For the capacitor charging experiment, use 1000  F capacitor with 100K potentiometer and 6V battery. (Not Power Supply). For the ammeter, use a Nuffield grey ammeter without any shunt. For the voltmeter, use a standard unilab orange digital meter. (In order to have high internal R) This set up discharges fully in about 90 sec. The pupils should see this so that it can be discussed afterwards. Demo if necessary. Teacher Notes It is good to give the kitchen foil capacitor demo a bit more teaching relevance by using the circuit below EHT A 1 1 A 2 2 Use separate switches, leaving the ones on the EHT switched on. (to give more rapid response) 1. Close switch 1. Watch ammeter. Discuss. 2. Open switch Close switch 2. Watch ammeter 2. Discuss

Capacitors in Electrical Circuits. A capacitor is a device which is designed to ‘store’ charge. At its simplest, a capacitor consists of 2 large conducting plates with an insulator between them, so the circuit symbol for a capacitor is: Now consider what will happen if a capacitor is put into an electrical circuit as shown below. When the switch is closed, a current will flow, building up positive charge on the left hand plate of the capacitor, and negative charge on the right. As the charge builds up, it will oppose the flow of further charge onto the plates, so the current will decrease. Eventually there will be so much charge on the plates that no more can flow onto them, so the current will be zero. The capacitor is then said to be fully charged. A Demo: Aluminium Foil & Bin liner capacitor & discuss.

Now consider what happens to the voltage across the capacitor. When the switch is first closed, the capacitor doesn’t oppose the flow of current, so it has very low ‘resistance’, so the voltage across it is very small. As the charge builds up, the capacitor starts to oppose the flow of current, so its ‘resistance’ gets bigger, and the voltage across it increases.. Eventually the capacitor prevents current flowing completely, so its resistance is infinite, and the voltage across it is the same as the EMF of the battery. V For the time being, our maths isn’t good enough to deal with this situation of changing current and voltage, so we have to be a bit cunning to devise a way in which we can find out how the charge stored on the capacitor varies with the voltage across it. We use a variable resistor instead of a fixed one. This means we can reduce the resistance of the resistor as the ‘resistance’ of the capacitor increases, thus keeping the overall resistance of the circuit constant. This means the current remains constant, so we can use the GCSE formulato calculate the charge on the capacitor. Q = It

Experiment: Connect up the circuit shown below. The ‘wiggly’ lead is a flying lead which can be connected either to charge the capacitor, or to discharge it through the ammeter and variable resistor. 1. Charge the capacitor and make sure the variable resistor is set to its maximum resistance 2. Move the flying lead to the discharge position and record the current on the micro ammeter. 3. Whenever the current starts to drop, adjust the variable resistor to make it go back to its original value. 3. Take readings of voltage every 10 seconds, making sure that the current is at its original value whenever you take a reading. (The voltage will be changing, so take the reading on each 10 second mark – teamwork !) 4. Fill in the table below and plot a graph of charge against Voltage. Time (s) Voltage (v) Charge (  C) (Q = It) Current =  A A V

Conclusions: The charge is proportional to the voltage. A capacitor with a steep gradient would store a large amount of charge for a small voltage. A capacitor with a shallow gradient would store a small amount of charge for a large voltage. A ‘good’ capacitor Not such a ‘good’ capacitor The quantity ‘capacitance’ tells us how much charge a capacitor can store per volt across it. It can be found from the gradient of the Q/V graph. And from the formula Coulombs Volt Farads Capacitance is measured in ‘Farads’, and a ‘Farad’ is a coulomb per volt.

Capacitors in Electrical Circuits. A capacitor is a device which is designed to ‘store’ charge. At its simplest, a capacitor consists of 2 large conducting plates with an insulator between them, so the circuit symbol for a capacitor is: Now consider what will happen if a capacitor is put into an electrical circuit as shown below. A

Now consider what happens to the voltage across the capacitor. V For the time being, our maths isn’t good enough to deal with this situation of changing current and voltage, so we have to be a bit cunning to devise a way in which we can find out how the charge stored on the capacitor varies with the voltage across it. We use a variable resistor instead of a fixed one. This means we can reduce the resistance of the resistor as the ‘resistance’ of the capacitor increases, thus keeping the overall resistance of the circuit constant. This means the current remains constant, so we can use the GCSE formulato calculate the charge on the capacitor.

Experiment: Connect up the circuit shown below. The ‘wiggly’ lead is a flying lead which can be connected either to charge the capacitor, or to discharge it through the ammeter and variable resistor. 1. Charge the capacitor and make sure the variable resistor is set to its maximum resistance 2. Move the flying lead to the discharge position and record the current on the micro ammeter. 3. Whenever the current starts to drop, adjust the variable resistor to make it go back to its original value. 3. Take readings of voltage every 10 seconds, making sure that the current is at its original value whenever you take a reading. (The voltage will be changing, so take the reading on each 10 second mark – teamwork !) 4. Fill in the table below and plot a graph of charge against Voltage. Time (s) Voltage (v) Charge (  C) (Q = It) Current =  A A V

Conclusions: The charge is proportional to the voltage. A capacitor with a steep gradient would A capacitor with a shallow gradient would The quantity ‘capacitance’ tells us It can be found from the gradient of the Q/V graph. And from the formula Capacitance is measured in

Questions Page 95 Charge V Capacitance