# Why is the current time graph for charging a capacitor the same as charging it?Watch

Announcements
#1
For discharging it's pretty obvious charge decreases therefore pd across a capacitor decreases therefore the current decreases but charging??

Edit: I meant why is the one for charging the same as the one for discharging. Why does it decrease?
Last edited by Presto; 6 days ago
0
5 days ago
#2
I think it is because it is a natural process, it is exponential, the capacitor is initially uncharged (not actually true as always in real life some residual charge)
then when you start charging it, it charges at the CR (source resistance x capacitance, time constant) and it never ever is fully charged (practically, gets to 99%, then 99.9%, then 99.99%, then 99.999999 etc etc) so Tau is chosen , as you can actually measure 68.2% or whatever that value is

then discharging, the load R x Capacitance gives teh discharge time constant, from 100% (approx) to 32.8(?)%,

there are some great YouTube explanations, and general Physics chats about these fundamental things

- a practical capacitor has so much other interesting stuff going on, ∈v= permittivity of free space in farads/meter = 8.85 × 10−12, fringe effects, dielectric constant, temperature coefficients (highly important) but they stick to CR & RC charge/discharge as it has a bit of natural log maths to it

last thing, charging source resistance does not necessarily = load discharge resistance, so the two Tau's might be different after all
0
#3
Sorry I didn't mention it's A Level

(Original post by LuigiMario)
I think it is because it is a natural process, it is exponential, the capacitor is initially uncharged (not actually true as always in real life some residual charge)
then when you start charging it, it charges at the CR (source resistance x capacitance, time constant) and it never ever is fully charged (practically, gets to 99%, then 99.9%, then 99.99%, then 99.999999 etc etc) so Tau is chosen , as you can actually measure 68.2% or whatever that value is

then discharging, the load R x Capacitance gives teh discharge time constant, from 100% (approx) to 32.8(?)%,

there are some great YouTube explanations, and general Physics chats about these fundamental things

- a practical capacitor has so much other interesting stuff going on, ∈v= permittivity of free space in farads/meter = 8.85 × 10−12, fringe effects, dielectric constant, temperature coefficients (highly important) but they stick to CR & RC charge/discharge as it has a bit of natural log maths to it

last thing, charging source resistance does not necessarily = load discharge resistance, so the two Tau's might be different after all
0
X

new posts
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• Arts University Bournemouth
Art and Design Foundation Diploma Further education
Sat, 25 May '19
• SOAS University of London
Wed, 29 May '19
• University of Exeter
Thu, 30 May '19

### Poll

Join the discussion

#### How did your Edexcel GCSE Maths Paper 1 go?

Loved the paper - feeling positive (535)
26.38%
The paper was reasonable (884)
43.59%
Not feeling great about that exam (395)
19.48%
It was TERRIBLE (214)
10.55%