parallel across a load of 3Ω. Calculate the effective current in the circuit.
- A battery of emf 12V and internal resistance 0.5Ω is connected to 1.5Ω and 4Ω series resistors. Calculate the terminal voltage of the battery.
- A cell of emf 1.5V is connected in series with a resistor of resistance 3Ω. A voltmeter connected across the cell registers 0.9V. Calculate the internal resistance of the cell.
- A cell of emf 1.5V and internal resistance 1Ω is connected to two resistors of resistance 2Ω and 3Ω in series. Calculate the current through the resistors.
- A current of 10A passes through a conductor for 10secs. Calculate the charge flowing through the conductor.
- Which of the following material is a conductor? (a)plastic (b)sodium (c)wood (d)glass
- A current of 10A passes through a conductor for 10sec. calculate the charge flowing through the conductor.
- A current of 2.0A passes through a conductor for 50secs. Determine the quantity of electric charge that flows through it in the given time
- The terminal voltage of a battery is 4V when supplying a current of 2A and 2V when supplying a current of 3A. The internal resistance of the battery is?
- A resistance R is connected across the terminal of an electric cell of internal resistance 2Ω and the voltage was reduce to 3/5 of its nominal value. The value of R is
- What is the value of R
Summit your answers in the comment section below