Physics Practice Work sheet 1 Topic : Current Electricity Class :VIII Prob 1. A student sets up the circuit shown in the fig in a laboratory at room temperature. The electromotive force (e.m.f.) of the battery is 12 V. The switch is closed. (a) The connecting wires in the circuit are made from copper covered by insulating plastic. State the name of the particles that flow in the copper and state the direction in which they are flowing at point Y in the circuit. (b) At room temperature, the resistance of component Z is 6.0 Ω. (i) Calculate the resistance of the whole circuit. (ii) Calculate the current measured by the ammeter. (iii) The current in the ammeter is IA, the current in the 1.5 Ω resistor is IR and the current in component Z is IZ. Write down an equation that relates IA, IR and IZ. (c) The 12 V battery is made from cells of electromotive force (e.m.f.) 1.5 V which are all in series. (i) Calculate the number of 1.5 V cells that make up the battery. (ii) State the e.m.f. of a battery made by connecting all of these cells in parallel. [ You can solve all the problems using the following important information given here, also you can take help from your lecture copy. 1. RT=R1+R2 (for series connection) 2. 1/RT = 1/R1 + 1/R2 (for parallel connection) 3. V=IR 4. In parallel circuit, current divides in parallel portion then again meets and same current w ill flow 5. In series connection of cells ,sum of the voltages of all the cells will work. 6. In parallel connection of cells one after another cell will be used so no calculation needed , just the voltage of individual cell will work one after another.] 2. A student sets up the circuit shown in the Fig. The electromotive force (e.m.f.) of the battery is 6.0 V. (a) The resistance of the variable resistor X is set to 1.8 Ω. Determine (i) the total resistance of the circuit, (ii) the current measured by the ammeter. (b) (i) State what is meant by the potential difference (p.d.) across a component in a circuit. (ii) Add to the Fig. the symbol for a component that is measuring the p.d. across the 2.8 Ω resistor. (iii) The resistance of X is increased. Explain why the p.d. across the 2.8 Ω resistor decreases. [The important information is ‘voltage is directly proportional to resistance’. More resistance of the resistor needs more voltage in the series circuit] 3. How you would connect three resistors, each of resistance 6 ohm so that the combination has resistance a) 9 Ω b) 4 Ω In case a) the combination is connected across the terminals of a battery with p.d. of 12 V (e.m.f) across its terminals Calculate c) the current through each resistor [ans of c) 0.67 A ,0.67 A , 1.33 A] d) the p.d across each resistor. [ ans of d) 4V, 4V, 8V] [You need to draw the circuit for a) and also for b)] [ Hints: if you connect two 6 Ω resistors in parallel and one 6 Ω in series with them ,then total resistance will be 9 Ω. If you connect two 6 Ω resistors in series and one 6 Ω resistor in parallel with them ,then the total resistance will be 4 Ω. ] 4. From the book ‘Physics for O level’ Solve the problems Worked example 18.1 , 18.2 , self-assessment question from page no 279, worked example 18.3 ,18.4, selfassessment question from page no 284, worked example 18.5 , worked example 18.6 ( hints: read the last para of page no 290), worked example 18.7 , 18.8, self-assessments from page no 296.